Reuben's Home Inspection Blog


How Serious Is A Cracked Heat Exchanger?

It's an industry standard: if a furnace has a cracked heat exchanger, it gets replaced.  The American Gas Association has even put this in writing - they say "Any visible crack or hole is reason for requiring replacement of the heat exchanger or furnace."  When I inspect a furnace and I find a cracked heat exchanger (and I find a lot of them), I always say to replace it.

So what's the big deal with cracks or holes?  The concern is that a cracked heat exchanger could allow exhaust gas from the furnace to contaminate the household air with carbon monoxide.   In order for this to happen, the furnace must be producing high levels of carbon monoxide AND the exhaust gas must be mixing with the household air.  For a good example of a hazardous heat exchanger, check out the photo below showing a large rust hole in the heat exchanger of this high-efficiency furnace that was only ten years old.

Rust Hole in Heat Exchanger

Cracks, on the other hand, I'm not so sure about.  With the majority of the cracked heat exchangers that I've seen, I've always been curious how the exhaust gas from the furnace could possibly leak out of those tiny cracks enough to contaminate the househouse air.  Of course, what I'm curious about doesn't matter... but sometimes my curiosity gets the best of me, and I have to find out for myself.

So I did. I took home a furnace that had a cracked heat exchanger, and I removed the heat exchanger cell that had the largest cracks.  You can see the cracks for yourself below - click on any of the photos for a larger version.  This first photo shows the cracks as seen from inside the heat exchanger cell - this is what we saw during our inspection.

Cracked Heat Exchanger Inside

These next two photos show the cracks from the exterior, or blower side of the heat exchanger cell.  This part of the heat exchanger is usually not visible during the course of a home inspection.

Cracked Heat Exchanger Outside Cracked Heat Exchanger Outside 2

I wanted to see if water would leak through these cracks, so I doused the outside of the heat exchanger and looked inside for any signs of leakage.  Nothing.

I've heard that penetrating oil, such as WD40, will get through the cracks, so I tried that next.  Nothing.

Feeling pretty disappointed at this point and determined to get some results, I filled the heat exchanger with water.  I laughed like a mad scientist at what happened next.

Water Test on Heat Exchanger

Water began to leak out of the factory seam in about ten different places, but the crack never leaked.

Why does this tiny crack mean the furnace should be replaced?  I've heardthat when heat exchangers get very hot, the metal expands and the cracks open up, allowing air to leak in to or out of the heat exchanger.  Just because my test didn't allow any water to leak doesn't mean that this furnace was safe, and it doesn't mean that a different furnace will behave the same way... but I sure found it amusing.

Maybe I need to get out more.


Reuben Saltzman, Structure Tech Home Inspections


Comment balloon 13 commentsReuben Saltzman • March 02 2010 06:23AM


Rueben ~ Very informative info on a cracked heat exchanger. Thanks for a thorough explanation!

Posted by Monique Hailer, South West Florida & Southern Maryland Homes (Schooner Bay Realty., Inc. & CENTURY 21 New Millenniun) about 9 years ago

Years ago that seemed to appear on every home inspection..."possible" cracked heat exchanger...anything is possible...when the carbon monoxide reader jumped ....whoops....oh yeah...crack somewhere and new furnace !

Posted by Sally K. & David L. Hanson, WI Realtors - Luxury - Divorce (EXP Realty 414-525-0563) about 9 years ago

Rueben, great pictures and explanation.  Overheating is a major cause of failed heat exchangers.  So make sure you keep those filters changes and keep all of the supply grilles in the rooms open during the heating season.  The furnace needs that air exchange to keep the exchanger cool.  Also stay away from those very restrictive High Efficiency filters that cost $15+ at Lowes and Home Depot.  They are very restrictive and dont allow good air flow.  Stay with a filter rated at a 6 MERV or less.


Posted by Lee Floyd (Race City Inspections Inc) about 9 years ago

Thanks Rueben, it's always nice to learn about something -- anything -- that can possible help explain things to my clients. 

Posted by Vickie Osoria (Keller Williams Realty) about 9 years ago

Monique - I glad I could help clear some of this mystery up.

Sally & David - I hate saying "possible" anything.  I've done it, and I still do it, but I use that word very sparingly.

Lee - you're completely right about the furnace filters. I preach the exact same thing to my clients at every inspection.  I smell a blog topic.

Vickie - hopefully this helps to put things into perspective.

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) about 9 years ago

If you right it before me you need to give me props. :-)

Posted by Lee Floyd (Race City Inspections Inc) about 9 years ago

Lee - no prob!

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) about 9 years ago

Reuben, some time things just make your head go bump in the night---very interesting.

Posted by Charles Buell, Seattle Home Inspector (Charles Buell Inspections Inc.) about 9 years ago

Interesting post. Typically if a heat exchanger has a crack, the crack will close rather than open up due to the expansion of the exchanger. Either way I bet you had fun taking it apart didn't you.

Posted by Mike Gillingham (Eastern Iowa Inspection Services LLC) about 9 years ago

Charles - yeah, this was a crazy experiment.

Mike - I've heard the opposite from most heating experts, including Ellis Prach at Heat Exchanger Experts.  That's not to say you're wrong... that's just what I've heard.  I've never been able to tell any difference.  You bet I had fun taking that furnace apart :)

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) about 9 years ago


My own furnace in my house in Lake Elmo (MN), long ago when I lived up there, started making a loud banging noise when the system engaged.  The HVAC tech came out and went straight to the heat exchanger, calling the noise a signature sound for a failed heat exchanger.  I never saw the old unit, but I suspect it had large cracks.

Thank you,

Richard Acree


Comments in this blog posting are the copyrighted intellectual property of Richard Acree, President, HABITEC Home and Building Inspections, LLC, and contributing members of the Active Rain Real Estate network, and are intended to educate and otherwise assist home owners, sellers and buyers, building owners, sellers and buyers, realtors, real estate investors, property managers, and lenders in the process of owning, buying or selling homes or commercial buildings.  HABITEC is a residential (home) and commercial building inspection company serving Middle Tennessee including Nashville, Brentwood, Franklin, Murfreesboro, Smyrna, Mt. Juliet, Hendersonville, Dickson, Belle Meade, Columbia, Spring Hill and more!  In addition to building inspections HABITEC offers Environmental Services for mold assessments, radon testing and water quality analysis.  Additional information about HABITEC can be found on our website at, or call 615-376-2753. 

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Posted by Robert Dirienzo, Home Inspections - Nashville TN (HABITEC Home and Building Inspections, LLC) about 9 years ago

Richard - that's wild.  I've never heard of being able to identify a cracked heat exchanger based on a noise.  Have you heard of anything like that ever since?

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) about 9 years ago

That was the only time I ever heard of that.  I suspect there was more than a crack in there.  More like a broken part.  I wish I had pursued it more at the time.


Posted by Robert Dirienzo, Home Inspections - Nashville TN (HABITEC Home and Building Inspections, LLC) about 9 years ago