A furnace is almost always a background player for your home, keeping you warm in the cold winter months. It regularly isn't noticed until a malfunction appears. 

One root cause could be that your furnace has a cracked heat exchanger. It can be a safety risk, so it’s critical 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 helps move heat from the combustion chamber in your furnace to the air that circulates inside the system. It generally does this using coils or tubes that heat up the air while acting as a barrier to keep the gasses produced in the combustion chamber, called flue gasses, from leaking out into your home.  

Is a Cracked Heat Exchanger Dangerous? 

Thanks to its central role, it’s no surprise that a cracked heat exchanger can pose a risk. A damaged heat exchanger can permit dangerous gasses – such as carbon monoxide, which can be lethal – to be distributed through your home. 

For this reason, do NOT use your heating if you suspect there's a crack in the heat exchanger, as letting it run could make your entire household sick. Call an HVAC professional right away if you believe your furnace has a cracked heat exchanger that needs repair. 

Four Warning Signs of a Cracked Heat Exchanger: 

  • Furnace switches off: A crack in your heat exchanger could cause your furnace to switch off. 
  • Strange Smells: If the air coming out of your furnace has an intense chemical scent, it might be an indicator that gas is seeping through cracks in your heat exchanger. These gasses, which can smell like formaldehyde, are a common warning sign. 
  • Carbon monoxide alarm is triggered or you notice poisoning symptoms: If a cracked heat exchanger is releasing carbon monoxide into your home, your carbon monoxide alarm should go off or family members could start experiencing signs of carbon monoxide poisoning. Side effects include headaches, dizziness, weakness, nausea, vomiting or feeling tired. If an alarm goes off or you feel sick, leave the home right away and then call for help. 
  • Soot: If you notice black sooty collecting near the exterior of your furnace, it’s an indication something might be seriously wrong.

What to Do if Your Furnace Heat Exchanger is Cracked 

If you worry your furnace has a cracked heat exchanger, contact a pro experienced in furnace installation as soon as possible so they can inspect your system and, if necessary, perform a furnace heat exchanger replacement. Costs will vary depending on the situation, but estimates often hover around $1,000 to $3,000. 

Fortunately, the good news is that heat exchangers are generally included in the warranty. It's a good idea to check the warranty paperwork on your furnace, as while the warranty might not cover the entire cost of repairs, it still may significantly lower your bill.  

How to Prevent a Cracked Heat Exchanger in Your Home  

One of the most convenient ways to prevent a problem in your furnace overall is with regular furnace maintenance. Furnaces provide the best possible return on investment when they work efficiently. Hiring a certified professional to check your furnace for worn-out parts, clogs in the air filters and other common problems can help you avoid getting a big bill later on.  

It’s also helpful to review your furnace filters every few months – it’s encouraged some filters be replaced every 90 days or sooner if they are dirty or grimy. While the filters are not part of the heat exchanger itself, the strain of drawing air through a clogged filter makes the entire furnace work longer to do its job. And the harder your furnace works, the more strain pieces like the heat exchanger will experience.