A furnace is almost always a background player for your home, keeping you warm in the cold winter months. It often isn't noticed until something breaks down. 

One root cause could be that your furnace has a cracked heat exchanger. It can be a safety risk, so it’s worthwhile to know the signs of a cracked heat exchanger and what you can do if you believe that might 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 moves through the ventilation. It typically accomplishes this through coils or tubes that warm the air while acting as a barrier to keep the gasses produced in the combustion chamber, called flue gasses, from leaking out into your home.  

Is a Cracked Heat Exchanger Dangerous? 

Given its important role, it isn't surprising that a broken heat exchanger can be hazardous. A crack in the heat exchanger can permit dangerous gasses – such as carbon monoxide, which can be lethal – to circulate through your home. 

For this reason, never use your heater if you believe you're dealing with a cracked heat exchanger, as letting it run could make the entire household sick. Reach out to 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 switches off: Cracks in the heat exchanger could cause your furnace to switch off. 
  • Odd Smells: If the air leaving your furnace has a powerful chemical smell, it could be evidence gas is seeping through cracks in your heat exchanger. These gasses, which may smell like formaldehyde, are a common warning sign. 
  • Carbon monoxide alarm is triggered or you notice poisoning symptoms: If a cracked heat exchanger is releasing carbon monoxide into your home, your carbon monoxide alarm should go off or household members might start experiencing signs of carbon monoxide poisoning. Side effects include headaches, dizziness, weakness, nausea, vomiting or feeling tired. If the alarm goes off or you feel unwell, exit the home immediately and then call for help. 
  • Soot: If you spot black sooty buildup around the exterior of your furnace, it’s more evidence something could be seriously wrong.

What to Do if the Furnace Heat Exchanger is Cracked 

If you suspect your furnace has a cracked heat exchanger, hire a pro well versed in furnace installation right away so they can take a look at your system and, if required, perform a furnace heat exchanger replacement. Costs should 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 often protected by the warranty. You’ll want to review the warranty paperwork on your furnace, since while the warranty won't always cover the entire cost of repairs, it can 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 through routine furnace maintenance. Furnaces work the best when they operate efficiently. Contacting a certified professional to inspect your furnace for old parts, clogs in the air filters and other likely problems can keep you from getting a big bill later on.  

It’s also helpful to inspect your furnace filters every few months – it’s recommended some filters be swapped out 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 your entire furnace work harder to do its job. And the harder your furnace has to work, the more strain components like the heat exchanger will experience.