A Guide to Furnace Heat Exchanger Troubleshooting, Inspection, & Leaks
InspectAPedia^® -

How to inspect and test furnace heat exchangers for leaks
Furnace heat exchanger leak testing
Furnace heat exchanger cleaning procedures
Carbon monoxide leak warnings for heating furnaces
Photographs show clues indicating leaky, dangerous furnace heat exchangers
How much combustion gas or CO leakage from a furnace leak exchanger is allowed?
Is there a difference in safety between leakage in a new heat exchanger and leaks that occur in use due to damage, rust, or a crack?
Details of the DeWerth furnace heat exchanger test method & leak calculations
Details of the Matzen furnace heat exchanger test method
Recommendations for reliable furnace heat exchanger testing procedures
Questions & answers about furnace heat exchanger leaks, cracks, damage, inspection & testing

Furnace heat exchanger inspection, troubleshooting, and leak testing guide: This heating system test article describes how to inspect furnace heat exchangers for leaks. We compare and evaluate the reliability of all of the various furnace heat exchanger testing methods, we explain just how much leakage is "acceptable" by industry standards, and we conclude with recommendations for reliable heat exchanger testing and inspection.

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We recommend that you never rely on visual inspection alone to determine the safety of a furnace heat exchanger. Readers of this document should also see CARBON MONOXIDE - CO and see BACKDRAFTING HEATING EQUIPMENT. More about carbon monoxide - CO - is at CARBON MONOXIDE WARNING. We include the text from historical articles on methods used for testing furnace heat exchangers for leaks, and the allowable or standards for heat exchanger cracks, holes, leaks, or carbon monoxide hazards from such leaks. Also see HEAT EXCHANGER CLEANING

Watch out: Dangerous carbon monoxide gas leaks, potentially fatal, can be present intermittently depending on variations in heating system operation and building conditions. This website answers most questions about central heating system troubleshooting, inspection, diagnosis, and repairs. We describe how to inspect residential heating systems to inform home owners, buyers, and home inspectors of common heating system defects.

Readers of this article should also see How to Inspect Heating Systems and those considering using instruments to test heat exchangers for leaks should review Recommendations for gas measurement instruments & gas detector tubes for indoor gas level tests. Also see DUCT & AIR HANDLER ODORS.

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How to inspect a furnace heat exchanger for damage or leaks & carbon monoxide CO gas hazards

Visual Inspection of the Furnace Heat Exchanger can Detect Some Leaks but Not All

In response to a reader who wrote that they have a G14Q3, installed in 1988, Lennox Pulse Furnace and who was wondering if there are any visual inspections for signs of a defective heat exchanger we provide the furnace heat exchanger inspection suggestions described in this article.

The photographs shown just above are two examples of rust and flame marks on a furnace that would be a basis for further inspection of the condition of the heat exchanger for cracks, rust perforation, or other unsafe conditions. But heat exchanger leaks can occur in a variety of locations and parts, including:

Cracks or open seams in the heat exchanger at various locations due to heat stress, mechanical stress, mechanical damage. Similarly, seams in the heat exchanger may have come apart or been improperly joined at manufacture, or the manufacturing process may have failed to fully crimp closed a heat exchanger seam
Rust perforations in the heat exchanger such as those shown above or simply thinning of the heat exchanger wall (to 50% or less of original thickness) due to rusting. Rust may occur due to condensate leaks onto the heat exchanger from an air conditioner coil, from humidifier leaks, or simply from location of the furnace in a damp or wet location.
Mechanical holes in the heat exchanger, for example due to lost or loose screws, or cleanout ports that were not closed properly or were lost during cleaning
Gasket or sealant leaks: other writers also cite these heat exchanger leaks: broken crimped rings, broken or leaking heat exchanger furnace seals or gaskets, including cemented seals

A Complete List of Methods Used to Find or Test for Leaks in a Furnace Heat Exchanger

Visual inspection of the heat exchanger for cracks: It is possible in some cases to see a damaged heat exchanger by spotting Cracks in the steel, discoloration, or soot. We illustrated and discussed visual inspection of heat exchangers just above.

Extending the heat exchanger visual inspection with light and mirror: One can try with a flashlight and mirror to extend the total area that can be seen above the Burner itself (with the burner off of course).

This photograph shows a significant hole in an oil fired heat exchanger. This damage was visible through the inspection door above the oil burner on the front of the furnace.

Using a telescoping mirror and flashlight one can inspect most of the interior of most oil fired furnace combustion chambers and most of the interior of the simple can-type oil fired furnace heat exchangers. But don't forget to use the other inspection methods discussed in this article.

Extend the heat exchanger inspection by examining from the supply air plenum: One can also often see the top section of a heat exchanger by inspecting it through an opening in the Supply air plenum (in an up-flow system).

Indirect heat exchanger inspections - signs of causes of heat exchanger damage:

One might inspect "indirectly" by observing external evidence that indicates a risk of heat exchanger damage, such as exposure of the furnace to wet conditions or flooding conditions, evidence of water leaks onto or into a furnace heat exchanger (say from a humidifier mounted above the heat exchanger, or a plumbing leak onto the system, or from an air conditioning condensate leak or an air conditioning system drip tray leak that may have sent water onto a heat exchanger leading to rust perforation.
Direct visual inspection of the heat exchanger using a powerful flashlight; but keep in mind that it virtually impossible to see all, or often even most of the heat exchanger during this type of inspection.

Inspect for Heat Exchanger Leaks by Evidence at the furnace burner - flame test: for evidence of a heat exchanger crack or leak by watching for a change in flame pattern or color when the furnace blower just starts to operate.

Sit (not too close) where you can see the gas flame on a gas-fired heating furnace, then have someone turn up the thermostat to cause the furnace to start. Normally the burner will ignite and burn for a time (seconds to a minute or so) before it is warm enough for the fan limit control to turn on the blower fan.

When you hear the furnace blower fan start to operate be especially alert for a change in the color or pattern of the flame at the furnace's gas burner. (You can't to this test with an oil fired heating furnace).

If the flame pattern suddenly jumps, wavers, or changes color this is a strong suggestion that the heat exchanger is damaged. In the photograph we show inconsistent gas flame patterns at a gas fired heating furnace - additional inspecting and testing for safety were needed.
Special flame test: to check for heat exchanger leaks makes use of a chemical sprayed into the blower compartment while the burner is operating. If the blower is sending air into the heat exchanger the flame changes color. (VAPO HEAT HT-1Q. )

Heat exchanger test measuring pressure changes: the magnehelic gauge test. The heat exchanger is sealed, a special gauge connected to the unit at a pressure sensing port, and the blower is operated. Gauge reading changes can indicate that the blower is pressurizing (leaking into) the heat exchanger.
Heat exchanger water spray test: (requires disassembly of the furnace) - the heat exchanger exterior is sprayed with water or a soap solution and its interior is examined for water entry
Heat exchanger test using a penetrating dye such as Magna Flux: liquid dye (red or fluorescent dye) is sprayed over an area of the heat exchanger expected of leakage and the interior is inspected for evidence of the dye. If using a fluorescent dye the heat exchanger is examined using a UV or "black" light.
Heat exchanger test using special gases: AGA has described a test gas mixture of nitrogen/methane inserted into a plugged heat exchanger (also may require disassembly in some cases). A less sophisticated gas test burns a small quantity of sulphur inside the heat exchanger while the inspector looks (subjectively?) for a sulphur odor in the living area, rising through the ductwork. [Not recommended.] More reasonable and safe is the use of refrigerant gases and a halogen leak detector such as the Using the TIF 5000 Gas Detector - very sensitive. Also see REFRIGERANT LEAK DETECTION.
Heat exchanger test using a "smoke bomb": a smoke bomb is lit inside the heat exchanger and the inspector watches for smoke escaping into the plenum. Properly used smoke bombs can be a very sensitive test as the smoke bomb pressurizes the heat exchanger with smoke when it ignites. Similar to the smoke bomb, but without the risk of placing a burning device into the heat exchanger is use of an external smoke source (chemical) that is used to blow smoke into the heat exchanger. We have read that some technicians used a lit piece of roofing felt inside the heat exchanger as a smoke source. We do not recommend this method.
Heat Exchanger Leak Detection Instruments measuring flue gases or carbon monoxide (CO) in the plenum: A flue gas leak or carbon monoxide detection instrument such as the TIF8800 combustible gas analyzer or other hand-held gas detectors, other carbon monoxide testing methods, or a home carbon monoxide detector (which should be installed in any home but especially homes using hot air heat and most especially in homes using natural gas or liquid propane (LP-gas) fired furnaces) can detect the presence of carbon monoxide or heating system combustion gases (even without carbon monoxide in the case of the two references above), but the absence of detection of flue gases or carbon monoxide at any particular time cannot be taken as a guarantee that the heat exchanger is not damaged and leaking. Similar tests check changes in the CO level or for oxygen levels greater than 0.5% in the flue. See Using the TIF 8800 Gas Detector.

Watch out: testing for flue gases or CO as a sole method for testing for heat exchanger leaks is unreliable. Depending on whether or not the blower fan is operating, where the leak is located, and burner adjustment, the heat exchanger may have a leak that goes undetected by instruments. Be sure to check measurements both before and after the blower fan has begun to operate.

Depending on where the damage is on the heat exchanger and whether or not the furnace blower is operating, a leak may be present but remain undetected.
Common heat exchanger leak, crack, or rust points: that should be included in an inspection include cracks at welds, seams, and on certain models, a heating service technician may know that defects have been found at a particular location.
Known causes of heat exchanger damage: in addition to the possibility of leaks and rust damage we've already discussed, other conditions can make a visible or hidden heat exchanger crack or opening more likely to have occurred. These include evidence that the system was dropped or damaged during shipment or installation, or knowledge of or evidence of overheating, such as the furnace's burner having been forced to operate past the normal high temperature limits of a fan limit switch.

We encountered this last damage after an inspector, knowing that once the blower comes on any furnace gas leak may be diluted or even reversed in direction, wired the furnace's gas burner to keep operating past the normal high limit on that safety control, so that s/he would have a longer period to test the system for leaks.
Known problem furnace or heat exchanger brands: Some heating furnace brands and models have become known to have frequent or specific safety concerns. If your furnace model has been recalled or has had a safety warning issued concerning it that information can often be found by searching the US Consumer Product Safety Commission's website or by asking your local heating contractor to check that information for you. Some examples of heat exchanger and carbon monoxide warnings about heating products include:

Lennox Pulse Furnace Safety Problems, Recall, Inspection, Advice
Weil McLain Model GV Gas Boiler/gas valve CPSC recall/repair
Goodman Furnace High Temperature Plastic Vent HTPV safety recall US CPSC notice
Home Heating System Should Be Checked [for proper venting and for CO Carbon Monoxide Hazards - DJF]

Watch out: SAFETY WARNING: Any evidence of furnace heat exchanger damage or of carbon monoxide or flue gas leaks should be taken seriously and those heating systems should be immediately checked by a professional.

Visual Inspection Alone is Unreliable for Detecting Furnace Heat Exchanger Leaks or Damage

Relying on visual inspection of heat exchangers: Given that a lot of the heat exchanger surface simply cannot be seen without completely disassembling the System, we would not rely on a visual inspection alone to decide if a system was damaged or not. There are other Tests using pressure testing or more commonly, tracer gas testing, that are more reliable.

Relying on gas detection instruments: Relying on gas detection instruments alone, without a visual inspection of the system is also dangerous and can falsely indicate that no problem in present when in fact the heating system is unsafe. We discuss the reasons for this at Some warnings about relying on instruments for detection of hazardous gases in buildings.

Install CO detectors in buildings, as well as smoke detectors. Ultimately the combination of expert inspection, testing, and the use of carbon monoxide detectors and smoke detectors will make a significant improvement in the safety of any home heating system.

Smells and odors in the building could be an indicator of heat exchanger leaks - see DUCT & AIR HANDLER ODORS.

What is the Allowable Amount of Gas Fired Heat Exchanger Leakage?

Some gas industry experts (Douglas DeWerth & others ) have published studies indicating that in a new gas fired furnace heat exchanger and system, the allowable flue gas leakage rate can be equal to about the leakage expected from a 1/8" diameter hole. This effort at making a reasonable standard recognized that there could be safe imperfections in heating system manufacture. [1] through [11].

Studies (DeWerth) and standards such as published by AGA (american gas assoc) may define the "allowable leakage" from a heat exchanger, but DeWerth's "1/8 equivalent leak hole size" standard quoted by Friedman in the ASHI Technical Journal article was intended to address the condition of NEWLY manufactured units and assumed particular equipment size, room size, air movement etc. that may not pertain to an in-use older or damaged heating system.
The actual leakage from a crack depends on variables during operation as well as its location - for e.g. when a blower is running air may go into the combustion chamber through the crack rather than the other way around.
We believe but would need to research it that any manufacturer will advise that if a crack has occurred the unit needs repair or replacement. (Sometimes repair by welding may be acceptable but it's doubtful).
Ultimately ANY crack in a heat exchanger that was caused by wear and tear or some event like improper operation is potentially dangerous because we don't know that it won't suddenly increase in size or change how it leaks flue gases into the building air as operating conditions change. For that last reason we'd be surprised if any HVAC contractor would risk the lives of the occupants (nor a lawsuit) by saying leaving the unit in place is OK.

Heat Exchanger Leak Detection Articles

Watch out: The following articles on methods for detecting leaks or level of damage to furnace heat exchangers originally appeared in The ASHI Technical Journal, Vol. 2 No. 1, July1991, (D. Friedman, Ed.) and should be used for background and historical purposes only.

