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

One source might be that your furnace has a cracked heat exchanger. It’s a potentially dangerous issue, so it’s important to learn the evidence of a cracked heat exchanger and what you should do if you are worried that may be the problem.  

What Is a Heat Exchanger in a Furnace? 

A heat exchanger helps transition heat from the combustion chamber in your furnace to the air that moves throughout the system. It usually does this via coils or tubes that heat up the air while acting as a barrier to keep the gasses produced in the combustion chamber, called flue gasses, from getting out into your home.  

Is a Cracked Heat Exchanger Dangerous? 

Given its important role, it’s no surprise that a cracked heat exchanger can be hazardous. Cracks in the heat exchanger can enable dangerous gasses – including carbon monoxide, which can be lethal – to flow across your home. 

For that reason, never run your heating if you think you're dealing with a cracked heat exchanger, as doing so could make the entire household ill. Contact an HVAC professional immediately if you believe your heating has a cracked heat exchanger that needs to be repaired. 

Four Signs of a Cracked Heat Exchanger: 

• Furnace shuts off: A crack in your heat exchanger can cause your furnace to turn off. 
• Odd Smells: If the air coming out of your furnace has a powerful chemical odor, it could be an indicator that gas is slipping 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 poisoning symptoms: If a cracked heat exchanger is emitting carbon monoxide inside your home, your carbon monoxide alarm may go off or household members could struggle with 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 immediately and then call for help. 
• Soot: If you spot black sooty collecting around the exterior of your furnace, it’s another sign something could be seriously wrong.

What to Do if the Furnace Heat Exchanger is Cracked 

If you worry your furnace has a cracked heat exchanger, contact a pro 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 will differ depending on the situation, but estimates can roughly suggest $1,000 to $3,000. 

Fortunately, the good news is that heat exchangers are generally protected by the warranty. You should check the warranty paperwork on your furnace, because while the warranty may not cover the entire cost of repairs, it can significantly reduce your bill.  

How to Prevent a Cracked Heat Exchanger in Your Home  

One of the most convenient ways to prevent a problem in your furnace overall is through consistent furnace maintenance. Furnaces provide the best possible return on investment when they run efficiently. Calling a certified professional to inspect your furnace for old parts, clogs in the air filters and other potential problems can help you avoid getting a big bill later on.  

It’s also a good idea to inspect your furnace filters every few months – it’s encouraged 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 drawing air through a clogged filter makes your entire furnace work harder to complete its job. And the harder your furnace has to work, the more deterioration parts like the heat exchanger will experience.