A furnace is often a background player for your home, helping keep you warm in the cold winter months. It frequently won't be noticed until something goes wrong. 

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

What Is a Heat Exchanger in a Furnace? 

A heat exchanger helps move heat from the combustion chamber inside your furnace to the air that moves throughout the air ducts. It typically handles this using coils or tubes that heat up 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? 

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

For this reason, don't ever use your heating if you suspect you're dealing with a cracked heat exchanger, as letting it run could make the entire family ill. Call an HVAC professional as soon as possible if you are worried your furnace has a cracked heat exchanger that should be repaired. 

Four Signs of a Cracked Heat Exchanger: 

  • Furnace switches off: Cracks in the heat exchanger may cause your furnace to shut off. 
  • Strange Smells: If the air leaving your furnace has an intense chemical scent, it might be evidence gas is slipping through cracks in your heat exchanger. These gasses, which may smell like formaldehyde, are a common warning sign. 
  • Carbon monoxide alarm goes off or you notice health problems: If a cracked heat exchanger is releasing carbon monoxide in your home, your carbon monoxide alarm could go off or household members may start experiencing signs of carbon monoxide poisoning. Side effects include headaches, dizziness, weakness, nausea, vomiting or feeling drowsy. If your alarm goes off or you feel unusually tired, leave the home right away and then call for help. 
  • Soot: If you find black sooty buildup on the exterior of your furnace, it’s more evidence something might be seriously wrong.

What to Do if Your Furnace Heat Exchanger is Cracked 

If you believe your furnace has a cracked heat exchanger, hire a professional well versed in furnace installation as soon as possible so they can inspect your system and, if necessary, start 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 generally covered by the warranty. It's a good idea to confirm the warranty paperwork on your furnace, because while the warranty won't always 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 easiest ways to prevent a problem in your furnace overall is through regular furnace maintenance. Furnaces work the best when they operate efficiently. Hiring a trained professional to check your furnace for worn-out parts, clogs in the air filters and other likely problems can help you avoid getting a big bill later on.  

It’s also beneficial to inspect your furnace filters every few months – it’s ideal some filters be changed 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 dragging air through a clogged filter makes your entire furnace work more vigorously to do its job. And the harder your furnace works, the more deterioration components like the heat exchanger will experience.