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In this article we discuss residential forced warm air furnace heat exchanger life in more detail, explaining the factors that impact the life of the heat exchanger, things you should do to maximize the life of the furnace heat exchanger, and similarly, things you should not do because they are likely to destroy your furnace before its time.
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Typical Residential Warm Air Heating Furnace Heat Exchanger Life
Furnace heat exchangers transfer heat from the burning fuel (typically LP gas, natural gas, or heating oil) into building air. In most designs combustion gases pass from the combustion source (oil or gas flame and combustion chamber) through the heat exchanger's interior tubing to an exhaust point that vents gases through the heater's flue and chimney (sketch at left).
What happens if a heat exchanger leaks?
If a heat exchanger develops a leak (through rust perforation or by cracking due to metal fatigue) the heating system is most likely unsafe and needs repair or replacement. There are two principal reasons to talk about furnace heat exchanger life:
1. Safety: a leaky heat exchanger can be dangerous, risking carbon monoxide poisoning.
2. Cost: although it is possible to replace a leaky heat exchanger on many furnace models (not all), the labor cost and total cost investment in such a replacement in an older furnace is questionable enough that more often the ultimate cost is that of a new heating system.
A while back at HEAT EXCHANGER LEAK TEST reader Marlin asked "what is the normal life of natural gas furnace?" to which we replied "Just about the warranty period - manufacturers know their equipment well; warranty periods run from 10 years to lifetime." But this was a bit too glib. Here we provide the details about the causes of leaks in furnace heat exchangers and about furnace heat exchanger life expectancy.
Why do we care about leaky furnace heat exchangers? Furnace heat exchanger leaks may vary from trivial - even below the limits of detection, potentially fatal.
Watch out: Dangerous carbon monoxide gas leaks (CARBON MONOXIDE - CO), potentially fatal, can be present intermittently depending on variations in heating system operation and building conditions. Actual risk of CO poisoning varies even when there is a leak in a furnace heat exchanger depending on where the leak is, its size, the fuel used (gas vs. oil) and other factors that we discuss in taxing detail in a companion article: HEAT EXCHANGER LEAK TEST.
The top two most commonly-cited causes of leaks or failures of a forced warm air furnace heat exchanger are corrosion and metal fatigue. To understand the actual causes of heat exchanger failures in a bit more detail is to know how to reduce the risk of this potentially dangerous and costly defect.
With all of these, and given that the manufacturers have a greater database of experience than the rest of us (and keep actual performance a trade secret), I figure we should quote anticipated furnace heat exchanger life as about that of its warranty period, excluding exceptional events. I will research for any updates to our list of scholarly articles on the topic and will include them in our article.
is evident when we see an add-on air conditioner evaporator coil atop an older furnace, questionable or leaky condensate handling, rust stains atop the furnace, and similar clues.
Inspecting inside the furnace below the A-coil or evaporator or cooling coil one might see actual rust damage to the heat exchanger. Even if damage is not immediately visible, don't assume that a visual inspection of the heat exchanger is comprehensive. It's not.
At below left we illustrate a hole burned/rusted through the heat exchanger of an old oil-fired furnace. At below right we were concerned about the burned fragments in the left side of the photo.
Our photo at below left shows variation in the gas flame of that furnace - a possible indicator of a heat exchanger leak. At below right we see what I call "exfoliating rust" (red arrow) that shows up in thick flakes or chunks - this is not superficial rust. Our blue arrow points to a pile of this rust debris beneath the gas burner in a furnace that was installed in a damp area - the heat exchanger is suspect even before a thorough inspection.
Our two photos below show a severely rusted gas furnace interior, and our red arrow (below right) points to a severe crack in the heat exchanger.
Just ask the equipment manufacturer. OK so everyone wants to sell you a new furnace or an expensive heat exchanger replacement. It's not just greed. No one wants to bear the risk of promising you that a repaired heat exchanger is safe.
Our photo at left shows the heat exchanger in a new gas furnace we installed in a Poughkeepsie NY Home.
We heard from folks describing repair attempts using high-temperature resistant epoxy, welding, and other low-cost solutions.
Any such repair is likely to void the manufacturer's warranty and each of them has a warning or three to keep in mind: epoxy may not bond reliably or may not withstand the furnace operating conditions;
MIG welding can avoid burning a big hole in the exchanger but if the welder is not an absolute expert the welding job may cause further metal fatigue and more heat exchanger cracking even larger than before; carbon monoxide gas is itself odorless and colorless; you may be depending entirely on carbon monoxide detectors for the life and safety of building occupants.
In fact Keith Prach, an expert on heat exchanger failures & testing notes that when a heat exchanger fails by cracking,
Prach continues to describe the initial state of heat exchanger cracks as small, difficult to see, but growing ever larger as the heater goes through heating cycles. Adding welds, in my OPINION means adding stress points for future heat exchanger cracks.
OPINION: I would not try to repair a furnace heat exchanger.
If the furnace is generally in good condition and operates at a good efficiency but has suffered a cracked heat exchanger, one might consider a heat exchanger replacement when the cost of the total job is about half the cost of a new furnace and of course also if the furnace heat exchanger remains covered by the manufacturer's warranty.
On older furnaces and non-factory warranty heat exchanger replacements we're left with some questions: is the new heat exchanger warranted? Is the work proper, complete, safe? Were the same furnace heat exchanger leak tests performed after the repair as before?
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
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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Frequently Asked Questions (FAQs) about about furnace heat exchanger life expectancy & warranty periods
Could you tell me what the normal life expectancy of a home furnace heat exchanger is? Many thanks. - J.F. 3/11/2013
Thanks for the question - it prompted us to expand our original furnace warranty information in the article above.
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