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Here we discuss how to inspect furnace heat exchangers for leaks. 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 & GAS HEAT ODORS and see Backdrafting Appliances. More about carbon monoxide - CO - is at CARBON MONOXIDE WARNING.

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

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.

A List of Ways to Find or Suspect a Leak 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.

• 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).

• Heat Exchanger Leak Detection Instruments: 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.
  
  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]

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, I 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.

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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

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

• Our recommended books about building design, inspection, and repair, and about indoor environment testing, diagnosis, and cleanup are at the InspectAPedia Bookstore.
• ...

More Information on Building Diagnostic Inspections and Repairs

• Inspection Procedures for Oil-Fired Heating Systems Detailed step by step approaches for inspecting complex systems]
• "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,

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