DeWerth's article was shared with ASHI for the information of home inspectors and for comment;

Matzen's article (published in - The ASHI Technical Journal, Vol. 2 No. 1, July1991) was submitted to DeWerth and the AGA and to other ASHI reviewers for information and comment as well. As of publication of the 1991 issue of the Journal neither group had replied to the other's paper.

A Technical Committee review of this paper starts on page 29 Heat Exchanger Testing and Test Devices: Who's Right? - The ASHI Technical Journal, Vol. 2 No. 1, July1991, (D. Friedman, Ed.)

The research summary for this study notes that while the methodology may not be the ultimate approach for assessing the need to replace furnace heat exchangers, in 1991 it was considered by the Gas Appliance Manufacturer Association's (GAMA) project advisory committee to be the most effective approach. Further, GAMA committee members want to and may already have initiated activity to have the methodology adopted into ANSI standards.

Three Step Method for Detecting Unacceptable Flue Gas Leakage from Furnace Heat Exchangers

Douglas W. DeWerth, P.E. - The ASHI Technical Journal, Vol. 2 No. 1, July1991, (D. Friedman, Ed.), Reprinted with permission, This paper was originally printed by the Gas Research Institute, GRI 84/0162.

In developing a three-step furnace heat exchanger test procedure, this article first comments on shortcomings of alternative contemporary procedures for testing heat exchangers. Next the author explains the derivation of the size of hole which should be detected in a failed heat exchanger and, based on some assumptions about house air changes per hour and similar factors, the equilibrium level of CO which would occur from such a leak is computed.

These data provide a basis for specifying the level of CO which must be detected in ppm. For inspectors not familiar with visual inspection techniques, the first two steps of the three step process will make interesting reading. The third step, using a tracer gas, provides an alternative which ASHI readers should evaluate against contemporary procedures. Readers who are not mathematically inclined should not be troubled by the equations in this paper. They are included for completeness but are not necessary to understand the material.

Overview of the Three Step Method for Detecting Unacceptable Flue Gas Leakage from Furnace Heat Exchangers

A three-step method for detecting unacceptable leakage of flue gases through furnace heat exchangers was developed and field tested. The first step of the method is thorough visual examination of the heat exchanger with a strong light and a mirror. Next a check of the burner flames is made with and without the circulating air blower operating. The final step is to trace the migration of a 14.3 percent methane in nitrogen mixture from the combustion chamber to the circulating air side of the heat exchanger if a leak is present. A portable combustion gas detector is used which can be calibrated to respond to a leakage rate equivalent to 200 ppm carbon monoxide.

The method was field tested by seven utility companies which compared the new three-step method to their normally used method. The majority of the utilities felt the new method was more reliable and accurate than their currently used method.

This study and report were prepared under contract by American Gas Association Laboratories and was provided for publication by the Journal.

Portions have been edited or abstracted. Readers will note that the first two steps of the method are visual only. The third method requires purchase of special equipment which is in the same price range or less costly than other flue gas detection methods discussed by Matzen. Use of step three of this method requires the mixing and purchase of a 14.3% methane/nitrogen mixture which is noncombustible and cannot be diluted with air to a combustible mixture.

Laboratory evaluation of existing leak detection methods concluded that none of the current methods were completely acceptable, being either too sensitive, potentially harmful to the appliance or home environment, or creating unrealistic conditions during the performance of the test.

Heat Exchanger Testing and Test Devices: Who's Right?

The research summary for this study notes that while the methodology may not be the ultimate approach for assessing the need to replace furnace heat exchangers, it is considered by the Gas Appliance Manufacturer Association's (GAMA) project advisory committee to be the most effective approach. Further, GAMA committee members want to and may already have initiated activity to have the methodology adopted into ANSI standards.

Background

Gas furnaces have earned a reputation for safe, environmentally benign performance. Steps have been taken in designing these furnaces to prevent flue products from leaking into the home environment. However, for infrequent situations where a heat exchanger has become excessively cracked or corroded, it is possible for some undesirable flue gas to leak into the circulating air system. For most furnace types, excluding those with power burners [forced draft conversion burners], such leakage can occur only for short, intermittent portions of the operational cycle. Leakage can occur in furnaces with power burners whenever the burner is on, because the flue gases are generally at a higher pressure than the circulating air side of the heat exchanger. If left uncorrected defective heat exchangers can become an annoyance and impact the comfort and well being of the indoor occupant.

Currently there are several procedures in use to detect leakage from a heat exchanger: for example, visual examination, smoke bombs, odor tracing, and salt spray as well as commercially available kits such as Leak-Seek<190> (lithium citrate traced through the heat exchanger). The effectiveness of these methods has been evaluated. Some have undesirable side effects, such as accelerated heat exchanger corrosion caused by salt spray. Other test methods are overly sensitive. It is important that the test method is not so severe that it detects minute, inconsequential openings, since this could induce a home owner to replace a sound furnace unnecessarily.

Some of the evaluated methods were partially successful, but the results of most were suspect and/or had undesirable side effects. In an effort to obtain a better technique the Gas Appliance Manufacturer's Association (GAMA) initiated a research project in 1980 which was continued with support from the Gas Research Institute (GRI).

The scope of the research program involved:

review of and evaluation of all existing leak detection methods
determination of normal operating conditions of furnaces from the standpoint of pressure differences across the heat exchanger
development and field testing of an acceptable test procedure
An accurate and reliable heat exchanger leak detection methodology has been developed. The method was tested by seven utilities during the 1982-83 heating season.

Review of existing leak detection procedures

Gas utility servicemen use a variety of procedures in attempts to identify potentially dangerous leaks in furnace heat exchangers. The procedures would be used in response to service calls for

odors
fluttering burner flames
suspected cracked heat exchanger
sooted heat exchanger
routine service

Thirteen different procedures [for testing furnace heat exchangers for leaks] were identified after surveying utility companies. Many of these methods do not give reliable results, thus often more than one method is used. Some methods are corrosive to the heat exchanger. Others are performed under unrealistic conditions (such as increased pressure in the heat exchanger), or are so sensitive that they indicate leakage even in new heat exchangers.

Visual Inspection of the Heat Exchanger

One of the most common methods used is a visual inspection of the heat exchanger with a strong light. The serviceman must then make a judgment as to whether any cracks or holes he finds are large enough to warrant replacement of the furnace. Generally any fault is reason for recommending that the home owner obtain a second opinion of the need for replacement of a heat exchanger.

Smoke Bombs for Heat Exchanger Tests

The use of smoke bombs is the second most common practice. With the flue outlet and burner access opening blocked a smoke bomb or candle is ignited in the heat exchanger. Observations for smoke are then made in the circulating air side of the heat exchanger. When the bomb explodes a positive pressure is created inside the heat exchanger forcing the smoke out of very small cracks.

This makes the test very sensitive. Precautions are also needed with this method so the smoke does not get into the house where it may stain certain paints, fabrics, and tiles. An air analysis test may be used in addition to the smoke test. This test detects combustion products in the circulating air stream as an indirect indication of heat exchanger cracks. If high readings are obtained there is no doubt that the heat exchanger has a crack or fault. With slight increases a more thorough examination of the heat exchanger may be required. The drawbacks of this method are that it is complicated and time consuming to perform. Also the detection device is expensive.

Editor's note: See Matzen and other articles [below] for an evaluation and survey of this equipment. Significant to Matzen et al, DeWerth does not say these tests are necessarily invalid. Odorants, such as sulphur candles and oil of wintergreen have been used for detecting leaks. A small quantity of the odorant is introduced into the combustion side of the heat exchanger; the hot air registers in the house are then checked for the characteristic odor. Any odor in the circulating air would indicate a heat exchanger leak. With the sulphur candle this test can be quite reliable, but very unpleasant smelling. The wintergreen is more pleasant, but the odor clings to the serviceman's hands and clothes, making the test unreliable.

Tracer Gas Tests for Furnace Heat Exchanger Leaks

Another test procedure is to release a small amount of carbon monoxide (CO) in the hot heat exchanger and then check for high concentrations of CO in the circulating air stream. The detector needed for this method is expensive and the use of CO is dangerous. Freon has been used as a tracer gas with a halogen leak detector. Freon gas is released in the combustion side of the heat exchanger and if the halogen leak detector senses halogens in the circulating air stream a leak is present. A problem with this method is phosgene gas (a deadly poison) can be generated if the freon passes through a flame. Also, freon is heavier than air.

Spray Dyes for Heat Exchanger Tests

A spray can of fluorescent solution is marketed which is sprayed into the heat exchanger with a suspected leak and penetrates even the finest crack. Then the crack is detected using an ultraviolet light; however the heat exchanger must be accessible for close examination of all surfaces.

Chemical Smoke Tests of Furnace Heat Exchangers

The American National Standards for Gas-Fired Central Furnaces (Z21.47-1978) outlines a test method using a fuming or smoking material such as titanium tetrachloride. The material is introduced into the combustion chamber, and if combustion products are discharged through door cracks or other openings, their presence will be revealed by observing the smoke. A drawback to this method is that titanium tetrachloride is very corrosive.

Another widely-used procedure involves spraying a sodium salt solution into the burner flames and checking the circulating air stream with a propane torch for the presence of sodium ions. [Detection of Cracked Heat Exchangers in Warm Air Furnaces , J.F. Wunderlin, Wisconsin Gas Co., GV-6, June 1978.]

If the blue flame of the torch turns yellow, this indicates the presence of sodium ions and a leak. As the solution is sprayed into the burner flame, visual inspection of the heat exchanger surface below the burner port is needed. If a solution of sodium chloride (table salt) is used, corrosion of the heat exchanger can be accelerated. Sodium bicarbonate salt solutions, on the other hand, are non-corrosive. The drawbacks to the sodium ion tracing method are that dust in the air can be mistaken for the sodium ion and acceptable leakages may be detected as the test is very sensitive.

A procedure similar to the sodium ion tracing is to trace the lithium ion, with the same drawbacks.

All of these methods have some undesirable drawbacks. The ideal method for detecting the leakage of flue gases into the circulating air stream would not be harmful to the furnace components of the home environment, would be performed under realistic operating conditions, and would not detect small amounts of acceptable leakage.

Furnace Heat Exchanger Pressure Measurements for Leak Detection

The pressure drop across the furnace heat exchanger surface has a significant effect on the tendency of flue gases to pass from one side of the exchanger to the other. Only if the flue gas side is more positive than the circulating air side will there be a tendency for flue gases to leak into the circulating air through any cracks or corrosion holes in the heat exchanger.

In order to determine the actual operating pressure inside and outside the heat exchanger, measurements were made with four furnaces with typical heat exchangers. ["A Simple Test That You Can Use to Check a Furnace for Leaks," American Artisan, September 1966.]

Two of the furnaces were equipped with atmospheric burners, one with an induced draft system, and the fourth with a power burner. Flue gas pressure taps were attached at two-inch intervals up the side of the heat exchanger from the burner port to the flue outlet. Circulating air taps were located adjacent to the flue gas taps.

Pressure measurements were taken with and without the circulating air blower in operation. The results showed that with atmospheric burners the average pressure on the flue gas side with the burners operating is about 0.02 inch water column (w.c.). The average pressure on the circulating air side of the heat exchanger is 0.3 inches w.c. when the blower is operating. Thus the air side of the heat exchanger is more positive than the flue gas side when the circulating blower is on. If there were a hole leakage would be from the air side to the flue gas side.

When the blower is off the air side pressure is less positive than the flue gas side and leakage would occur from the flue gas side to the air side. This condition exists only during the short heat up period before the blower starts.

An induced draft system has a negative pressure of about 0.2 inch w.c. or less on the flue gas side of the heat exchanger at all times. The flue products are drawn through the combustion chamber causing the flue gas side to always be under a negative pressure. A positive pressure on the air side of the heat exchanger, caused by the circulating air blower, further insures that the flue gases stay in the combustion chamber.

With a power burner system the flue gas side of the heat exchanger must have a positive pressure of as much as 0.4 inch w.c. Thus, the pressure on the flue gas side is more positive than the air side, whether the circulating air blower is on or off. The only exception is the top of the heat exchanger where the circulating air blower may impinge directly onto the heat exchanger. Air side pressure would be more positive than the flue gas side at these points. If there were a hole or crack, flue gases would leak into the circulating air stream at all times except if the hole or crack were at the top of the heat exchanger.

Table 1

summarizes the various systems and the leakage potential due to the pressure drop across the heat exchanger. In a furnace with atmospheric burners flue gases could only leak into the circulating air system if there were a hole or crack during the time the blower was not operating. Induced draft furnaces would not allow any leakage through the heat exchanger as they operate under a negative pressure. A power burner system is more critical in that the flue gas side of the heat exchanger is always more positive than the circulating air side when the furnace is operating. Thus, if there were a hole or crack flue gases could leak into the circulating air stream.

This analysis of flue gas leakage based on the pressure difference across the heat exchanger is true for relatively small holes. For heat exchangers with large perforations flue gases may leak regardless of the pressure differential. Large perforations would be detected by visual observation of the heat exchanger or by the flame pattern. No further test procedure would then be needed.

Test methods should not subject the heat exchanger to abnormally high pressures, and should not detect insignificant leakage which may occur from pin holes in welded seams or other manufacturing imperfections.

Minimum Hole Size Allowable in a Furnace Heat Exchanger

It is almost impossible to construct a heat exchanger that is entirely air tight. Therefore any test method developed to detect flue gas leakage needs to have quantitative aspects. It would not be desirable to identify as unacceptable any heat exchanger leakage that meets the requirements for heat exchanger joints according to the American National Standards.

