A furnace is often a background player at home, ensuring you're warm during the cold winter months. It frequently isn't noticed until something breaks down. 

One cause may be that your furnace has a cracked heat exchanger. It’s a potentially dangerous issue, so it’s worthwhile to know the evidence of a cracked heat exchanger and what to do if you believe that is the problem.  

What Is a Heat Exchanger in a Furnace? 

A heat exchanger transfers heat from the combustion chamber of your furnace to the air that circulates through the system. It generally accomplishes this through coils or tubes that heat the air while functioning as a barrier to keep gas formed in the combustion chamber, called flue gasses, from getting out into your home.  

Is a Cracked Heat Exchanger Dangerous? 

Because of its important role, it’s no surprise that a broken heat exchanger can pose a risk. Cracks in the heat exchanger can permit dangerous gasses – including carbon monoxide, which can be lethal – to circulate through your home. 

For this reason, never run your heating if you suspect there's a crack in the heat exchanger, as doing so could make the whole household ill. Contact an HVAC professional as soon as possible if you are worried your heating has a cracked heat exchanger that should be repaired. 

Four Warning Signs of a Cracked Heat Exchanger: 

  • Furnace switches off: Cracks in the heat exchanger may cause your furnace to turn off. 
  • Strange Smells: If the air leaving your furnace has an intense chemical odor, it could be an indicator that gasses are seeping through cracks in your heat exchanger. These byproducts, which can smell like formaldehyde, are a significant warning sign. 
  • Carbon monoxide alarm goes off or you notice health problems: If a cracked heat exchanger is relieving carbon monoxide inside your home, your carbon monoxide alarm should go off or family members could experience signs of carbon monoxide poisoning. Symptoms include headaches, dizziness, weakness, nausea, vomiting or feeling sleepy. If an alarm goes off or you feel sick, leave the home immediately and then call for help. 
  • Soot: If you see black sooty accumulating around the exterior of your furnace, it’s another sign something may be seriously wrong.

What to Do if Your Furnace Heat Exchanger is Cracked 

If you believe your furnace has a cracked heat exchanger, call a pro well versed in furnace installation right away so they can examine your system and, if required, handle a furnace heat exchanger replacement. Costs often vary depending on the situation, but estimates run in the neighborhood of $1,000 to $3,000. 

Estimates aside, the good news is that heat exchangers are regularly covered by the warranty. You’ll want to check the warranty paperwork on your furnace, since while the warranty might not cover the entire cost of repairs, it still may significantly reduce your bill.  

How to Avoid a Cracked Heat Exchanger in Your Home  

One of the best ways to avoid problems in your furnace overall is with routine furnace maintenance. Furnaces provide the best possible return on investment when they operate efficiently. Calling a skilled professional to examine your furnace for old parts, clogs in the air filters and other potential problems can keep you from getting a big bill later on.  

It’s also helpful to review your furnace filters every few months – it’s recommended some filters be swapped out every 90 days or sooner if they are dirty or grimy. While the filters aren't connected to the heat exchanger itself, the strain of pulling air through a clogged filter makes the entire furnace work longer to accomplish its job. And the harder your furnace needs to run, the more strain components like the heat exchanger will sustain.