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

One root cause might be that your furnace has a cracked heat exchanger. It can potentially be hazardous, so it’s worthwhile to learn the symptoms of a cracked heat exchanger and what you can do if you believe that may 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 system. It typically accomplishes this using coils or tubes that warm the air while serving as a barrier to keep gas 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’s no surprise that a broken heat exchanger can be very dangerous. A damaged heat exchanger can enable dangerous gasses – including carbon monoxide, which can be lethal – to flow through your home. 

For that reason, never use your heater if you suspect you're dealing with a cracked heat exchanger, as this could make your entire household ill. Call an HVAC professional right away if you are worried your heating has a cracked heat exchanger that needs repair. 

Four Signs of a Cracked Heat Exchanger: 

  • Furnace switches off: Cracks in the heat exchanger may cause your furnace to switch off. 
  • Unusual Smells: If the air leaving your furnace has an intense chemical smell, it could be an indicator that gas is seeping through cracks in your heat exchanger. These byproducts, which can smell like formaldehyde, are a major warning sign. 
  • Carbon monoxide alarm goes off or you notice symptoms of poisoning: If a cracked heat exchanger is relieving carbon monoxide into your home, your carbon monoxide alarm may go off or family members might experience signs of carbon monoxide poisoning. Symptoms include headaches, dizziness, weakness, nausea, vomiting or feeling drowsy. If an alarm goes off or you feel unwell, leave the home as soon as you can and then call for help. 
  • Soot: If you find black sooty collecting around the exterior of your furnace, it’s an indication something might be seriously wrong.

What to Do if the Furnace Heat Exchanger is Cracked 

If you worry your furnace has a cracked heat exchanger, call a professional well versed in furnace installation right away so they can examine your system and, if needed, start a furnace heat exchanger replacement. Costs should 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 included in the warranty. It's a good idea to review the warranty paperwork on your furnace, since 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 best ways to prevent a problem in your furnace overall is with routine furnace maintenance. Furnaces provide the best possible return on investment when they operate efficiently. Contacting a trained professional to examine your furnace for broken-down parts, dirty filters and other common 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 aren't connected to the heat exchanger itself, the strain of pulling air through a clogged filter makes the entire furnace work harder to accomplish its job. And the harder your furnace has to work, the more wear and tear components like the heat exchanger will endure.