The standards for Gravity and Fan Type Direct Vent Wall Furnaces (Z21.44.1981), and for Direct Vent Central Furnaces (Z21.64-1978), requirement for tight joints in heat exchangers is met if the combustion chamber-vent section does not leak more than 2% of the flue gases. This test is conducted with the internal pressure of the heat exchanger raised to 0.1 w.c. static pressure.

The leakage from the minimum allowable hole is calculated from:

size 12 L sub C = .02^V^I> where:

size 12 L sub C = maximum allowable flue gas leakage rate, cubic feet per hour,

size 12 V - 15 cubic feet of flue products per 1000 BTU, assuming 50% excess air,

size 12 I - Input rate, 1000 BTU/hour For an 80,000 BTU/hour furnace:

size 12 L sub C = left ( {.02} right ) left ( {15} right ) left ( {80} right )

size 12 L sub C = <_> 24 cubic feet per hour of flue gases allowed to <_> <_> <_> <_> leak.

Figuring the diameter of a heat exchanger leak opening from the area concentration of leaked gas that will occur as a result

It is possible to calculate the hole size needed to leak 24 cubic feet of gases using an orfice flow equation assumed equal to a particular or specified opening diameter. Passing 24 cubic feet of flue gases per hour would require a leaking orfice of 0.051 sq.in. For a pressure drop across the orfice of .4 inch w.c. the orfice would be 0.025 square inches.

0.51 sq.in. computed Leak Opening for a Furnace Heat Exchanger

The diameter of an orfice for .051 sq.in. was found to be the computed size of an acceptable equivalent total flue gas leakage opening as follows:

DeWerth [8] used a desired rate of flow in cubic feet per hour, a constant of 1658.5 to convert units, a .9 orfice factor, the area of the orfice in sq. in., a pressure drop across the orfice of 0.1 in. w.c. - which is conservatively high for all but power burners, and the specific gravity of flue gases assumed equal to 1.

Passing 24 cubic feet of flue gases per hour would require a leaking orfice of 0.051 sq.in. For a pressure drop across the orfice of .4 inch w.c. the orfice would be 0.025 square inches.

The diameter of an orfice for .051 sq.in. is computed as follows:

Square root of (4 x Area divided by Pi), as .254 in. and the diameter of an orfice of .025 sq.in. is .180 in. [GRI 84/0162 p. 10]

1/4" diameter Hole in a Heat Exchanger - Unacceptable at 80,000 BTUH example

This calculation implies that it would take a 1/4 inch diameter hole in the heat exchanger to leak an unacceptable amount of flue gases for our example of an 80,000 BTUH furnace with an internal pressure of 0.1 inch w.c. In an effort to select a conservative condition in the development of the test method, a 1/8 inch hole was selected as the minimum size hole that should be detected. A hole with one half the diameter results in an area one fourth that of the 1/4 inch diameter hole, which would reduce the flow by a factor of four according to the orfice equation. [2].

Maximum Leakage Allowed

After conservatively determining that the minimum hole size representing an unacceptable condition was a 1/8 inch diameter hole, it was necessary to select the maximum CO concentration to be allowed in the flue gases leaking through the 1/8 inch hole. A 100% safety factor was used on the maximum 400 ppm air-free CO allowed by the American National Standards for Furnaces Z21.47-1983. Thus, 200 ppm air-free CO was used as reference for detection.

The appropriateness of the selection of 200 ppm CO can be verified using an analysis based on the maximum allowable room concentration of CO. An equation was developed

Carbon Monoxide Hazards From House Heaters Burning Natural Gas, G.W. Jones, et al., Technical Paper 337, Department of the Interior, Washington, D.C. 1923. which relates the room concentration to emission levels over time. Figure 1 [next page] shows this equation together with all the involved factors.

From this equation it can be seen that if assumptions can be developed to define

size 12 CO sub A , size 12 N,V, > < >and

size 12 R> < >it would be possible to calculate

size 12 CO sub R> < >after a time

size 12 T > < >< >representing equilibrium. The

size 12 CO sub R > < >value would be the CO concentration in the house at equilibrium.

Editor's note: the equation computes COR, the CO in the room, in ppm, based on the CO in air-free products of combustion ppm, the number of air changes per hour in the room, the volume of the room, the volume of dry air-free combustion products per 1000 BTU of fuel burned (8.52 cu. ft. for natural gas), the burner input rate in BTU/hr., a Naperian logarithmic base of 2.7133, and T, the time needed to reach a given concentration (of CO) in hours.

House Air Changes Per Hour

A review of literature on tightness of house construction was used to determine a value of

size 12 N> , < >room air changes per hour.

size 12 CO sub R = {CO sub A left ( {1 - 1 over e sup NT} right )} over NV > <_> <_> where

size 12 CO sub R> <_> = CO in room, ppm

size 12 CO sub A> <_> = CO in air-free products of combustion (in FAF <_> <_> <_> <_> <_> duct), ppm

size 12 N> <_> <_> <_> = Number of room air changes per hour

size 12 V> <_> <_> <_> = Volume of room, cubic feet

size 12 P> <_> <_> <_> = Volume of dry air-free combustion products, <_> <_> <_> <_> <_> cu. ft. per 1000 BTU of fuel burned (8.52 <_> <_> <_> <_> <_> <_> <_> cu. ft. for nat. gas)

size 12 R> <_> <_> <_> = Input rate, thousands of BTU/hr.

size 14 e> <_> <_> <_> = Naperian logarithmic base (2.7133)

size 12 T> <_> <_> <_> = Time to achieve a given concentration, hours.

A great deal of what is written on house infiltration cites an old rule of thumb that leakage rates vary from 0.5 to 1.5 air changes per hour (ACPH) and average about 1.0 ACPH.

This overall average is shown in the ASHRAE Handbook of Fundamentals and is also used by the EPA in its proposed text for inclusion in DOE's rule making for the Residential Conservation Service (RCS) program.

The current generation of houses being built, however, appear to have an average infiltration rate between 0.6 and 0.86 ACPH. Assuming that the average ACPH for houses built two decades ago had an average of 1.0 ACPH, the average has dropped 15 to 20 percent over that time. Current air infiltration studies generally support the 0.6 to 0.86 ACPH figure.

Natural Ventilation of Modern Tightly Constructed Homes, AGA/LIGT Conference on Natural Gas Res. & Tech., Chicago, IL, July 1982.

Air Leakage Characteristics and Weatherization Techniques for Low Income Housing, DOE/ASHRAE Conference on Thermal Envelopes, Orlando, FL December 1979.

Residential Air Infiltration , ASHRAE Technical Paper, Philadelphia, PA 1979

Building Energy Data Compilation Analysis and Demonstration , DOE Contract W-7405-ENG-48, Lawrence Berkeley Laboratory, 1980.>

[Discussion of contents of the references by the author is in his original report.]

House Air Change Rate

Based on the above data and references, a realistic, conservative house room air change rate would be 0.5

size 12 (N = 0.5 size 12 )>

House Volume: A small house with 9000 cubic feet (1125 sq.ft. x 8 ft. ceiling) was assumed.

size 12 (V = size 12 9000)>

Furnace BTU Rate: The house was equipped with an 80,000 BTU/hour furnace with a 50 percent load factor.

size 12 R = size 12 .50(80,000)>

Flue Gas Spillage Destination: 100 percent of the flue gases generated were assumed to spill into the house (which would never occur under realistic conditions)

Time to Equilibrium: From exercising the equation it takes about eight hours for equilibrium conditions to be reached.

size 12 T = size 12 8>

CO Level at Equilibrium: size 12 CO sub R = {( 200 ) ( 8.52 ) ( 40 ) left ({ 1 - 1 over {e sup (.5)(8)}} right )} over {( 0.5 ) ( 9,000 )}>

size 12 CO sub R = size 12 14.9> < >ppm.

Using the size 12 CO sub R> < > equation above, this would mean that after a period of eight hours the house CO concentration would be no more than 15 ppm.

Allowable CO Exposure in buildings

The CPSC, in its report on Health Effects of Carbon Monoxide evaluated the scientific basis for suggesting long term exposure limit for CO and concluded that the value should be no more than 15 ppm as a time weighted average. OSHA Concentration Limits for Gases as shown in the Federal Register (Volume 39, Number 125, 6/27/74) specifies a maximum eight hour weighted average of 50 ppm. Much higher than the 15 ppm allowed here.

The American Conference of Governmental Industries Hygienists also discusses their recommended Threshold Limit Value of 50 ppm in Documentation of the Threshold Limit Values (3rd Ed., 1971). This source also reports that the CO limit in the USSR is 18 ppm and in Czechoslovakia 30 ppm.

Therefore by developing a leak detection method that would allow no more than 200 ppm CO to leak from a crack or hole in a furnace heat exchanger the home environment should remain safe if 100 percent of the emissions from a properly adjusted furnace were released into the indoor air.

Specification of the Test Gas for Detecting Heat Exchanger Leaks

A mixture of 14.3 percent methane in nitrogen was used as the tracer gas as this mix cannot be diluted in air to obtain a combustible mixture.

[We've omitted an interesting but lengthy section which explains the reason for selection of the particular test gas, including an explanation of why certain concentrations of combustible gas, including high concentrations, will not support combustion (insufficient oxygen).-Ed.]

Description of the Test Procedure for Heat Exchanger Leaks

The developed method traces the migration of 14.3 percent methane in nitrogen from the combustion side to the air side of the heat exchanger. The presence of the gas mixture in the circulating air side is detected with a combustible gas detector which is calibrated to respond to about 200 ppm CO, the maximum leakage concentration chosen.

Figure 2 shows the set-up for the Three-Step Method.

Step One - Visual Inspection

Step One is to conduct a thorough visual examination of the heat exchanger. Clean any loose particles on the visible surfaces of the heat exchanger, use a mirror and a strong flashlight. Inspect the internal sections for signs of split seams, open cracks, severe deterioration.

Examine joints between flue gas passages of the heat exchanger and other parts of the furnace. If construction is such that a portion of the heat exchanger or radiator is in the cold air return compartment, special care should be given when examining these parts.

Access for visual inspection of the heat exchanger is frequently limited by evaporator coils, etc. therefore a removable inspection plate, access panel, or heat register on the plenum would be helpful to visually examine the exchanger from the air side. Any visible crack or hole in the heat exchanger is reason for requiring repair.

Step Two - Flame Observation

The furnace is then turned on and Step Two: an observation of the flames before and after the circulating air blower comes on is made. After the unit is hot, the gas and electrical power to the furnace are shut off (the blower is not operational for the rest of the test).

Observe the flame pattern for floating flames and flame rollout or any flame distortion. These observations indicate a possible split seam, open crack, severe deterioration of the heat exchanger or mechanical separation of the heat exchanger from the jacket. Disturbance of the flame by the blower is a reason for requiring repair of the heat exchanger.

Step Three - Tracer Gas

Step three is then performed: the tracer gas is injected into the combustion chamber and the calibrated gas detector is used to check for the presence of methane on the air side of the heat exchanger.

Prepare an access hole in the plenum over the heat exchanger, as close to the heat exchanger as possible. If you cannot get within 3 inches, any opening as close as possible will be acceptable but you'll have to allow more time for reaction.

Allow the furnace to operate at least 5 minutes, then quickly conduct the rest of the procedure while the heat exchanger is warm.

Check the vent connector for any blockage

Turn off the main burner and pilot and power supply to the unit

Insert the gas detector probe into the selected area in the plenum and null out any background disturbance

Place the injector probe for the tracer gas in the bottom of a heat exchanger section. Adjust the flow rate of the tracer gas to seven cubic feet per hour. Maintain this flow rate through the balance of the test. For multiple section heat exchangers do one section at a time.

As the heat exchanger is flooded move the gas detector probe to cover the top of the heat exchanger section for at least two minutes.

If an unacceptable leak is present the calibrated indicator

If the tic rate increases during the probing period but the light does not go on, there is no unacceptable leak. But it may be desirable to further investigate if the tic rate increase is substantial.

If the light goes on, the leakage rate is unacceptable and the source of the leak should be investigated by further probing. This is a reason for requiring replacement of the heat exchanger.

Repeat the procedure for the remaining heat exchanger sections

Any access openings made in the furnace plenum to conduct the test must be closed or sealed.

If no reason for corrective action is indicated, re-light the pilot and turn the furnace back to its ready condition in accord with the manufacturer's rating plate or instructions. Seal the hole in the plenum with a small piece of sheet metal.

Combustible Gas Detector Specifications

Alkaline battery, low battery indicator light, hand held, portable, maximum weight two pounds. Warm up time maximum 30 seconds. Maximum operating temperature for probe and instrument: 150 degF. Must detect CH4 and CO. Calibration: internal is desirable with a tick indicator at a low level (less than 20 ppm CO); an indicator light for a low level (200 ppm) of combustible gas; an alarm light and audible signal at 50 percent of the LEL for gas leak detection. Calibration should be plus or minus 5%.

The combustible gas detector can be purchased from

J&N Enterprise, PO Box 188, Wheeler IN 46398 219-759-1142
Pragmatics Inc., PO Box 737, Manchester MO 63011 314-225-6786
Sierra Monitor Corporation, 1050K Duane Ave., Sunnyvale, CA 94086 408-746-0188
The calibration gas (200 ppm CO/N2) and working tracer gas (14.3% CH4/N2) from
Matheson, Dayton OH 45424 513-236-3021
AIRCO, Chicago, IL 60628 312-468-4200.

