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

One cause might be that your furnace has a cracked heat exchanger. It’s a potentially dangerous issue, so it’s important to familiarize yourself with the symptoms of a cracked heat exchanger and what to 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 of your furnace to the air that moves through the ventilation. It typically handles this with coils or tubes that warm the air while serving as a barrier to keep the gasses formed 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 isn't surprising that a cracked heat exchanger can be hazardous. Cracks in the heat exchanger can enable dangerous gasses – such as carbon monoxide, which can be lethal – to circulate throughout your home. 

For that reason, do NOT run your furnace if you suspect it has a cracked heat exchanger, as this could make the whole household ill. Call an HVAC professional as soon as possible if you believe your heating has a cracked heat exchanger that needs repair. 

Four Warning Signs of a Cracked Heat Exchanger: 

  • Furnace shuts off: Cracks in the heat exchanger can cause your furnace to turn off. 
  • Unusual Smells: If the air escaping your furnace has an intense chemical smell, it might be a sign gas is leaking through cracks in your heat exchanger. These byproducts, which may smell like formaldehyde, are a significant warning sign. 
  • Carbon monoxide alarm is triggered or you recognize poisoning symptoms: If a cracked heat exchanger is emitting carbon monoxide in your home, your carbon monoxide alarm may go off or family members may experience signs of carbon monoxide poisoning. Symptoms include headaches, dizziness, weakness, nausea, vomiting or feeling tired. If the alarm goes off or you feel unwell, get out of the home as soon as you can and then call for help. 
  • Soot: If you notice black sooty collecting around the exterior of your furnace, it’s more evidence something might be seriously wrong.

What You Should Do if a Furnace Heat Exchanger is Cracked 

If you believe your furnace has a cracked heat exchanger, call a pro experienced in furnace installation as soon as possible so they can inspect your system and, if needed, start a furnace heat exchanger replacement. Costs will vary depending on the situation, but estimates can roughly suggest $1,000 to $3,000. 

Estimates aside, the good news is that heat exchangers are regularly covered by the warranty. It's a good idea to check the warranty paperwork on your furnace, since while the warranty might not cover the entire cost of repairs, it still may significantly reduce your bill.  

How to Avoid a Cracked Heat Exchanger in Your Home  

One of the easiest ways to prevent a problem in your furnace overall is via regular furnace maintenance. Furnaces work the best when they operate efficiently. Calling a certified professional to inspect your furnace for broken-down parts, clogged filters and other common problems can help you avoid getting a big bill later on.  

It’s also helpful to take a look at your furnace filters every few months – it’s recommended 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 drawing air through a clogged filter makes your entire furnace work more vigorously to complete its job. And the harder your furnace has to work, the more wear and tear pieces like the heat exchanger will endure.