Furnace heat exchangerFurnace heat exchanger

You notice a strange smell coming from your furnace – could it be a cracked heat exchanger? Upon further inspection, you see that there is rust along the exterior of the system. You wonder: Is this something I need to call an HVAC company about?

The answer: Yes, immediately. Both of those signs could point to a cracked heat exchanger, which, if not handled soon, poses a serious health and safety risk to your family.

The heat exchanger is the largest component of your home’s heating system, and over the course of its lifespan, it can begin to develop cracks. When a crack is formed in your furnace, the system may not be able to completely combust fuel. This can lead to high and dangerous levels of carbon monoxide entering your home.

What Does a Heat Exchanger Do?

Made up of metal shells and tubes, the heat exchanger is the part of the furnace responsible for generating hot air. But, how does it do that? When your furnace burns natural gas or propane, its exhaust byproducts enter and travel through the heat exchanger.

This heats the metal as it passes through to the furnace’s exhaust outlet. At the same time, the hot metal heats the air that blows across the heat exchanger. From there, the system distributes the warmed air throughout your home through air ducts.

What Causes a Cracked Heat Exchanger?

The air inside the gas furnace and around its heat exchanger gets extremely hot, causing the metal inside the exchanger to expand and contract. Well-maintained heat exchangers and systems on the front end of their working life are able to withstand this heat.

Over the course of several years, however, it’s fairly common for the metal expansion and contraction to cause stress cracks in the heat exchanger.

Heat exchangers typically last between 10 to 20 years. If yours is within that age range, any cracks within the heat exchanger are likely from normal use. If your heat exchanger is less than a decade old, however, other factors may be to blame, including:

Limited airflow to your gas furnace

Too much pressure may build up inside the furnace if you have several blocked registers. An overly clogged or dirty furnace filter or obstructed air ducts can also cause heat exchanger cracks.

The furnace is too large for the home

Improperly sized furnaces may turn on and off frequently. This is known as “short cycling.” Short cycling causes your heat exchanger to expand and contract more than necessary. This overuse of your system may eventually lead to cracks before you’d normally see them if your heating system was the correct size for your home.

Why Are Cracked Heat Exchangers a Problem?

If you suspect your furnace has a cracked heat exchanger, it’s important to call a heating and cooling technician to diagnose the issue immediately. A cracked heat exchanger poses serious health risks for a couple of reasons:

Carbon monoxide leaks

The heat exchanger contains and burns off carbon monoxide, sulfur dioxide, and nitrous oxide. A leak means they could escape into your home’s air ducts. This is known to cause illness, including flu-like symptoms, or, in extreme cases, death. 


When a mixture of gas and air combust in the heat exchanger, uncombusted gas builds up until it reaches a high enough volume to be explosive. When it becomes explosive, it combusts and forces flames and air pressure out of any accessible hole, such as a cracked heat exchanger. According to the National Fire Protection Association (NFPA), furnace fires occur about 2,600 times per year.

What Do You Do If Your Heat Exchanger is Cracked?

You may be unaware of a cracked heat exchanger unless your carbon monoxide detector beeps, but you may also notice the signs mentioned above. Unfortunately, you can’t really know for sure that your heat exchanger is cracked without an HVAC technician’s inspection.

Our service technicians will be able to identify a cracked heat exchanger using specialty tools and video inspection systems to examine it closely.

If you have a cracked heat exchanger, you’ll likely need to have it replaced with a new furnace sooner than later. Because a heat exchanger is more likely to crack late in your furnace’s life, consider a system upgrade or replacement.

Schedule a Furnace Check-up Today, and Avoid a Cracked Heat Exchanger

One of the best things you can do if you suspect a cracked heat exchanger is to call in a pro for help. Even if your heat exchanger is new, we can still perform routine maintenance to make sure your furnace is safe to operate this winter. Call or contact us online to set up a heating tune-up today, or schedule HVAC service on your other heating, air conditioning, or indoor air quality equipment.