References

American Gas Association, 1515 Wilson Boulevard, Arlington, VA 22209
Gas Appliance Manufacturer's Association, 1901 North Fort Myer Drive, Arlington, VA 22209
Gas Research Institute, 8600 West Bryn Mawr Avenue, Chicago, IL 60631
Douglas W. DeWerth, P.E. is an ASHRAE Member and is Group Manager, Residential Appliances/GATC, Research and Development, for the American Gas Association in Cleveland, OH. Readers should also note two relevant and interesting articles provided to the Journal by R. Matzen and A. Carson, respectively:
John N. Kirkpatrick, MD, Occult Carbon Monoxide Poisoning,The Western Journal of Medicine , (January 1987 146:52-56) available from the author, The Mason Clinic, PO Box 900, Seattle, WA 98111-0900
F. Steel, Airtight Houses and Carbon Monoxide Poisoning,Canadian Building Digest , Division of Building Research, National Research Council Canada, Ottawa, Canada K1A OR6 ISSN 0008-3097 UDC 614.8:728 pages 222-1 - 222-4.

Gas Furnaces and Indoor Air Quality

Richard C. Matzen

As houses are made increasingly energy efficient, heating system combustion byproducts become important health considerations. High on the list of pollutants is carbon monoxide (CO) an odorless, tasteless toxic gas. In 1982 the U.S. Consumer Products Safety Commission reported 340 carbon monoxide deaths; 290 (85%) resulted from gas appliances in the home. From 1979 through 1986 the average number of carbon monoxide deaths has dropped to 62 per year. Further, these figures do not reflect the number of families who suffer poor health or who have other complaints ascribed to exposure to carbon monoxide. A significant number of home inspectors elect to exceed the ASHI Standards of Practice by concerning themselves with this topic. This article reviews the mechanism of carbon monoxide poisoning, defines acceptable CO levels, and suggests techniques for testing common natural gas furnaces for heat exchanger failures. Carbon monoxide monitors and monitor use are also discussed.

The equipment and methods discussed in this article are based on information from various manufacturers and on the author's experience. This discussion may not be technically complete. Material for this article has been submitted to the American Gas Association and other experts for comment. Feedback will be published in subsequent issues of the ASHI Technical Journal. A Technical Committee review of this paper starts on page 29.>

What Is Carbon Monoxide Poisoning?

Hemoglobin is an oxygen-bearing protein found in the blood of animals and man. When hemoglobin combines with carbon monoxide, carboxyhemoglobin is formed. Carboxyhemoglobin cannot carry oxygen. Therefore, if an animal or person is exposed to carbon monoxide long enough, the person will have insufficient levels of hemoglobin-carrying oxygen and serious illness or death can result.

Carbon monoxide and hemoglobin have a tremendous attraction to one another and in fact carbon monoxide is absorbed by hemoglobin at 210 times the rate of oxygen absorption. Blood volume is relative to body mass, small people (especially children) and pets will absorb carbon monoxide and lose their critical oxygen carrying capacity more quickly than large people. People with heart disease who cannot increase the rate of oxygen delivery are candidates for early symptoms of carbon monoxide poisoning.

Carboxyhemoglobin converts back to hemoglobin shortly after exposure, but if exposure is continuous carboxyhemoglobin begins to accumulate. For example, when an individual smokes, exposure lasts several minutes and carboxyhemoglobin converts to hemoglobin prior to the next cigarette. When the smoker has a defective muffler in his car, he gets carbon monoxide from two sources. When the smoker has a bad muffler and sits in a traffic jam morning and evening and goes home to a defective furnace, carboxyhemoglobin levels accumulate.

It is common for families with a defective furnace to suffer headaches, nausea, fatigue, poor concentration, sleep disturbance and palpitations. Such a family will cease to thrive but the failure to thrive will occur well before the family visits a doctor.

Other air quality problems, such as inadequate makeup air, can cause complaints <197> not the subject of this paper.

In addition to carbon monoxide, natural gas combustion produces formaldehyde (HCHO). Formaldehyde is similar to carbon monoxide's molecular structure except it contains additional hydrogen atoms. Formaldehyde is a colorless gas that can cause nausea, watery eyes and burning sensations in the eyes and throat.

Irritation from formaldehyde starts at a far lower level than carbon monoxide. By comparison, the industrial level for exposure to carbon monoxide is 50 parts per million (ppm) for a healthy worker for 8 hours and exposure to formaldehyde is 3 ppm. Formaldehyde irritates most people at about 1 ppm and many at .5 ppm. Industrial standards are far above the level at which health is affected by both gases.

Furnaces which contribute very low levels of carbon monoxide into a home most likely contribute formaldehyde gas. The potential for formaldehyde gas makes the lowest levels of carbon monoxide a serious warning.

Acceptable Carbon Monoxide Levels in buildings

Carbon Monoxide is odorless, tasteless and leaves the blood within hours after exposure ends; factors which make carbon monoxide extremely difficult to study.

In order to address the question of how much is too much, the limits published by government and health organizations are shown in

Table 1.

The standards in the chart[Table 1] were published only after scientific data was produced. It is notable that all standards have footnotes describing the limits of knowledge or the limits of research. The footnotes point to the newness of carbon monoxide issues in our homes.

These limits of knowledge and research are reflected in the position of the American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE). In 1984, ASHRAE published a standard of 5 ppm as the acceptable level for carbon monoxide in an indoor environment. In 1989 ASHRAE deleted the standard. The reason for the deletion was that ASHRAE's standard was arbitrary, not based on scientific study and that other published standards varied notably. Hence the ASHRAE standard contributed little to the public well-being.

The national ambient air quality standard, set by the Environmental Protection Agency (EPA), of 9 ppm is based on studies of nervous system and heart changes after exposure to carbon monoxide. Some controversy has resulted from these standards and more study seemed necessary.

Subsequently, the EPA funded a 1989 medical study of treadmill performance and electrocardiogram changes during carbon monoxide exposure performed by the St. Louis University Medical School. In that study heart patients reported pain early in treadmill tests after exposure to carbon monoxide. The higher the carbon monoxide, the sooner pain and electrocardiogram changes began. The study concluded that low levels of carbon monoxide (below 9 ppm) affected treadmill and electrocardiogram performance. In an accompanying editorial, researchers stated that the safest level would be the lowest level achievable.

Researchers stated that exposure to carbon monoxide for 8 hours at a rate of 9 ppm allows enough carbon monoxide to take 2 percent of the body's hemoglobin and these levels result in measurable changes in the body.

As a practical matter it is never desirable for a furnace to contribute carbon monoxide to a home. The American National Standards Institute (ANSI) holds as its standards that furnaces shall not contribute exhaust (carbon monoxide) to the room environment.

The Properties of Natural Gas Natural gas occurs below the earths surface as the result of decomposition of organic matter which may or may not be associated with petroleum. Natural gas is composed of methane (CH4) about 80% to 95%, ethane (C2H6) about 5% to 15% and methyl mercaptan, an odorant, about .1-.3%.

Natural gas is burned by combining gas with air (oxygen) in an exact ratio of 10 parts air to 1 part gas. In theory, when burned, pure methane (CH4) combined with oxygen (O) will produce three products; heat, carbon dioxide (CO2) and water (H20)<196>

Figure 1.

Hence it is easy to imagine that natural gas is perfectly clean, safe and non-toxic.

In practice several factors are not so simple as perfect combustion. We do not burn pure oxygen, we burn air containing oxygen at 20% and nitrogen at 80%. We don't burn methane, we burn methane, ethane, and mercaptan with oxygen and nitrogen. Carbon monoxide is a combustible gas formed by incomplete combustion of hydrocarbon fuel. In perfect combustion one atom of carbon combines with two atoms of oxygen and the end product after combustion is carbon dioxide (CO2). If one atom is missing from the formula, the end product is carbon monoxide <196>

Figure 2.

Carbon monoxide is combustible, however, more heat and oxygen must be added to burn carbon monoxide and a cool burning gas furnace leaves notable quantities of carbon monoxide unburned.

In natural gas combustion the final exhaust products will be carbon dioxide and water.

Figure 3.

But if oxygen is in short supply carbon monoxide, hydrogen, hydrocarbons, and free carbon will be the final products of combustion. If combustion is incomplete or if mixing of fuel and air are incomplete or the temperature too high or low, a succession of chemical compounds will result. These compounds are called aldehydes and include formaldehyde.

Gas furnaces have a complex exhaust, however, furnace mechanics deal only with carbon monoxide emissions when tuning a burner. Mechanics strive to tune furnaces such that the exhaust which flows to the rooftop contains less than 100 ppm carbon monoxide, but exhaust may be laden with nitrogen dioxide (NO2), sulfur dioxide (SO2) and formaldehyde, all of which have undesirable health effects.

The Traditional Gas Furnace The following is a description of the traditional natural gas furnace found in the majority of homes

Figure 4.

Gas furnaces manufactured after 1972 may not be as simple as the earlier furnaces because new furnaces have become more complex in order to achieve higher efficiency.

The simplest old furnaces and the new furnaces share the same potential to mix exhaust gases with room air and are monitored for heat exchanger failure using the same inspection method.

First, the control valve simply allows the gas to flow from the gas piping system through the control valve and into the burner area. The gas passes into the burner through several ports or openings where the gas is mixed with the room air. These tubes are arranged inside the furnace heat exchanger. At the exterior of the furnace is either an electrical ignition device or a conventional (continuous) pilot light which ignites the gas-air mixture.

The flame rises vertically into the heat exchanger from each burner opening. The heat exchanger is open at the bottom to allow fuel into the combustion area and it is open at the top to allow the exhaust from the combustion process to be collected in a draft hood, routed to the flue and up to the rooftop of the house into open air.

Heat exchangers operate at about 1600 degF. Comparatively speaking 1600 degF is cool - hence the heat exchanger is thin enough to allow temperatures near its exterior to rise to about 400 degF. There room air is circulated by the blower through the openings between the bellows of the heat exchanger and onwards into the rooms of the house as warm air of 100 degF to 120 degF. This is the critical area of a natural gas furnace. The heat exchanger must be thin enough to allow for the transfer of heat from the open flame, a thickness of about 3/32 of an inch. But also the heat exchanger must be durable enough to withstand heating and cooling multiple times each day for the life of the furnace.

Heat Exchanger Failures

A heat exchanger in the traditional furnace is made from rolled steel of two mirror image parts seamed together like a clam shell. Many furnaces fail by developing cracks in the sheet metal, cracks along welded seams, or holes due to rust or corrosion.

Occasionally holes formed by rust can be seen with the eye or with the aid of a mirror, but only 20% of the total surface of the heat exchanger is visible to view even with a mirror. Holes or cracks often are visible only when thermal expansion causes the cracks to open. Visual inspection is not normally possible when the furnace is in operation.

Many heat exchangers fail by becoming overheated. A heat exchanger is protected from overheating by a carefully adjusted upper limit device. The upper limit device causes the furnace to cycle to its off position when the temperature of the air in the plenum above the furnace exceed the limit set by the mechanic.

A notable number of heat exchangers fail from abnormal rust accelerated by the presence of chlorinated compounds. A chlorinated compound is any compound to which a chlorine molecule is attached. Many household products are chlorinated, among them; solvent, paint thinners, detergents and bleach.

When these compounds mix with humidity, hydrochloric acid is formed and is drawn into the furnace where the acid produces rust and salt deposits. The salt deposits re-combine with moisture from the air in garages, basements and crawl spaces to continue the corrosive process and rapidly ruin a heat exchanger.

Plugged air filters accelerate heat exchanger failure. A furnace filter neglected for several heating seasons will block the flow of air through the heat exchanger. The internal temperature of the furnace may exceed the continuous operating design temperature without reaching the high limit. Broken welds and cracks may result.

Carbon Monoxide Monitors

A carbon monoxide monitor is a sophisticated electronic device which functions by exposing carbon monoxide to two electrodes. If the two electrodes contact carbon monoxide, electrical current will flow between them in direct proportion to the carbon monoxide concentration. The signal from the electronic cell is amplified and displayed on a liquid crystal display (LCD).

Carbon monoxide monitors are regulated by the Occupational Safety and Health Administration (OSHA) and their range (threshold of sensitivity) and accuracy (stated as a percentage of error) are readily advertised by their manufacturers. The best equipment is sensitive to concentrations from 0-2000 ppm and are accurate to within 1-3 ppm.

Carbon Monoxide Monitoring

Using a carbon monoxide monitor requires that a base number be established prior to sampling furnace air. The base reading will be the carbon monoxide in the outdoor air prior to entering the house. A second reading will be taken in the house to demonstrate that cooking, fireplaces and cigarettes are not altering the base number.

At the firing of the furnace, it is important to sample the air at the front of the furnace not at the flue. Flue gas spillage at chimney/draft hood at initial startup (30 seconds) is common. Flue gas spillage from blocked chimneys is a life and death issue, but is not a heat exchanger failure. Don't confuse these gas sources.

Room monitoring should continue through a heating cycle and after the burner has shut down. Many furnaces produce readings only after they have become hot enough to open cracks by thermal expansion. Early heat exchanger failure produces minor carbon monoxide contamination which can be detected by monitoring high in the room to read the content of the warm exhaust against the ceiling and comparing the reading to the cool air near the floor containing less exhaust and hence, less carbon monoxide.

After the burner is shut down the combustion draft will no longer pull exhaust into the flue and chimney effectively. The fan, by blowing house air over holes in the exterior of the heat exchanger, may then create a draft drawing residual carbon monoxide into the room <196>

Figure 5 below.

