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This article explains how to recognize and fix combustion air defects on heating appliances such as boilers, furnaces, and water heaters. Lack of adequate combustion air causes improper heater operation, increased maintenance cost, and risks dangerous production of carbon monoxide gas.

This article series explains how to recognize & diagnose problems with residential heating boilers, including loss of heat, heating boiler noises, leaks, odors, or smoke, and high heating costs. This website answers most questions about central hot water heating system troubleshooting, inspection, diagnosis, and repairs. Also see Combustion Air Defects where we list symptoms of lack of adequate combustion air for oil fired heating equipment.

If you have no heat and need help diagnosing the cause, see HEATING LOSS DIAGNOSIS-BOILERS or HEATING LOSS DIAGNOSIS-FURNACES. Our photo at page top shows an oil fired furnace installed in a closet with an airtight door; there was no outside combustion air supply. The heating system could not work properly nor safely in this home.

Watch out: inadequate combustion air supply to a gas burner (and less often to an oil burner) is very dangerous and can produce potentially fatal carbon monoxide. If you suspect unsafe heating system operation or a carbon monoxide problem be sure everyone leaves the building immediately and then call your local fire department for assistance.

COMBUSTION AIR for TIGHT buildings explains how to provide outside combustion air for tight buildings. See COMBUSTION GASES & PARTICLE HAZARDS for an explanation of the dangers of inadequate combustion air. See COMBUSTION PRODUCTS & IAQ for the relationship between fuel burning appliances and building indoor air quality. More about carbon monoxide - CO - is at CARBON MONOXIDE - CO and at CARBON MONOXIDE WARNING. Also see the safety warnings at BACKDRAFTING HEATING EQUIPMENT.

The photograph shows a heating furnace with soot blow-back around the oil burner probably means there is backpressure in the combustion chamber - an improper operating condition that may be unsafe. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

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How to Recognize & Diagnose Inadequate Combustion Air, Sooting, or Burn Marks at Oil-Fired or Gas-Fired Heating Systems

Lack of adequate combustion air can be indicated by or can result in these heating system operating and safety worries:

• Gas burner sooting or odors: SAFEY WARNING: Small amounts of soot or flame marks right at the gas burner also indicate an operating problem but may not be producing carbon monoxide. But soot produced at a gas fired appliance such as chunks of soot found around a gas flue vent or draft hood is a RED DANGER FLAG as dangerous carbon monoxide may be produced and a chimney may be blocked. Turn off the equipment and contact your heating service company or utility company immediately. See CARBON MONOXIDE WARNING and see SPILL SWITCHES
• Improper oil burner system operation such as noises, rumbling, etc. discussed at OIL BURNER NOISE SMOKE ODORS
• Oil burner sooting or dirty operation (the photo at above-left)
• Burn marks on the boiler (photo at above right), furnace, or water heater, (these conditions may be caused by a blocked exhaust flue and inadequate venting).
  
  Safety Warning: burn marks on a heating boiler or furnace (shown in our page top photo) can also be due to collapse or damage to the combustion chamber liner - a serious fire hazard needing immediate attention. Notice that in our photo at right, the brown burn marks around the oil boiler combustion chamber inspection port (that rusty round door above the oil burner) have been cleaned-off.
  
  The presence of these burn marks does not necessarily mean that the oil burner backpressure problem remains - in this case the system has been cleaned and adjusted, but no one has re-painted the front of the boiler. But if you see fresh peeling paint or soot in such an area further investigation is needed.
  
  That's why we recommend that after repairing a back-pressure problem at heating equipment the service technician should clean the boiler or furnace exterior - to remove confusing debris and to make it easier to see if the problem recurs.
• Heating equipment located in a small utility room with no provision for combustion air intake. When the service technician adjusts the system she probably worked with the utility room door open, but when the service tech left the job he may have closed the door - completely changing the availability of combustion air for the equipment. We need about one square inch of un-louvered (unobstructed) combustion air intake per 1000 btuh of the oil fired heating boiler, furnace, or water heater.
• Increased heating system operating cost, spending more on heating oil than necessary
• Damage to oil burner components (backpressure heat can destroy an ignition transformer),
• Higher and more frequent heating service costs,
• Loss of heat, the heating system goes off on safety reset
• Noises when the boiler, furnace, or water heater starts, is operating, or is shutting down
• Smoke indoors,coming out of the heating equipment or its draft regulator
• Potentially, the production of carbon monoxide or other flue or combustion gases which escape into the building - potentially dangerous. It is harder to produce with oil heat than with gas heat, but not impossible.

Also see Unsafe Air Conditioning or Heating Duct Openings which describes the risks of reduced combustion air on hot air heating systems when certain return air duct defects are present, and also see CARBON MONOXIDE - CO and CHIMNEY INSPECTION DIAGNOSIS REPAIR

Guide to a Simple Combustion Air Safety Check for Gas Fired Heating Appliances

Thanks to Tjernlund Products who recommended this procedure, we describe a simple combustion air safety check that can be performed by a homeowner or by a heating service technician.

