A furnace is almost always a background player at home, ensuring you're warm across the cold winter months. It often won't be noticed until a malfunction appears. 

One source might be that your furnace has a cracked heat exchanger. It can potentially be hazardous, so it’s critical to familiarize yourself with the evidence of a cracked heat exchanger and what you can do if you are worried that might be 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 throughout the air ducts. It typically does this with coils or tubes that heat up the air while serving as a barrier to keep byproducts formed in the combustion chamber, called flue gasses, from leaking out into your home.  

Is a Cracked Heat Exchanger Dangerous? 

Because of its important role, it’s no surprise that a damaged heat exchanger can be hazardous. Cracks in the heat exchanger can permit dangerous gasses – including carbon monoxide, which can be lethal – to be distributed across your home. 

For obvious reasons, do NOT run your heating if you suspect you're dealing with a cracked heat exchanger, as this could make your entire household sick. Reach out to an HVAC professional immediately if you think your heating has a cracked heat exchanger that should be repaired. 

Four Symptoms of a Cracked Heat Exchanger: 

  • Furnace switches off: A cracked heat exchanger could cause your furnace to turn off. 
  • Unusual Smells: If the air leaving your furnace has a powerful chemical odor, it could be evidence gas is leaking through cracks in your heat exchanger. These byproducts, which may smell like formaldehyde, are a common warning sign. 
  • Carbon monoxide alarm goes off or you notice symptoms of poisoning: If a cracked heat exchanger is relieving carbon monoxide inside your home, your carbon monoxide alarm may go off or household members could struggle with signs of carbon monoxide poisoning. Side effects include headaches, dizziness, weakness, nausea, vomiting or feeling drowsy. If your alarm goes off or you feel sick, leave the home immediately and then call for help. 
  • Soot: If you find black sooty buildup around the exterior of your furnace, it’s an indication something may be seriously wrong.

What to Do if a Furnace Heat Exchanger is Cracked 

If you worry your furnace has a cracked heat exchanger, call a professional with extensive experience in furnace installation as soon as possible so they can take a look at your system and, if necessary, handle a furnace heat exchanger replacement. Costs often vary depending on the situation, but estimates often hover around $1,000 to $3,000. 

Estimates aside, the good news is that heat exchangers are generally covered by the warranty. You should review the warranty paperwork on your furnace, because while the warranty might not cover the entire cost of repairs, it can significantly lower your bill.  

How to Prevent a Cracked Heat Exchanger in Your Home  

One of the most convenient ways to avoid problems in your furnace overall is via consistent furnace maintenance. Furnaces provide the best possible return on investment when they run efficiently. Hiring a skilled professional to examine your furnace for old parts, dirty filters and other likely problems can keep you from getting a big bill later on.  

It’s also a good idea 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 dragging air through a clogged filter makes your entire furnace work more vigorously to complete its job. And the harder your furnace works, the more strain pieces like the heat exchanger will sustain.