These monitoring techniques will identify early failure where small quantities of 2-3 ppm are in evidence. Grossly damaged heat exchangers may admit 10-30 ppm in the room air in one cycle of the furnace.

As a result of the energy crisis of the 1970s, houses have become increasingly resistant to air changes. Indoor air quality has become an important consideration. Fortunately, during the same period, technological improvements have made testing instruments available and affordable. Consequently, carbon monoxide detection is becoming a common procedure in the home inspection profession.

References

American Gas Association and the National Fire Protection Association, National Fuel Gas Code, Quincy MA (1988).
American Society of Heating, Refrigerating and Air Conditioning Engineers, Inc., ASHRAE, ASHRAE Standard 62-1989, Atlanta GA (1989).
American Society of Heating, Refrigerating and Air Conditioning Engineers, Inc.,
ASHRAE Handbook-Equipment, Atlanta GA (1988).
The North American Manufacturing Company, North American Combustion Handbook, Cleveland OH (1952).
EPA Indoor Air Division, The Inside Story - A Guide to Indoor Air Quality, Washington D.C. (1988).
U.S. Consumer Product Safety Commission, Carbon Monoxide Poisoning Involving Vented Gas Space Heaters, Washington D.C. November 1982.
John N. Kirkpatrick M.D., Occult Carbon Monoxide Poisoning,
Western Journal of Medicine (January 1987). [Of particular interest regarding effects of chronic exposure to flue gases.]
American Gas Assoc., Fundamentals of Gas Combustion, Arlington VA (1985).
American Gas Assoc. Fundamentals of Gas Appliances, (1976).
Ernest J. Oppenheimer, PhD., Natural Gas: The New Energy Leader
Pen and Podium Productions. New York NY (1981).
National Indoor Environmental Institute, Your House and Your Health, Plymouth Meeting PA (1983).
Elizabeth W. Allred, Et al., Short Term Effects of Carbon Monoxide Exposure on the Exercise Performance of Subjects With Coronary Artery Disease,
The New England Journal of Medicine, (November 23, 1989).

Richard Matzen is a member of the American Society of Home Inspectors in Seattle, WA. He is a frequent writer and lecturer on the topics with which this article is concerned. Illustrations were prepared by Baadh Design, Seattle, imported and scaled for use in this article by the Journal.

Choosing and Using a CO Monitor for Gas Leak Detection

Daniel Friedman

This article identifies several devices and methods of CO detection, lists some sources of equipment, and offers sample measurement report text. Readers should be sure to see Matzen's description of use of calibrated electronic sensors in the preceding article. Matzen argues vigorously, and we agree, that for common inspection purposes as described in his article, electronic CO monitor sensing devices are the superior tool. However many inspectors and heating professionals use other methods. Accuracy and cost of these alternatives vary widely, and some very accurate but relatively low-cost alternatives are available. Some of the good, the bad, and the very ugly are discussed in this article.

TECHNICAL REVIEW: The equipment and methods discussed in this article are based on information from various manufacturers and on the author's experience. This discussion is not technically complete. Material for this article will be submitted to the American Gas Association and to the equipment manufacturers for comment.

Journal review staff comments are noted at the end of this article. Feedback from outside reviewers will be published in subsequent issues of the 1991 Journal. Also see the DeWerth article in this issue for an in-depth treatment of derivation of leakage levels, leak hole sizes, and for a description of AGA's approved three step visual, flame, and tracer-gas heat exchanger inspection method.

CO testing is considerably beyond the required scope of ASHI home inspections. While professional inspectors do make these tests, the choice to make CO measurements and the selection of equipment and methodology are not decisions to be made lightly - a mistake could mean someone's death. Regardless of disclaimers of scope and responsibility, the actions and written report of the last building professional to enter a building before an accident or death will be exposed to careful scrutiny.

Neither ASHI nor the authors can control the selection and use of test equipment in the field. Readers who address these life-safety concerns are responsible for seeking proper guidance from various expert sources on the definition of hazardous conditions, proper test methods, and proper use of equipment.

Electronic CO Monitors

Matzen's article in the 1991 Journal issue describes the operation, use, and accuracy of the type of CO monitoring devices which he's found appropriate and reliable for testing heat exchanges. These devices vary in price and features. When a test for CO level monitoring has been made, or when evidence suggests it may be appropriate,

AGA's Gas Combustion p.17 indicates that "A check for CO in the air should be made whenever a customer has been overcome or complains of chronic headaches or nausea. Matzen's Northwest ASHI Seminar, March 31, 1990, noted that these same complaints are indications of clinical depression, and that CO poisoning can only be proven by a carboxyhemoglobin test. Home inspectors should, of course, avoid attempts at medical or psychological diagnosis.

A check should be made if house plants are dying, or if there is a chronic odor whose source or cause cannot be located. Condensation on cool surfaces in the house can be a sign of the presence of flue products, which may lead to danger. Only qualified trained personnel should use CO detecting devices.> these tools provide quick, accurate response. Two which Matzen has tested are listed below: both offer models with digital readout of CO levels in parts per million - ppm.

The Draeger Model 100 Gas Monitor,

[previous page] a shirt-pocket sized device, is a three-electrode electrochemical gas sensor that produces, according to its manufacturer, extremely accurate, very specific, fast measurements. The CO model, #4510131 with digital display, reads measurements from 0-2000 ppm with an accuracy of within 2 ppm. While the unit lists for $575. Optional features can add as much as $250. to cost. A lower-cost model without the digital display sounds an alarm to warn of hazardous gas levels. Draeger Model 100, pocket-sized personal gas monitors are available for CO and other gases.

The Gas Tech Model CO-82 detector, similar in size to the Draeger unit, also includes a digital display reading levels from 0-500 ppm CO linear, continuing to 1999 ppm. Alarms are also included. A two-electrode electrochemical cell is used for detection. (Do not confuse Gas Tech Corporation with GasTec Corporation. GasTec, discussed below, is a manufacturer of colorimetric sensing tubes and is based in Yokohama, Japan. These companies do not like being mistaken for one another.)

The Lynn 7400 CO analyzer, also a hand-held device can be used with a rubber aspirator bulb or attached to a Lynn analyzer - a motor driven pump system.

This detector, factory calibrated at 500 ppm CO, was designed to measure heating system efficiency by measuring the level of CO in flue pipes for heating equipment, and can tolerate probe temperatures of 1000 degF. It reads CO levels from 0 to 1999 ppm. Average test time is one minute, sensor life is typically ten years.

Test procedures for this type of electrochemical device are described in the previous article by Matzen.

Furnace or Boiler Gas or Oil Combustion Efficiency Kits

Bacharach Instruments manufactures a wide range of manual, chemical, and automated testing kits for gas and oil burner combustion testing. Some of their equipment includes relatively low-cost devices, such as their Monoxor indicator, which measures CO levels in percent. The Monoxor uses an aspirator pump to feed a flue gas sample into a collecting bladder which in turn draws the gas through a glass indicator tube. This is a colorimetric process similar to others described below - a yellow chemical turns dark brown in the presence of CO, and the length of the stain indicates the CO level.

The operating range of these tools is designed for measuring in the flue vent pipe of gas or oil-fired devices, not for air sampling in the living area, though some of this equipment can measure at sensitivities approaching the standards for allowable limits. The most sensitive test ranges typically measure 0-0.2%.

Percent vs ppm: the percent multiplied by 10000 yields a measurement in ppm. See Bacharach literature. Bacharach indicates that for gas-fired equipment CO should not exceed 400 ppm in the flue gases. See Bacharach's "Combustion Analysis with Fyrizers", bulletin 24-9070R - 3/81 p.7.

Bacharach explains that conversion burners and gas-designed equipment, if over-fired, can have flame impingement on cold surfaces and may produce CO even if they are being run with excess air and even though CO2 and O2 are at acceptable limits.

Excess air is introduced in gas combustion to assure that no CO remains in the flue products. See Matzen's article for details about "complete combustion". Reference: Bacharach Instruments publication 20M-AL 2/82

RW Beckett Corporation also produces combustion and flue-gas measurement equipment. However a review of literature they provided to us did not find equipment specifically for CO measurements. Beckett's tests measure oxygen percentage.

Other companies such as MSA, ISC, Monitox, and MDA also produce equipment which may be useable for these tests; these companies did not provide literature for this article.

Aspirators & Colorimetric Detection for Gas Detection

At least two companies, Gas Tech and Draeger, manufacture relatively low-cost devices to detect carbon monoxide using color changes based on a tube developed by the National Bureau of Standards.

This introduction is based on information on gas combustion provided by the American Gas Association, AGA. Readers should see the

Gas Engineer's Handbook as well as detailed literature provided by the various manufacturers for more detailed explanation of various gas testing methods.>

Color Change Methods

A glass tube containing a chemically treated silica gel is opened by breaking off two ends of the tube. Using either a simple rubber aspirator bulb an air sample is drawn through the tube. If the gas being measured is present, the chemical in the tube changes color. For CO tests, the tube changes from yellow to green.

The intensity of the color change permits comparison of the tube against a reference card, based on the number of times the aspirator bulb was squeezed, and the amount of CO present can be read at levels from 0.001 to 0.1 percent.

Other detectors use activated iodine pentoxide which changes from gray-white to green, with a sensitivity range of 0.1 to 1.0 percent CO. Finally, a detector ampoule is available which can be crushed, hung in the air for ten minutes, and checked for color change. The color of the cotton in the bulb changes to gray in the presence of CO and again the level is estimated by comparison of the colored ampoule to a reference card.

Sensidyne/GasTec Aspirator

The Sensidyne/GasTec pump method for measuring gas levels uses a Sensidyne calibrated piston-type pump with glass colorimetric tubes manufactured by GasTec. The tips are broken off of a fresh detector tube, the tube is inserted into the pump unit, and a number of pump strokes, ranging from 1/2

There's a clicking latch to permit calibrated half-strokes> to an upper limit depending on the specific gas tube being used.

GasTec measurements are not a part of our normal home inspection procedure, but are offered on request as an additional service. We've made frequent use of CO and CO2 measuring tubes in the past five years.

Gas sensing tubes are available for a very wide range of other gases.> Measurements have been made in investigating the adequacy of fresh makeup air in public buildings and private homes (CO2) and in investigating furnaces suspected of having flue gas leaks (CO).

Draeger Aspirator

National Draeger, Inc. is a supplier of at least 230 detector tubes capable of measuring over 350 different gases. They also produce sampling equipment including a modestly priced bellows pump for use with their colorimetric tubes. Their handbook is itself a text on this topic.

Air investigations and technical gas analysis with Draeger Tubes, Detector Tube Handbook, 6th Ed, May 1985. The operating procedures are quite similar to the equipment described above. Draeger's pump is also a calibrated design, but constructed using a rubber bellows rather than machined tube and piston.

Aspirator & Colorimetric Tube Accuracy

The accuracy of pump and tube systems depends on two carefully controlled items: the volume of air collected by the pump and the chemical consistency and performance of the sampling tube. The pump is manufactured to close tolerances and is designed to give a measured 100 ml stroke. The handle will lock open in the half or full stroke position.

The sampling tube is calibrated as an integral part of its manufacturing process. Calibration and accuracy tests are performed using combinations of standard reference gases of known concentration and dynamic gas flow technique as gas source, and nondispersive infrared adsorption or gas chromatographic technique as standard analysis. The chemistry and operation of the tube are well known and generally reliable.

In Gastec's CO tube potassium palladosulfite is reduced by CO to metallic palladium which produces a dark brown stain. The amount of CO determines the length of stain on an indexed glass sampling tube. The test can be interfered with by carbon disulfide, halogens, mercaptans, phospine, phosgene, acetylene, hydrogen sulfide, sulfur dioxide, nitrogen dioxide, ethylene, and hydrogen in varying amounts. Gastec indicates that interferences result in plus error - a "safe" direction of error.>

With each type of detector tube the manufacturer provides instructions indicating the accuracy ranges, the number of calibrated pump strokes, and the appropriate corrections to be made based on the air temperature in the measured

We usually make no correction for temperature. Higher temperatures than 68 Deg. F would justify a small upwards correction in the estimated CO level. For example, if the tube read 500ppm and the temperature was 40degC (104degF) the actual measurement would be 55ppm. But if you're finding CO at those levels the precise number is irrelevant, and the presence of a problem is unequivocal.>

Draeger CO measuring tubes are available in sensitivity ranges from 2 to 3000 ppm or .1 % and up by volume. The bellows pump draws 100 cc of gas per stroke and can be use with a wide range of tubes. For home inspectors using the Draeger bellows pump Draeger recommends the CO 5/c detector tube to monitor CO levels between 5 and 700 ppm. Draeger tubes are a larger diameter than Gastec tubes. The two are not interchangeable.

As a word of caution to the client, we identify the manufacturer's claim for accuracy of the particular equipment being used. This is less accurate than the electronic digital readout equipment described earlier, but capable of identifying CO levels as low as 1 ppm.

In addition to following procedures described by the equipment manufacturer, our measurement procedure and measurement interpretation have been assisted by consulting with hazardous substance and health physicist consultants available on the SAFETY net forum.

The SAFETY net forum is a special interest group organized on Compuserve, a public computer information utility.>

Aspirator & Colorimetric Tube Procedure:

Draeger's recommendation for heating system testing includes measuring CO from the plenum as the most representative indication of CO escaping from the heat exchanger. If that area is inaccessible they suggest measuring at the closest register to the plenum.

