CRACKED heat exchanger? What does that mean, and what do you do now?

You’re a rather vigilant person, getting your annual furnace maintenance done.  Or perhaps your furnace isn’t working properly and you’ve called in the professionals.  Your HVAC service technician has just informed you that you have a cracked heat exchanger.  What?!?

Unfortunately, heat exchangers cannot be repaired.  If your furnace is running okay, you’d probably rather ignore the bad news.  However, the reality is that your furnace can become a real danger to your home and family.  When a heat exchanger is cracked or rusted, it must be replaced, either through a complete furnace replacement or changing out the heat exchanger.  There is no repairing it, and ignoring it is not a solution.

Let’s step back and take a look at what a heat exchanger is and does.  The heat exchanger plays a central role in any furnace.  In very simple terms, the burning fuel warms the heat exchanger, which in turn warms the air in your home.  In the case of a forced air furnace, it separates the warming flame from the air in the home.  The return air from your home and the hot exhaust from the combustion of gas are separated by sheet metal or tubing that typically snakes back and forth, to allow for the maximum amount of air to pass over it and the most heat to be exchanged between the two streams.

Whew, that was a mouth full.  Why is all of this info important?  Inside of your furnace there is actually a fire burning.  Natural gas or propane is being burned, and the byproduct of that process is energy in the form of heat, along with water vapor, carbon dioxide, as well as carbon monoxide, pure carbon (soot) and some Nitrogen Oxides.  With the exception of water vapor, all of these are considered pollutants and all of them can cause sickness or be fatal in high enough quantities.

So why do heat exchangers crack?  They’re designed to provide safe operation of the furnace for its full life expectancy – usually 15-18 years.  However, many things can accelerate the wear and tear on a heat exchanger, causing it to fail much sooner.

Sometimes it’s just age and use.  Where we live, the heat exchanger heats up and cools down several thousand times per year.   Normal usage, normal life span.

More commonly, the cause of a cracked heat exchanger is misapplication or poor maintenance of the furnace.  If the furnace is grossly oversized or used incorrectly, the heat exchanger is put at risk to fail prematurely.  When a furnace is oversized, short cycling or cycling on limit can cause the heat exchanger to heat up and cool down far more often than if the furnace was properly sized.

Another heat exchanger killer is lack of airflow.  The main cause of low airflow in a furnace is a dirty filter.  If the filter gets clogged with dust and dirt, the air can’t get through and the furnace will cook itself.  The heat won’t be able to get out and will most likely cycle on its high limit until it ceases to function.  Poor airflow can also be the product of poor ductwork design, poor installation, or not having enough ductwork to support the size of the furnace.  The heat exchanger will be over-stressed from being constantly overheated and cooled.

Cracks in a heat exchanger are sometimes easy to see.  Sometimes they might seem too small to warrant attention or are located in an area that doesn’t seem critical to furnace operation.  Either way, the problem needs to be faced head on.  Upon finding a crack in a heat exchanger, our recommendation will be to replace the heat exchanger or the furnace.  Your family’s health and safety are too important to make guesses as to how dangerous the crack may or may not be.