This procedure is only intended for buildings where LP gas or natural gas heating appliances (heating boiler, warm air furnace, water heater) are installed AND where a flue gas spillage safety switch such as a Tjernlund UC1 Universal Control, MAC1E or MAC4E auxiliary controls for gas fired equipment or Field Controls Gas Spillage Sensing Kit Model GSK-3, GSK-4, GSK-250M switches are installed on those appliances. (Similar products are provided by other manufacturers.)

1. Close all doors and windows of the building. If the gas fired heating appliance is installed in a utility room or closet, close the entry door to that room. Close all fireplace dampers. Turn on the clothes dryer (if installed) and all exhaust fans such as range hoods, bathroom exhaust vent fans, whole house fans, and radon mitigation fans, running all fans at their maximum speed.
2. Turn on and place in operation the gas fired heating appliance on which the flue gas spillage sensor switch to be tested has been installed. Set the appliance thermostat for continuous operation. If other gas fired appliances are installed in the same building run those appliances at the same time.
3. Allow the fans turned on in step 1 to operate for fifteen minutes.
4. If the flue gas sensor spill switch trips during the fifteen minute test period this means that an unsafe condition exists. Check all of the appliances for a venting malfunction and check for adequate combustion air. (The occurrence of the flue gas spillage safety switch tripping may indicate inadequate combustion air but it could also indicate an unsafe chimney or some other unsafe condition.) Also see CHIMNEY INSPECTION DIAGNOSIS REPAIR.
5. TURN OFF FUEL SUPPLY to the gas fired appliance that caused the safety switch to trip.
6. DO NOT OPERATE THE GAS APPLIANCES UNTIL THE UNSAFE VENTING CONDITION HAS BEEN INVESTIGATED BY A PROFESSIONAL CONTRACTOR or GAS UTILITY COMPANY SERVICE PERSONNEL. See CARBON MONOXIDE WARNING.
7. Return all windows, doors, fans, to their previous conditions of use.

SAFETY WARNING: If AT ANY TIME your gas fired equipment has shut down in SAFETY OFF position it may be due to a resettable flue gas spill sensor switch. Check with your heating service company - you might think you can avoid a costly heating service call, BUT BEWARE: because flue gas spillage is very dangerous, including the production of potentially fatal carbon monoxide gas, don't simply reset the system without finding out what caused the problem in the first place.

MORE SAFETY WARNINGS: in addition to our safety warning above, Tjernlund explains that flue gas safety switches are intended to alert the building occupants to a potentially dangerous condition.

But flue gas spillage safety switches are not a substitute for a regular chimney safety inspection nor do they replace regular heating appliance inspection and maintenance by a trained technician. Those steps must be taken as well.

What causes flue gas spillage:

Please see SPILL SWITCHES for a discussion of the causes of flue gas spillage in buildings and for a description that contrasts inadequate combustion air with other causes of dangerous flue gas leakage into buildings.

Bachrach Corporation, a manufacturer of heating system test equipment opines that gas fired equipment is more likely to have flue gas spillage from a blocked chimney than from building depressurization due to inadequate combustion air supply. We're not sure what data supports that view.

Certainly home inspectors find many heating appliances installed in tiny closets with no outside combustion air and a door that, when shut, blocks off air to the appliance. We have also observed that gas fired heating equipment operated just fine in a building until a new owner installed a whole house ventilation fan system.

Combustion Air for Woodstoves - How is It Provided?

The link to the original Q&A article in PDF form immediately below is followed by an expanded/updated online version of this article.

• Q&A on Outside Combustion Air for Woodstoves- PDF version, use your browser's back button to return to this page. Original article, Solar Age Magazine, November, 1985, adapted and updated for InspectAPedia.com December 2010.

The question-and-answer article below paraphrases, quotes-from, updates, and comments an original article, (see links just above) from Solar Age Magazine and written by Steven Bliss.

Question: how do I provide combustion air for a woodstove?

I've often read about the necessity of providing combustion air to a woodstove, but there's usually not much explanation of how it's done. Is it just a pipe that ends near the stove inlet? - Chris Rich, New Castle VA

Answer: hard-ducted outside combustion air required for some woodstoves - in manufactured housing

Many wood-stove manufacturers provide a special line of stoves intended for manufactured housing. These woodstoves must have an external combustion-air inlet hard-ducted directly into the stove.

Often the combustion air supply duct comes up through the floor under the woodstove (or coal stove or airtight wood-burning fireplace), making it almost invisible from inside the room where the woodstove is located.

This makes it attractive for the homeowner, and easy for the builder to install.