Air samples are taken in the same locations described in Matzen's preceding article. In addition, air samples can be taken directly over the heat exchanger in a plenum by drilling a 1/4" hole. We find or drill an opening

WARNING: do not drill holes if you don't know what you're doing: you may damage the equipment or make it unsafe.> into the heating plenum, after assuring we're drilling in a location which is certain to avoid any damage to the heat exchanger, furnace controls, or other equipment. Following testing such holes can be sealed with a snap-in button manufactured precisely for this purpose and available from heating equipment suppliers. We also seal test holes with gummed foil tape.

When measuring over the heat exchanger in a plenum we take air samples before the system operates, after the burner has operated but before the blower fan is running, and after the blower fan is running. After making a number of measurements with other instruments with the blower fan running, Tom Byrne (ASHI NY) pointed out to us that the blower pressurizes the area around the heat exchanger, making it less likely that flue gases will escape into the house air - depending on the location of any defect.

DeWerth explains this same point in his Three Step Method article in this 1991 Journal issue. Further, the added volume of house air significantly dilutes gases leaking from the heat exchanger. Thus a more proper diagnostic test is taken with the heat exchanger hot (small cracks may have opened) but before the fan runs.

Watch out: ARNING: we've heard procedures suggesting disabling the blower fan and running the system "hot" (protected of course by the limit switch) in order to obtain longest test interval in the plenum. This is a very tempting procedure for users of the Gastec method since a test requiring three pump strokes may take nine minutes.

Many furnace plenums heat so rapidly that the system will not run for nine minutes without operating the fan. But in our opinion there is risk of causing damage to a marginal heat exchanger by this procedure: bringing the heat exchanger to excessive temperatures beyond those intended by the manufacturer for normal operation may cause buckling or even cracks at welded seams. Such damage can render the system unsafe or inoperable. Therefore during normal inspection procedures we never run the furnace with the blower disabled.

The measurement range of the detector tube depends on the number of pump strokes taken. For example, with the Sensidyne pump and GasTec tube, 8-150 ppm requires three pump strokes. 25-500 ppm requires one pump stroke. 500-1000 ppm requires 1/2 pump stroke. Rather than waste tubes by starting with three, we examine the tube color after the first stroke and stop there if the CO level has run off the scale.

Equipment cost, less than the electronic devices described earlier, is balanced by a considerably longer time to make measurements, depending on the number of pump strokes required and the rate at which gas flows through the sampling tube. Typically the pump handle is pulled back and locked on a full stroke, giving a precise volume of air being pulled through the tube.

The operator then must wait for the vacuum created in the pump body to be satisfied by pulling sample air through the tube - which can take several minutes. Given the fan delay problems discussed above, this test method, when used

in heating plenums is less tractable than those using the electronic sensing devices recommended by Matzen, and is slower than the electronic methods in all cases.

Broad-Spectrum Analyzers for Gas Leak Detection

TIF 8800 Combustible Gas Detector

Perhaps the most widely used diffusion-type electronic sensor among home inspectors, the TIF 8800 is both versatile and inexpensive. Typical cost is under $200., considerably less than the CO monitors discussed earlier and using no disposable tubes like the pump systems discussed above.

Gas Leak Detection Instrument accuracy

While wide range sensitivity to many problem gases and low cost may be a source of the popularity of this instrument, Matzen and others argue strongly that this instrument is not suitable for testing heat exchangers because its response level is rated at about 100 ppm of CO. Matzen has pointed out

Northwest ASHI Seminar Notes, March 31, 1990, Richard Matzen> that because the TIF 8800 works on a dispersal theory it will respond to virtually any substance miscible in air including all fuels from alcohol to gasoline, as well as simple humidity.

This response to humidity probably explains why the user may observe that the tic-rate of the TIF alarm actually falls off when moved from the (more humid) basement air into the (drier) warm air in a plenum above the heat exchanger. The air in a plenum, unless there's a (misplaced) humidifier right there, will drop to very low levels when the furnace is operating.

The manufacturer indicates [TIF instruction manual L1-147 Rev. 9/85] that the instrument's sensitivity range is 50-1000 ppm with a one minute warm up time.

Matzen asserts: Northwest ASHI Seminar Notes, IBID, that at the worst furnace he's seen, a ramshackle building in South Seattle, 58 ppm was entering the living area - a level at which the TIF performance was questionable. He adds that the majority of defective furnaces he's studied produced readings of less than 10 ppm in living space.

TIF 8800 Test procedure:

Following a warm up period (electrode in the sensing tip has to heat up) an adjustable dial sets a tic noise to a steady but modest rate. The manufacturer says to use a rapid rate - which is a more sensitive setting. As the sensing tip is moved into test locations the operator listens for an increase in the tic rate. The faster the tic the higher the concentration of whatever is being detected. Your breath will make the instrument respond, even if you're sober - because of its humidity. Gas leaks in any serious quantity will promptly change the tic rate to a continuous siren.

If the siren sounds before the leak source can be identified, the air in the area is contaminated with heavy concentrations of gas. The user can desensitize the tip by turning the control knob to a slower tic rate. When looking for small leaks, a high tic rate must be used.

An example of field use of this instrument is at Case History: LP Gas Leak

TIF8800 Field Use Experience

Daniel Friedman

We've used the TIF8800 extensively for six years. As a tester for natural gas leaks, LP gas leaks, sewer gas leaks, this is an outstanding and economical instrument. We regularly find LP and natural gas leaks at unions, couplings, and we find trace level leaks (mild response on the TIF alarm) at the base of the pilot control on 50-75% of the gas-fired domestic water heaters we examine. We've found leaks in

outside gas lines by detecting gas leakage into building basements at the point where a buried gas line enters the basement through the foundation wall. We've also proven that an uncapped basement sewer pipe was leaking sewer gases into a basement from an old supposedly "abandoned" septic.

We've also found confounding sources of response including the organic solvents in pipe dope on new piping, fumes from some plastics, the cat box in a basement, and in one instance, a flatulent client.

We agree with Matzen who says that as a heat exchanger tester this is not the best tool for such tests. We've had unreliable results, and following Matzen's argument we no longer use this instrument for testing heat exchangers. If you're using the instrument and get a positive response there

may be a problem, and any prudent inspector would call it to the attention of his/her client. But the absence of a response on the TIF is no guarantee that a heat exchanger is not defective. And false alarms are possible.

In one case [Friedman, File 1468807, July, 1988] the TIF did not detect flue gas leakage in the plenum over a heat exchanger which had a hole so large that we could put four fingers through it.

This was a steel oil-fired furnace in a wet basement. The hole was a rust and burn opening above the burner tube in the wall of the heat exchanger. Based on the external condition (bad rust), age (more than 20 years old), and environment (wet basement), we expressed strong caution that the system might not be useable, and we referred the client to a local HVAC contractor for more careful inspection. The local contractor reported that the system was safe and useable.

At start of the heating system (three months later) the serviceman from a different oil company checked again (as we had suggested to the client) and found the large hole we described. At no time did the TIF detect this opening, possibly because of the operating manner of the furnace, whose blower ran nearly immediately whenever the thermostat called for heat - itself an unusual condition. Generally draft was into, rather than out of the heat exchanger. The system was replaced.

Reporting Test Results

The following inspection report language has been reviewed/approved by absolutely no ASHI authority. However it has been read and used by several thousand inspection clients. To date we've been lucky: no deaths, no injuries, no lawsuits, or as a famous food critic said once said of a dessert made entirely of chemical additives, ... no lemon, no cream, just pie. Comments and criticism should be addressed to the Journal editor. Feedback will be discussed in future issues.

When we use an instrument to test heating systems we report the following:

Using a Gastec<190> multi-stroke gas sampling pump with a Gastec<190> Carbon Monoxide low-range detector tube No.1La we tested for heating system exhaust gas leaks into the air circulating system. Presence of carbon monoxide might well indicate both failure of the heat exchanger and presence of a potentially very hazardous condition.

We indicate the method of testing:

We tested at an opening into the heating plenum before the blower fan operated - a sensitive testing technique.

We tested at a register, as there as no opening into the plenum chamber. Plenum chamber tests are more sensitive. Therefore our test was less reliable.

We no longer use the TIF for this test. When we did, if we found evidence of CO leakage using the TIF8800, and recognizing that other factors could cause it to "squawk" we reported:

Watch out: Safety Hazard: Performing this test we found evidence of possible combustible gases at these locations. We caution you that our test instrument is extremely sensitive. None the less this presents a possible hazard and could indicate that the heat exchanger is or will soon be defective. Significant expense may be involved.

If we find evidence of CO leakage using a Draeger electronic CO monitor or using the Gastec pump and tube we report:

Watch out: Safety Hazard: Performing this test we found evidence of possible Carbon Monoxide (CO) Gas at these locations. This represents a safety hazard and could indicate that the heat exchanger is or will soon be defective. Significant expense may be involved. Technical descriptions of our test for CO and of the test instrument are attached to your report.

Before using the system, have a qualified heating expert check the equipment to assure that it is not leaking flue gases, safe, and working properly.

We also report other observations which support our conclusion or which provide a basis of warnings to the client, such as

Rust on the furnace heat exchanger may indicate present or imminent failure of the heat exchanger on this system. If system replacement is required, a significant expense will be involved.

Soot stains around most of the hot air registers may indicate present failure of the heat exchanger on this system, or that the system has been circulating dirty air - indicating poor filter maintenance and clogged circulating fan.

Heavy rust on the system's outer jacket is a possible indication of internal rust and shorter than normal life expectation.

If we were unable to make any test for gas leakage, particularly on a rusted suspect system, we report:

The heat exchanger was not tested as there was no opening into the plenum. We discussed making such an opening and performing this safety test prior to the heating season. A failed heat exchanger can leak flue gases into the living area - which is dangerous, and would possibly require replacement of the system - an expensive step.

Provided there was at least visual access to the system we'd add,

We did not see external signs of such failure such as visible damage or significant rust on the equipment. However visual inspection alone cannot assure the condition and safety of a heat exchanger.

And of course if external signs such as those listed above raised obvious doubts of system condition, we'd make a similar warning as if we had found CO present. This should probably be the fall back position for any inspector who makes the ASHI Standard visual only examination of a dubious furnace.

Case History: LP Gas Leak Detected Using the TIF 8800

DJ Friedman

During the ASHI 1990 Annual Conference in Phoenix we received a frantic telephone call: There's been a gas explosion at your rental house in Poughkeepsie... The author headed straight for the airport, immediately lining up at the Quantas ticket counter for the next flight to Australia.

Fortunately, it wasn't that bad. A gas leak in the base of an LP gas cook stove had caused a flame rollout which badly frightened the tenants. There had been no explosion. And no damage. They had reason to be scared, having been burned out of their previous home, an apartment, when children on an upper floor played with matches by dropping lit ones into a hole in the wall.

Flames rolled out of the oven

What actually happened? An LP gas oven was in use. The occupants heard a noise, saw a ball of fire and flame roll out of the bottom of the stove at the broiler door. They called the fire department who shut off the LP gas tanks outside. No damage anywhere. Of course, now there was no domestic hot water and no cook stove.

From Phoenix, an experienced, well qualified plumber was called in New York <197> with express instructions: go to the property, find the gas leak, fix it. If you cannot find it, make sure there is a sound shutoff valve in the gas line at the stove, shut it off, and re-light the water heater pilot so that the occupants can have hot water.

On the author's return from Australia the tenants described the procedure of the experienced, qualified plumber. While our event did not make the news, a similar one did

NY Poughkeepsie Journal, 26 February, 1991, p. 2B and the news article came in handy. More on that later.

The experienced, well qualified plumber did not come to the house. He sent an employee who was not experienced, and not qualified. Putting it politely. We'll call him Bud. Bud stood in the center of the kitchen, flicked his BIC, and when there was no explosion, informed our tenant No gas leak here, lady. Really.

There was additional discussion between tenant and Bud about the stove. Bud asserted that it was all fine, that maybe the pilots on the stove were set too low. (This stove has no pilots, it's got electronic ignition, but Bud refused to believe that detail.) Bud left all gas lines on. The stove continued it's small leak. The tenant, wisely, did not use it, and thought it had been turned off as we'd instructed.

Home from traveling and following a visual inspection for obvious damage to piping and valves from gas source to the appliance, we made use of the TIF at every joint, elbow, connector, and control from gas source to, through, and inside the stove, with the sensitivity dial set high.

Using the TIF8800 Combustible Gas Analyzer we found that the stove had a double fault: the oven control never completely shut off gas to the broiler, and the thermostatic safety valve at the broiler never completely shut off gas flow into the burner. As the cost of repair parts plus service charges was close to the cost of a new stove, the

Checking further outside we also found and had corrected a long-standing leak at the stem of the shutoff valve on the LP tank. The local gas supplier was reluctant to repair this item as they considered it both safe (it was outdoors after all) and good business.

Two weeks later, after calming down, we saw a news article reporting Millerton Blaze leaves 4 homeless.

[Police Beat clipping at left.] The report described a fire that started from a faulty gas line and which caused extensive damage to a home. The fire apparently started in the kitchen on a gas line behind the cooking range, according to the fire department. Forty fire fighters, two ambulances, no deaths. Lucky.

We called our plumber friend and directed his attention to the news article. We let him sweat a moment before explaining that no, this was not the house to which we'd sent him a couple of weeks before. But it could have been! Following a refund of the $135. charged by the plumber for his service call, we presented a mini-class to his service people on gas leak detection.

This class outline has not been reviewed nor approved by anyone. Criticism is welcome and should be addressed to the

Journal editor. Feedback will be reported in future issues.> At that class we learned that the plumber has a TIF8800, though it was not used at our tenant's house. Perhaps the mechanic was unfamiliar with its operation.

