What are the Common Causes of Heat Exchanger in a Gas furnace Cracking?


Have you been told the heat exchanger in your furnace is cracked?  That’s a problem that usually ends the life of an older furnace.  In a newer model, they are sometimes replaced, but it’s an expensive repair—often $600-$900.  Many heat exchangers have a lifetime warranty on the part, but if the labor isn’t also covered, the bill can be close to $500, or even higher.

What causes the heat exchangers to crack?  That’s what we answer in this guide.

Why Heat Exchangers Crack

Old Age: This is the primary reason that heat exchangers crack.  The metal of the heat exchanger is constantly heated, expanding the metal, and cooled, contracting it.  Eventually, after 100,000+ cycles, the heat exchanger may become brittle and crack.  Typically, heat exchanger cracks cannot be welded because the weld won’t withstand the expansion and contraction.

Rust: Corrosion is the second most common reason that heat exchangers crack.  The byproducts of combustion can be very corrosive.  They include water vapor, nitrogen oxides and sulfur dioxide. Eventually the rust can be bad enough to eat a hole in the heat exchanger causing it to crack.

Poor Air Flow: The heat exchanger gets extremely hot during a heating cycle – as high as 1,000 degrees F during normal operation. If the blower motor in the furnace isn’t functioning as it should, unheated air won’t be moved quickly enough across the heat exchanger.  Plugged furnace filters and undersized duct work can also cause poor air flow. The result is that the heat exchanger can become hotter than intended, causing it to warp or expand more than is typical.  Cracking is usually the result.

Poor Quality/Defects: If the metal used to make the heat exchanger is poor quality, it will crack much more easily.  In addition, if the burner isn’t manufactured properly, the flame may actually touch the heat exchanger, literally burning a hole in it. If too much gas is being supplied to the burner through a defective gas valve or monitor, the power of the combustion can eventually destroy the heat exchanger.

Preventing a Heat Exchanger from Cracking

The best prevention is to have your furnace installed and set up by a qualified contractor with good experience.  The quality of the installation plays a major role in how well the furnace will function through the years. Once it is properly installed, have it checked, cleaned and maintained at least every 2 years.  The furnace technician will check flame height and quality so conditions that lead to cracks may be eliminated.  On your own, be sure to keep the furnace filter changed every one to two months.  With proper installation and maintenance, you may get 20+ years of service from your furnace without a cracked heat exchanger.


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