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Cracked heat exchanger on furnace

Furnace Inspections: How to Find A Cracked Heat Exchanger

A Cracked Heat Exchanger Is Nothing To Ignore

By: Greg R. Wayman, ASHI Certified Inspector

What is a heat exchanger? 

The heat exchanger is the metal wall or tubing that is heated up when the burners are ignited.  The inside of the heat exchanger allows the toxic flue gases produced from the burners to exhaust out through the furnace flue.  The outside of the heat exchanger is where the cold air passes over, becomes warmed, and is blown throughout the duct work of the home.  The heat exchanger is the only wall separating the toxic flue gases from the supply air.  If a crack develops in the heat exchanger, there is a potential for carbon monoxide gas to leak over to the supply side and be blown throughout your rooms.  Carbon monoxide gas is odorless.  The human senses will not detect its presence.  However, there are signs.  Light-headedness, nausea, and flue-like symptoms are all signs of CO gas leaking into your home.  On an extreme case, if the crack in your furnace is bad enough and the conditions are right, you and your family may enter into a permanent sleep.


Gas forced air furnaces last approximately 20 years, some more, some less, depending on how well maintained the unit is.  Some conditions that may shorten the life of your furnace are: the evaporator unit of the air conditioner leaking onto the heat exchanger causing it to rust out, dirt/dust building up on the high-limit control switch forcing your furnace to turn on and off more frequently, a dirty filter that drastically reduces air flow which also will force your furnace to kick on and off more, or dirt on the burners producing inefficient flames causing excess soot buildup on the heat exchanger.  If a furnace is poorly maintained, it is not uncommon to find a cracked heat exchanger much sooner than 20 years. 

Having your furnace annually serviced is one way to help extend its life.  When your furnace reaches 10-12 years, it is strongly recommended that you hire a licensed HVAC (Heating Ventilation-Air Conditioning) company to perform a heat exchanger inspection.  This test should be performed annually from this point forward until the furnace is replaced.  Why? Just as a piece of metal is bent over and over again eventually fatigues and breaks, a similar force is being exerted on your furnace’s heat exchanger.  During operation, the thin metal of the heat exchanger is being heated to high temperatures (sometimes over 175 degrees Fahrenheit) and cooled down to room temperature (70 degrees F).  Each time, the metal expands as it warms up and contracts as it cools.  Eventually, the metal fails and a crack is formed.  A crack will always develop in a heat exchanger.  How soon depends on the conditions it has been subjected to over its lifetime.

Types of Heat Exchanger Tests

There are three basic types of heat exchangers: clam shell, Serpentine, and tubular.  Clam shells are most commonly found on the older models (typically installed before 1990.)  The Serpentine & tubular exchanger started to come into existence in the mid ’80’s and are predominantly the types manufactured today. These designs are more efficient and are found in the 80% and 90%+ efficiency units whereas the clam shells were typically 50%-70% efficient. Popular furnace brands that utilitize a Serpentine heat exchanger are Carrier, Bryant, Lennox, Armstrong Air, & Trane. The tubular heat exchanger can be found in furnace brands such as Rheem, RUUD, Amana, & Goodman.

Clam Shell Heat Exchanger

The most definitive way to identify if a crack is present and has fully formed through the metal is to perform a hydro test. The hydro test consists of removing the blower, utilizing a pump sprayer with soapy water, and spray the outside of the heat exchanger while looking on the inside to see if the solution bleeds through anywhere. Yes, there are times when the crack is so large and obvious that this test isn’t even necessary. Most times, however, the inside of the heat exchanger is dark, rusty, & dirty making it difficult to identify a crack.

If someone only uses a carbon monoxide detector to determine if a crack exists, like the local utility company, then they aren’t truly looking for a cracked heat exchanger. They are only trying to identify if there is eminent danger of carbon monoxide poisoning that instant they are present in that home at that exact time. They can’t determine when pressures will change in the house and carbon monoxide might start seeping through that crack and be blown throughout the home.

Serpentine Heat Exchanger

To find a crack in a Serpentine design, it has to be visual only. One needs to gain access to the heat exchanger typically by removing the blower and physically sliding into the blower compartment with a mirror and flashlight or by lifting the A-coil from above and looking down in. If an HVAC Tech tries to spray a Serpentine heat exchanger, then you need to remove them from your house. The reason is that the eyelets on a Serpentine design are not air tight. Spraying a soapy water solution over them will leak through even on brand new furnaces. The only way to inspect a Serpentine heat exchanger is with your eyes.

I’m not going to get into where to specifically look as that entails having very technical knowledge and training like you would receive from Heat Exchanger Experts as I have. If you want to receive that training, then I highly encourage you to attend one of their Certification courses. The cracks will form where the metal fatigues. Certain brands develop in certain areas almost always.

Furnace Inspection: Cracked Heat Exchanger
Vertical crack found during our Home Inspection

Tubular Heat Exchanger

Like a Serpentine design, the tubular heat exchanger needs to be inspected visually. These tubes are very durable and seem to hold up much longer than most Serpentine designs based on my 19+ years of experience. Usually, the cracks develop on the curves of the tubes where the metal heats up and cools down the most.  

Heat Exchanger Warranties

Typically, the hi-efficiency (90%+) furnaces installed today come with a limited lifetime warranty and the 80% furnaces have a limited 20-year warranty.  Warranties vary by manufacturer.  A furnace that was installed 15 years ago may not have any warranty remaining, but it is always a good idea to find out if one still exists.  Today’s warranties are normally transferable from one homeowner to the next if the house is sold.  However, a limited lifetime warranty is usually reduced to a 20-year limited warranty if a house transaction occurs.  If the heat exchanger is replaced, the warranty is only good for the time remaining on the original warranty.  One important and costly fact to remember is the warranty does not cover the cost of labor to remove the old heat exchanger and install the new.  Labor for this repair runs in the ballpark of $600-$1,200!  Choosing this route of action is not always the best decision for a homeowner.  You end up with a new heat exchanger on a furnace full of old parts that are no longer covered by a warranty.  For specific warranty information, please contact your respective HVAC dealer.