Looking for Gas Leaks

The National Fuel Gas Code and other publications from the American Gas Association and the Gas Appliance Manufacturers Association contain more complete details of proper detection and repair procedures for gas piping and equipment.

Traditional "Tests" for Gas Leaks

Matches and Lighters

We spend time talking about it because, no joking, people still do this. Don't do it! It's prohibited by the AGA & NFPA.

National Fuel Gas Code, ANSI Z223.1-1988 and NFPA 54-1988, Para. 4.1.5 p. 64> It's dangerous. It looks stupid to customers. It's bad for business. Even if the gas in the area is not enough to support an explosion, and even if you think it's quick and dirty but ok you cannot flick your BIC in every location where a test is needed. You face certain and total loss of any lawsuit involving damages.

Soap Solution Bubble tests are the traditional method for checking piping. There are some limitations: results are inconsistent, testing is slow, and you cannot cannot see every place where bubbles may occur. Soap cannot be used in all locations - eg. at air shutter near gas valves.

Pressure tests of gas piping were not discussed though this may be the procedure in new installations

Using the TIF 8800

This instrument detects combustible gases at 50-1000 ppm, including Methane, Ammonia, Acetone, Acetylene, Alcohol, Benzene, CO, Ethane, Hydrogen, Hexane, Isobutane, N-Butane, Pentane, Propane.

Common LP Gas Leak Points

In addition to a careful visual examination of all gas piping, fittings, valves, from gas-source to points of final use, spend some extra time with the TIF at:

Unions, at the pilot control and at right-hand side of many DHW gas controls, at the flare fittings and valves in gas appliances.

Note that sometimes leaks are present at so many fittings in old gas lines that you may not believe the instrument. Believe it unless you're testing at fresh pipe dope.

Watch for perforated flexible gas lines at appliances, leaky appliance controls and pilots, valves, and unions. Also check LP tank main valves, and flare fittings on LP lines.

False alarms with the TIF 8800 testing for LP Gas Leaks

These are not LP/Natural gas leaks, but can cause the TIF to respond: <_>moisture, < >flue gases, < >pipe dope, < >especially if recent, dead cats (methane), < >methane from sewer gas or flatulence, < >auto exhaust, < >any organic/combustible solvent.

Turning on the TIF

Go outside for a reference signal, in non-contaminated air. Let the instrument warm up. It can take a long time in cold weather, if there's a bad battery, low charge, or bad sensing tip.

Turn the adjusting knob (there's only one so you can't go wrong) slowly to full sensitivity to verify that it will signal, and listen for gradual increase in tic rate. If you cannot get a range of tic rates, the instrument is not working properly. If it won't tic in the intermediate ranges there's a bad battery or bad tip.

Adjust for slow steady tic. Rapid tics mean more sensitive. Use higher sensitivity for small leaks. Test to see that the instrument responds at a known source. Follow gas from source to final use, checking every connection, every component.

How close to piping? From touching it to 1/2" away, around every fitting/component being tested. Don't contaminate the tip with soil or pipe dope.

Take advantage of the flexible tip probe to reach to valves and controls inside ovens and heaters.

Final Note

If customer has reported odors, flame rollout, or some other event, and you cannot find and repair a leak, leave the gas shut off at the source and verify that it has been shut off. Document what you said and what you did, in writing. It can be as simple as a note written on a bill.

References

American Gas Association, Fundamentals of Gas Combustion, 7th printing, September 1985. Prepared by American Gas Association Laboratories for American Gas Association, 1515 Wilson Boulevard, Arlington, VA 22209 and Gas Appliance Manufacturers Association, Inc., 1901 North Fort Myer Drive, Arlington, VA 22209.
National Fuel Gas Code, NFPA 54 1988 and ANSI Z223.1 1988, available from AGA at the address above, or from the National Fire Protection Association, Batterymarch Park, Quincy MA 02269
NFPA and AGA, National Fuel Gas Code Handbook, Z223.2, same source as above.
DJ Friedman (editor of InspectAPedia.com) served as a professional home inspector and forensic investigator of building failures from, 1986 to 2010. He previously operated a heating and cooling service and repair company and a residential construction and renovation firm.
Bacharach Instruments, 301 Alpha Drive, Pittsburgh, PA 15238 412-782-3500.
Gas Tech, Inc., 331 Fairchild Drive, Mountain View, CA 94043 415-967-6794.
Lynn Products Company, 400 Boston St., Lynn MA 01905 617-593-2500 and 617-596-0430 FAX.
National Draeger, Inc. 101 Technology Drive Pittsburgh, PA 15275 412-878-8383 412-787-2207 FAX
R.W. Beckett Corporation, PO Box 1289, Elyria, OH 44036 216-327-1060
Sensidyne Corp. 12345 Starkey Rd. Suite E, Largo, FL 33453 813/530-3602. Sensidyne does not sell retail. Their equipment is marketed by Industrial Products Company, 21 Cabot Boulevard, Langhorne, PA 19047 800-523-3944 or in PA 800-562-3305. This distributor also can supply GasTec colorimetric tubes used with the Sensidyne calibrated pump.
TIF Instruments,Inc., 9101 NW 7th Avenue, Miami, FL 33150. Some ASHI Members operate inspection equipment companies and offer varieties of gas detection equipment:
Professional Equipment, 143 Plainview Road, Woodbury, NY 11797 800-334-9291.
The Specialty Tool Company, 145 D Grassy Plain St., Bethel, CT 06801 800-456-4605.
Draeger equipment is marketed to home inspectors by Kirsopp Home Inspections Inc., Belleville, IL 618-397-3057.
Dan Friedman is an ASHI Member in New York, NY Metro ASHI President, chairs the ASHI Technical Committee, and edits the ASHI Technical Journal. He has owned and operated a heating equipment service and repair company.

Heat Exchanger Testing and Test Devices: Who's Right?

- Daniel Friedman

For an accurate understanding of how we should look for an unsafe heat exchanger the pass-fail criteria must first be defined. Matzen's heat exchanger test procedure looks for levels of CO as low as 2-3 ppm at the heat exchanger. DeWerth's heat exchanger test procedure looks for levels of 200 ppm at the heat exchanger. Can we reconcile these views? Both articles are carefully reasoned; both articles cite authoritative sources for exposure levels. How are readers to interpret these differences? The Matzen and DeWerth articles provide food for serious thought. Here Journal staff suggests some interim opinions and some questions. The reviewers end with a call for inspection priorities.

Matzen's Test Method for Heat Exchanger Leaks

Matzen calls for testing at very low levels of CO, in part because of the concern, beyond that of CO hazards, for the presence of formaldehyde (HCHO) which may be present in flue gases, and to which people have reactions at lower levels than those permitted for CO.

He describes using a calibrated CO monitor, testing for CO, with the presumption that if CO is present, HCHO may be as well.

Matzen recommends use of a CO monitor with digital readout to permit explicit level monitoring and reporting. He measures both at the heat exchanger, and in the house air - an important additional measurement which looks at actual levels rather than an inferred level based on measurement only at the heater.

However this procedure does not deal with the question of actual CO exposure in the house at equilibrium. At such an inspection, data regarding actual house air changes per hour is generally not obtained.

Further, DeWerth argues that very low levels of CO found at the heat exchanger may be within acceptable limits and may not indicate equipment failure.

DeWerth's Test Method for Heat Exchanger Leaks

DeWerth's test for flue gas leaks at the heat exchanger of a gas-fired appliance looks for a level of 200 ppm flue product. Readers should pay particular attention to the careful sequence in which DeWerth develops his argument.

Is This Heat Exchanger Safe?

In testing any hypothesis, such as "This Heat Exchanger is Unsafe" there is more than one type of error possible.

Accepting for use a heat exchanger which is dangerous is not permissible since human life could be at risk. For this reason, DeWerth would be expected to be quite cautious in choosing the assumptions for his methodology. Readers should look for, and will find, some examples of caution and significant margins for error (100% safety factor is commonly used) in DeWerth's assumptions throughout his analysis. DeWerth himself, has not presented the cumulative effect on risk of all of the conservative steps in his process.

Rejecting for use a heat exchanger which is safe, and which meets or exceeds industry manufacturing standards is also an error. While the consequences are less dire, the consumer's interests are not served by requiring replacement of an acceptable heat exchanger. Few heating companies will actually make such a replacement. Usually the entire furnace is replaced, for a typical cost around $2000.

Derivation of Test Criteria for Heat Exchanger Leaks

DeWerth's analysis does not test house air. Rather, his measurement standard, 200 ppm to be tested at the heat exchanger is derived from answering the following:

What is the smallest hole in a heat exchanger which should be detected? Answer, 1/8" diameter.
What CO concentration should we assume is in the flue gas? Answer, 200 ppm.
ANSI Standard Z21.47-1983 allows 400 ppm air-free CO. 200 ppm affords a 100% safety factor.
What is the most CO that will leak out of a hole of that size? Answer, 200 ppm.
How much CO will be found in the house air, at equilibrium, from a leak of this size? Answer: 15ppm at equilibrium in the house.

A Test Methodology for Furnace Heat Exchangers - Conclusions

DeWerth's description of Visual Inspection and Flame Pattern Inspection procedures should be studied by every home inspector, regardless of whether or not s/he uses other more sophisticated techniques. These first two, of DeWerth's Three Step Procedure, are visual-only, and should be followed by anyone inspecting a gas-fired heater in compliance with ASHI Standards of Practice.

DeWerth's third step involves purchase of special equipment, as does that of Matzen. However, in addition to a flue gas analyzer device, the DeWerth method requires purchase and use of a tracer gas and a gas regulating device. The procedure is involved and time consuming. It's an appropriate procedure for a heating company expert who is called in to offer a second opinion regarding the condition of a heat exchanger.

Combining Both Methods of Testing for Heat Exchanger Gas Leaks

The choice to offer services which exceed the ASHI Standards of Practice is permitted by the Standards. Inspectors should beware of offering advanced services if they are not qualified. However this should not be taken as a suggestion by ASHI that an inspector may not make him/herself qualified in additional areas.

Every inspector should follow the first two (visual) steps of DeWerth's procedure

Qualified inspectors may follow a combination of the DeWerth Procedure with the Matzen suggestions for use of a CO monitor, substituting the CO Monitors for the more cumbersome tracer gas, as a step beyond visual inspection of heat exchangers.

CO Monitors were not dismissed as inaccurate. DeWerth raised questions of cost (now add the cost and time to use a gas cylinder/regulator) and a question of rejecting a good heat exchanger by a test which is too sensitive.>

Digital readout CO Monitors are now available. The inspector is free to elect the American Gas Association's standards of acceptability (or others if identified) while using this simpler and faster test instrument.

Inspectors who prefer Matzen's standards of acceptability will report any CO levels found in a building.

Inspectors who follow DeWerth's standard, itself a cautious one, would not call out a heat exchanger as "failed" if it leaked less than 15 ppm CO into the house living area at equilibrium conditions.

Based on DeWerth's explanation of the mechanism of failure and leak rates, a very low leakage detected

in the plenum at the heat exchanger, such as 2-3 ppm CO, should probably be reported as a caution rather than an assertion. Inspectors detecting the presence of
any levels of CO

in the house air should report it as a concern.

A call for Priorities:

reviewers (A. Carson and D. Friedman) agreed that some perspective is needed in considering heat exchanger tests. Little data was provided, and only by Matzen, regarding deaths caused by faulty heat exchangers. It is possible that some deaths and many complaints traced to high CO levels in homes are due to backdrafting and spillage from blocked or otherwise improper flues and vent connectors. Similar complaints may be attributed to inadequate supply of fresh makeup air in buildings, often seen as high CO2.

The concern for testing for leaks in heat exchangers and in some cases the methods used in the field are often poorly reasoned. If ASHI professionals are to expend training and inspecting time on life-safety issues perhaps we should first obtain loss data which would allow us to properly prioritize our efforts. Virtually every home safety survey cites tripping hazards as the number-one cause of injuries in homes. Future research (and Journal articles) ought to address this question of priority. But if you test, do it correctly!

As a final note, report the authoritative sources used for your standards, identify your equipment, and state the procedures used and the quantitative measurement obtained. This may avoid embarrassment and will add credibility to your findings. For inspectors who feel that documenting these data is too much work, we suggest skipping instrumented tests entirely.

It's little effort to pre-print a standard explanation identifying the test instrument, it's levels of performance, and the inspection standard used. The only "custom" writing necessary is the actual level measured in the building.

