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

One source may be that your furnace has a cracked heat exchanger. It’s a potentially dangerous issue, so it’s worthwhile to learn the symptoms of a cracked heat exchanger and what to do if you are worried that is 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 throughout the system. It typically handles this via coils or tubes that heat the air while functioning as a barrier to keep byproducts produced in the combustion chamber, called flue gasses, from leaking out into your home.  

Is a Cracked Heat Exchanger Dangerous? 

Thanks to its key role, it shouldn't come as a surprise that a cracked heat exchanger can be very dangerous. Cracks in the heat exchanger can allow dangerous gasses – including carbon monoxide, which can be lethal – to circulate throughout your home. 

For that reason, do NOT run your furnace if you suspect there's a crack in the heat exchanger, as doing so could make your entire household ill. Contact an HVAC professional as soon as possible if you think your furnace has a cracked heat exchanger that should be repaired. 

Four Warning Signs of a Cracked Heat Exchanger: 

  • Furnace turns off: Cracks in the heat exchanger may cause your furnace to switch off. 
  • Odd Smells: If the air coming out of your furnace has a powerful chemical smell, it may be an indicator that gasses are leaking 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 poisoning symptoms: If a cracked heat exchanger is emitting carbon monoxide in 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 your alarm goes off or you feel unusually tired, get out of the home right away and then call for help. 
  • Soot: If you spot black sooty collecting near the exterior of your furnace, it’s another sign something may be seriously wrong.

What You Should Do if the Furnace Heat Exchanger is Cracked 

If you suspect your furnace has a cracked heat exchanger, contact a professional well versed in furnace installation right away so they can inspect your system and, if needed, start a furnace heat exchanger replacement. Costs often differ 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 protected by the warranty. You should review the warranty paperwork on your furnace, as while the warranty won't always cover the entire cost of repairs, it still may significantly shrink your bill.  

How to Prevent a Cracked Heat Exchanger in Your Home  

One of the best ways to avoid problems in your furnace overall is via routine furnace maintenance. Furnaces provide the best possible return on investment when they run efficiently. Contacting a trained professional to check your furnace for worn-out parts, dirty 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 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 your entire furnace work harder to do its job. And the harder your furnace needs to run, the more wear and tear pieces like the heat exchanger will endure.