A Guide to Combustion-Air-Related Heating Equipment Malfunctions & Their Implications

• Noises & soot buildup can lead to a potentially dangerous "puffback" which can damage the heating equipment and blow soot and smoke throughout the building.
• Lack of adequate combustion air can result in improper system operation, sooting, loss of heat, noises, smoke, and potentially, the production of carbon monoxide or other flue or combustion gases which escape into the building - potentially dangerous

• Leaky oil at an oil burner ignition transformer: watch for back pressure in combustion chamber--watch out for blocked heating flue or blocked chimney - these would be unsafe, or the heater may be unreliable. An unreliable heater can lead to frozen burst pipes and costly building damage. A blocked chimney could lead to a potentially fatal carbon monoxide hazard.
• Odors: while carbon monoxide is odorless and colorless, when it is being produced by heating equipment it may be mixed with flue gases that indeed have a distinctive odor. If you smell oil, gas, or other unidentified odors in a building, be sure that the safety of all heating equipment and chimneys is included in your investigation.
• Unusual interior moisture conditions, noises, smoke, or odors during heating season--watch for blocked chimney or poor flue vent connections
• Equipment recalls: Specific heating system problem brands Repco, Blueray etc.
  [Do not confuse Repco™ heating boilers, which had fire chamber and other failures, with other products (REPCO pumps, REPCO controls, REPCO water conditioners, or water treatment that carry the Repco name. Those products are distinct from REPCO heating boilers and are produced/distributed by the R.E. Prescott Company. Further, we are unaware of any remaining warranty or replacement support for failed or antiquated Repco heating boilers.]
  
• BLUE vs YELLOW COMBUSTION FLAMES - the Blueray history and blue flame vs. yellow flame combustion, flame color & combustion efficiency, & how oil competes with gas as a heating fuel.
• Blueray Heating Boilers/Furnaces Safety Recall by the US CPSC, with additional history, photos, and technical details; Blue Flame Blue Ray
• Carbon Dioxide Gas Toxicity - an explanation of exposure limits and toxicity of this common gas in air
• Carbon Monoxide Gas Toxicity, exposure limits, building inspection for CO hazards
• DUST CONTAMINATION FROM HVAC? An Investigation of Indoor Dust Debris Blamed on a Heating/Cooling System Reveals Carpet Dust
• Goodman Furnace High Temperature Plastic Vent HTPV safety recall US CPSC notice
• Heating System Check Recommended for Carbon Monoxide - CPSC Release 88-92
• Home Inspection Report Language Library: Defects in Oil Tank Installations, Tanks, and Heating Oil Piping
• 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
• Plastic Heating Vent Pipe & Other Heating Safety Recall Notices
• SPILL SWITCHES - safety switches intended to detect dangerous flue gas spillage, where and how are they installed, how are they reset.
• Weil McLain Model GV Gas Boiler/gas valve CPSC recall/repair

Questions & Answers regarding this article

Questions & answers about combustion air requirements and safety hazards for oil and gas fired heating appliances

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Technical Reviewers & References

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• Additional technical contributors & reference sources for this article are listed below.

Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

CARBON MONOXIDE - CO
CARBON MONOXIDE WARNING

CHIMNEY INSPECTION DIAGNOSIS REPAIR
CHIMNEYS & Flues - Asbestos Transite Pipe
COMBUSTION AIR DEFECTS
COMBUSTION AIR for TIGHT buildings
COMBUSTION GASES & PARTICLE HAZARDS
COMBUSTION PRODUCTS & IAQ
COMPLETE COMBUSTION, Stoichiometric

INDOOR AIR QUALITY & HOUSE TIGHTNESS
INDOOR AIR QUALITY IMPROVEMENT GUIDE

• Field Controls provides instructions for the installation of LP and Natural Gas spill sensor switches, for example for their Gas Spillage Sensing Kit Model GSK-3, GSK-4, GSK-250M switches. Contact your heating service technician directly, or contact Field controls at fieldcontrols.com for more information. These switch models include a manual reset switch. Field Controls, Kingston NC 28504 - Tel 252-522-3031.
• Tjernlund Products provides instructions for the installation and use of their controls, including the WHKE Millivolt Interlock Kit for use with their UC1 Universal Control, MAC1E or MAC4E auxiliary controls for gas fired equipment. This document also describes Tjernlund's recommended combustion air safety check which we recommended in this article. Contact Tjernlund Products at tjernlund.com or at 800-255-4208.
• Bachrach Corporation, www.bachrach-training.com provides education for HVAC technicians. We found their web pages hanging during loading -01/2009. Readers may want to contact the company directly at: bacharach-inc.com or at 800-736-4666.
• R.E. Prescott Company, a producer of other REPCO™ residential and industrial products as well as a provider of design-build engineering services is at 10 Railroad Avenue, Exeter, NH 03833. Tel: 603-722-04321 or 888-786-7482. Trish O'Keefe from Prescott informed us (10/7/2009) that their company had nothing to do with the failed Repco heating boilers discussed at InspectAPedia.com. She wrote:
  • Our company manufactures & distributes residential water treatment equipment, including our Repco line of conditioners. We have a plumbing & heating supply dept as well. We are mistaken for the Repco Boiler company on a regular basis, most frequently in Oct. and Nov.
  • Any information you could give me on the other Repco company would be appreciated. We've assumed it is no longer in business, and we'd like to know that for sure. Many callers want to know where to get replacement parts and I would be glad to direct them if I knew...plus there's always a chance they might be interested in a Crown Boiler instead

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.

• 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 I, 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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