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Technical Reviewers & References

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HEAT EXCHANGER LEAKS

[1] "Three Step Method for Detecting Unacceptable Flue Gas Leakage from Furnace Heat Exchangers", Douglas DeWerth, P.S., The ASHI Technical Journal, Vol. 2 No. 1, July1991
[2] "Heat Exchanger Testing, Who's Right?" Dan Friedman, The ASHI Technical Journal, Vol. 2 No. 1, July1991. DJ Friedman (editor of InspectAPedia.com) served as a professional home inspector and forensic investigator of building failures from, 1986 to 2010. He previously operated a heating and cooling service and repair company and a residential construction and renovation firm.
[3] "Case History: LP Gas Leak - Using the TIF 8800," Dan Friedman, The ASHI Technical Journal, Vol. 2 No. 1, July1991
[4] "Choosing and Using a Carbon Monoxide CO Monitor," Dan Friedman, The ASHI Technical Journal, Vol. 2 No. 1, July1991
[5] American Gas Association, Fundamentals of Gas Combustion, 7th printing, September 1985. Prepared by American Gas Association Laboratories for American Gas Association, 1515 Wilson Boulevard, Arlington, VA 22209 and Gas Appliance Manufacturers Association, Inc., 1901 North Fort Myer Drive, Arlington, VA 22209.
[6] National Fuel Gas Code, NFPA 54 1988 and ANSI Z223.1 1988, available from AGA at the address above, or from the National Fire Protection Association, Batterymarch Park, Quincy MA 02269
[7]NFPA and AGA, National Fuel Gas Code Handbook, Z223.2, same source as above
[8] "Residential Gas Furnace Heat Exchanger Testing", Douglas DeWerth, P.E. American Gas Association (AGA) Laboratories under SAM number 630-92 9/86. , Refrigeration Service Engineer’s Society’s Service Application Manual (SAM)" The manual outlines and explains the many test methods that are used and have been used to inspect and test heat exchangers. This manual also goes into detail about the test equipment used in the 3 step method that the AGA developed. RSES members can view the SAM minus the photographs and diagrams at http://www.rses.org. If you are not a member or want to purchase a complete copy call RSES (800)297-5660 for availability and cost.
[9] Furnace Heat Exchanger Leak Test, American Gas Association .
- Step 1 Visual inspection.
- Step 2 Burner flame deviation test.
- Step 3 injecting a nitrogen/methane gas mixture into the burner chamber. The heat exchanger outlet of the heat exchanger is plugged and a combustible gas detector is used to check for gas leakage on the exterior of the heat exchanger. A detailed fact sheet on the AGA test procedure is of the heat exchanger.
- Source: http://www.aga.org/NR/rdonlyres/A156C36A-9324-4978-91B4-E78DB63DAD7D/0/8612FURNHEATEXCHNGLEAKTEST.pdf
[10] Jennifer Moore, Sales Administrator, Nextteq, LLC, Tampa FL, www.nextteq.com 813-249-5888. Nextteq is the master Distributor for Gastec in the United States. According to the company's website, Gastec Gas Sampling Pumps are the industry’s first and only pumps to provide on-the-spot measurement of ambient temperature. [Private email, JM to DF 5/23/08]
[11] AGA mixed gas test method for heat exchangers: see http://www.aga.org/pdf/publicinfo/co...facts8612b.pdf also see the gas test kit available from http://www.testproductsintl.com/gas.html
Chimneys, Flues, Woodstoves & Fireplaces: Safety Concerns, safe and proper venting of combustion gases, carbon monoxide hazards
GAS EXPOSURE EFFECTS, TOXIC Toxic Gas Exposure Hazards and Test Protocols including links to our toxic gas exposure screening and gas testing protocols.
Gases: Toxic gases, indoor exposure levels, testing, identification
A Toxic Gas Testing Plan: A Gas Sampling Plan for Residential and Commercial buildings lists some of the toxic indoor gases for which we test, depending on the building complaint and building conditions
Gas Exposure Hazard Levels: for Toxic Gas Exposure to Ammonia, Arsine, Arsenic, Bromine, Carbon Dioxide, Carbon Monoxide, Hydride, Ozone - allowable exposure levels and hazard levels
Carbon Dioxide Gas Toxicity hazard level, poisoning symptoms, & testing
Carbon Monoxide Gas Toxicity hazard levels, poisoning symptoms, & testing
Formaldehyde: US EPA. UFFI (Urea Formaldehyde Foam Insulation) was previously considered a hazard (formaldehyde outgassing). Subsequent research virtually closed concern regarding this material; however formaldehyde appears to remain a health concern for sensitive individuals.
Ozone Warnings - Use of Ozone as a "mold" remedy is ineffective and may be dangerous.
Sampling for gases in air such as VOC's, MVOC's, toxic chemicals, and combustion products.
Unfortunately no single test or tool can detect all possible building contaminants. We use methods and equipment which can test for common contaminants. If the identity of a specific contaminant is known in advance we can also test for a very large number of specific contaminant gases in buildings.
We use gas sampling equipment provided by the two most reliable companies in the world, Draeger-Safety's detector-tubes and Drager accuro bellows pump, the Gastec cylinder pump and detector-tube system produced by Gastec. We also have used gas detection tubes by Gastec previously marketed for use with Sensidyne pumps but Sensidyne pumps now use Kitagawa gas detection tubes. We also use Sensidyne's Gilian air pump. For broad screening for combustibles and a number of other toxic gases and for leak tracing we also use Amprobe's Tif8850 and 8800, and the TIF 5000 automatic halogen leak detector (for air conditioning and cooling system refrigerant leak detection). All of these instruments, their applications, and sensitivities (minimum detectable limits) for specific gases are described in our Gas Sampling Plan online document.

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.

The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop, and from the InspectAPedia bookstore. The 2010 edition of the Home Reference Book is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. InspectAPedia.com ^® author/editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.

More About Carbon Monoxide

Carbon Monoxide Gas Toxicity, exposure limits, poisoning symptoms, and inspecting buildings for CO hazards
Heating System Check for Carbon Monoxide recommended by the US CPSC
Toxicity of Gases & Indoor Testing Suggestions
Plastic Heating Vent Pipe & Other Heating Safety Recall Notices

More about How Furnaces Work

For details about the setting, re-setting, or function of the controls and switches commonly found on hot air heating systems see these articles:

Hot Air Heating Furnace Basic Operating Steps
What is the Function of the Hot Air Furnace Fan Limit Switch?
Furnace Cad Cell Relays Explained
Furnace Stack Relay Switches Explained
FURNACE CONTROLS & SWITCHES describes warm air furnace controls and switches
AIR CONDITIONING CONTROLS & SWITCHES provides a description of each air handler switch and control
FURNACE OPERATION DETAILS

Carbon Monoxide Gas Toxicity, exposure limits, poisoning symptoms, and inspecting buildings for CO hazards
DUST CONTAMINATION FROM HVAC? An Investigation of Indoor Dust Debris Blamed on a Heating/Cooling System Reveals Carpet Dust
Fuel Oil & Oil Heating Magazine, 3621 Hill Rd., Parsippany, NJ 07054, 973-331-9545
Goodman Furnace High Temperature Plastic Vent HTPV safety recall US CPSC notice
Home Heating System Should Be Checked [for proper venting and for CO Carbon Monoxide Hazards - DJF]
Inspection Procedures for Oil-Fired Heating Systems Detailed step by step approaches for inspecting complex systems]
Lennox Pulse Furnace Safety Inspection/Warranty Program: Carbon Monoxide Warning
Oil Tanks - The Oil Storage Tank Information Website: Buried or Above Ground Oil Tank Inspection, Testing, Cleanup, Abandonment of Oil Tanks
Oil Tanks Above Ground, UL Standards, guidance for home owners, buyers, and inspectors
Plastic Heating Vent Pipe & Other Heating Safety Recall Notices
Weil McLain Model GV Gas Boiler/gas valve CPSC recall/repair
Domestic and Commercial Oil Burners, Charles H. Burkhardt, McGraw Hill Book Company, New York 3rd Ed 1969.
National Fuel Gas Code (Z223.1) $16.00 and National Fuel Gas Code Handbook (Z223.2) $47.00 American Gas Association (A.G.A.), 1515 Wilson Boulevard, Arlington, VA 22209 also available from National Fire Protection Association, Batterymarch Park, Quincy, MA 02269. Fundamentals of Gas Appliance Venting and Ventilation, 1985, American Gas Association Laboratories, Engineering Services Department. American Gas Association, 1515 Wilson Boulevard, Arlington, VA 22209. Catalog #XHO585. Reprinted 1989.
The Steam Book, 1984, Training and Education Department, Fluid Handling Division, ITT [probably out of print, possibly available from several home inspection supply companies] Fuel Oil and Oil Heat Magazine, October 1990, offers an update,
Principles of Steam Heating, $13.25 includes postage. Fuel oil & Oil Heat Magazine, 389 Passaic Ave., Fairfield, NJ 07004.
The Lost Art of Steam Heating, Dan Holohan, 516-579-3046 FAX
Principles of Steam Heating, Dan Holohan, technical editor of Fuel Oil and Oil Heat magazine, 389 Passaic Ave., Fairfield, NJ 07004 ($12.+1.25 postage/handling).
"Residential Steam Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
"Residential Hydronic (circulating hot water) Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
"Warm Air Heating Systems". Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
Heating, Ventilating, and Air Conditioning Volume we , Heating Fundamentals,
Boilers, Boiler Conversions, James E. Brumbaugh, ISBN 0-672-23389-4 (v. 1) Volume II, Oil, Gas, and Coal Burners, Controls, Ducts, Piping, Valves, James E. Brumbaugh, ISBN 0-672-23390-7 (v. 2) Volume III, Radiant Heating, Water Heaters, Ventilation, Air Conditioning, Heat Pumps, Air Cleaners, James E. Brumbaugh, ISBN 0-672-23383-5 (v. 3) or ISBN 0-672-23380-0 (set) Special Sales Director, Macmillan Publishing Co., 866 Third Ave., New York, NY 10022. Macmillan Publishing Co., NY
Installation Guide for Residential Hydronic Heating Systems
Installation Guide #200, The Hydronics Institute, 35 Russo Place, Berkeley Heights, NJ 07922
The ABC's of Retention Head Oil Burners, National Association of Oil Heat Service Managers, TM 115, National Old Timers' Association of the Energy Industry, PO Box 168, Mineola, NY 11501. (Excellent tips on spotting problems on oil-fired heating equipment. Booklet.)
Links to our list of additional information on heating system inspection, repair, maintenance
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Free Encyclopedia of Building & Environmental Inspection, Testing, Diagnosis, Repair

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How to inspect a furnace heat exchanger for damage or leaks & carbon monoxide CO gas hazards

Visual Inspection of the Furnace Heat Exchanger can Detect Some Leaks but Not All

A Complete List of Methods Used to Find or Test for Leaks in a Furnace Heat Exchanger

Visual Inspection Alone is Unreliable for Detecting Furnace Heat Exchanger Leaks or Damage

What is the Allowable Amount of Gas Fired Heat Exchanger Leakage?

Heat Exchanger Leak Detection Articles

Three Step Method for Detecting Unacceptable Flue Gas Leakage from Furnace Heat Exchangers

Overview of the Three Step Method for Detecting Unacceptable Flue Gas Leakage from Furnace Heat Exchangers

Heat Exchanger Testing and Test Devices: Who's Right?

Background

Review of existing leak detection procedures

Visual Inspection of the Heat Exchanger

Smoke Bombs for Heat Exchanger Tests

Tracer Gas Tests for Furnace Heat Exchanger Leaks

Spray Dyes for Heat Exchanger Tests

Chemical Smoke Tests of Furnace Heat Exchangers

Furnace Heat Exchanger Pressure Measurements for Leak Detection

Minimum Hole Size Allowable in a Furnace Heat Exchanger

Figuring the diameter of a heat exchanger leak opening from the area concentration of leaked gas that will occur as a result

0.51 sq.in. computed Leak Opening for a Furnace Heat Exchanger

1/4" diameter Hole in a Heat Exchanger - Unacceptable at 80,000 BTUH example

Maximum Leakage Allowed

House Air Changes Per Hour

House Air Change Rate

Allowable CO Exposure in buildings

Specification of the Test Gas for Detecting Heat Exchanger Leaks

Description of the Test Procedure for Heat Exchanger Leaks

Step One - Visual Inspection

Step Two - Flame Observation

Step Three - Tracer Gas

References

Gas Furnaces and Indoor Air Quality

What Is Carbon Monoxide Poisoning?

Acceptable Carbon Monoxide Levels in buildings

Heat Exchanger Failures

Carbon Monoxide Monitors

Carbon Monoxide Monitoring

References

Choosing and Using a CO Monitor for Gas Leak Detection

Electronic CO Monitors

The Draeger Model 100 Gas Monitor,

Furnace or Boiler Gas or Oil Combustion Efficiency Kits

Aspirators & Colorimetric Detection for Gas Detection

Color Change Methods

Sensidyne/GasTec Aspirator

Draeger Aspirator

Aspirator & Colorimetric Tube Accuracy

Aspirator & Colorimetric Tube Procedure:

Broad-Spectrum Analyzers for Gas Leak Detection

Gas Leak Detection Instrument accuracy

TIF 8800 Test procedure:

TIF8800 Field Use Experience

Case History: LP Gas Leak Detected Using the TIF 8800

Looking for Gas Leaks

Traditional "Tests" for Gas Leaks

Pressure tests of gas piping were not discussed though this may be the procedure in new installations

Using the TIF 8800

Common LP Gas Leak Points

False alarms with the TIF 8800 testing for LP Gas Leaks

Final Note

References

Heat Exchanger Testing and Test Devices: Who's Right?

Matzen's Test Method for Heat Exchanger Leaks

DeWerth's Test Method for Heat Exchanger Leaks

Is This Heat Exchanger Safe?

Derivation of Test Criteria for Heat Exchanger Leaks

A Test Methodology for Furnace Heat Exchangers - Conclusions

Combining Both Methods of Testing for Heat Exchanger Gas Leaks

Every inspector should follow the first two (visual) steps of DeWerth's procedure

Inspectors who prefer Matzen's standards of acceptability will report any CO levels found in a building.

Based on DeWerth's explanation of the mechanism of failure and leak rates, a very low leakage detected

A call for Priorities:

Questions & Answers regarding this article

Ask a Question or Search InspectAPedia

Technical Reviewers & References

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

More About Carbon Monoxide

More about How Furnaces Work