A furnace is almost always a background player for your home, ensuring you're warm across the cold winter months. It often won't be noticed until something breaks down. 

One source might 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 might be the problem.  

What Is a Heat Exchanger in a Furnace? 

A heat exchanger transfers heat from the combustion chamber in your furnace to the air that circulates through the air ducts. It typically handles this with coils or tubes that heat up the air while functioning as a barrier to keep the gasses produced in the combustion chamber, called flue gasses, from getting out into your home.  

Is a Cracked Heat Exchanger Dangerous? 

Thanks to its central role, it isn't surprising that a broken heat exchanger can be hazardous. A damaged heat exchanger can enable dangerous gasses – including carbon monoxide, which can be lethal – to circulate through your home. 

For obvious reasons, don't ever turn on your heating if you believe it has a cracked heat exchanger, as doing so could make the entire household sick. Contact an HVAC professional immediately if you believe your furnace has a cracked heat exchanger that needs repair. 

Four Warning Signs of a Cracked Heat Exchanger: 

  • Furnace turns off: Cracks in the heat exchanger may cause your furnace to shut off. 
  • Unusual Smells: If the air leaving your furnace has a powerful chemical odor, it might be an indicator that gas is leaking through cracks in your heat exchanger. These gasses, which may smell like formaldehyde, are a significant warning sign. 
  • Carbon monoxide alarm goes off or you recognize health problems: If a cracked heat exchanger is relieving carbon monoxide inside your home, your carbon monoxide alarm should go off or family members may experience signs of carbon monoxide poisoning. Side effects include headaches, dizziness, weakness, nausea, vomiting or feeling drowsy. If your alarm goes off or you feel unwell, exit the home as soon as you can and then call for help. 
  • Soot: If you spot black sooty collecting around the exterior of your furnace, it’s another sign something could be seriously wrong.

What You Can Do if the Furnace Heat Exchanger is Cracked 

If you believe your furnace has a cracked heat exchanger, contact a pro experienced in furnace installation as soon as possible so they can examine your system and, if necessary, handle a furnace heat exchanger replacement. Costs will differ depending on the situation, but estimates run in the neighborhood of $1,000 to $3,000. 

However, the good news is that heat exchangers are often included in the warranty. You’ll want to review the warranty paperwork on your furnace, as while the warranty might not cover the entire cost of repairs, it could significantly lower your bill.  

How to Avoid a Cracked Heat Exchanger in Your Home  

One of the best ways to prevent a problem in your furnace overall is via routine furnace maintenance. Furnaces offer the most benefits when they operate efficiently. Hiring a certified professional to check your furnace for broken-down parts, clogs in the air filters and other likely problems can help you avoid getting a big bill later on.  

It’s also helpful to inspect 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 aren't a part of the heat exchanger itself, the strain of drawing air through a clogged filter makes the entire furnace work harder to accomplish its job. And the harder your furnace works, the more strain parts like the heat exchanger will experience.