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

One root cause could be that your furnace has a cracked heat exchanger. It’s a potentially dangerous issue, so it’s critical to learn the evidence of a cracked heat exchanger and what to do if you suspect that is the problem.  

What Is a Heat Exchanger in a Furnace? 

A heat exchanger transfers heat from the combustion chamber inside your furnace to the air that flows inside the system. It generally does this using coils or tubes that heat up the air while functioning as a barrier to keep the gasses created in the combustion chamber, called flue gasses, from getting out into your home.  

Is a Cracked Heat Exchanger Dangerous? 

Thanks to its key role, it isn't surprising that a broken heat exchanger can be hazardous. A crack in the heat exchanger can allow dangerous gasses – such as carbon monoxide, which can be lethal – to be distributed throughout your home. 

For that reason, never run your heating if you suspect there's a crack in the heat exchanger, as doing so could make your entire household ill. Reach out to an HVAC professional immediately if you think your heating has a cracked heat exchanger that should be repaired. 

Four Signs of a Cracked Heat Exchanger: 

  • Furnace switches off: A crack in your heat exchanger may cause your furnace to shut off. 
  • Strange Smells: If the air leaving your furnace has a strong chemical smell, it may be a sign gasses are slipping through cracks in your heat exchanger. These byproducts, which will often smell like formaldehyde, are a common warning sign. 
  • Carbon monoxide alarm goes off or you notice health problems: If a cracked heat exchanger is releasing carbon monoxide inside your home, your carbon monoxide alarm should go off or family members may struggle with signs of carbon monoxide poisoning. Symptoms include headaches, dizziness, weakness, nausea, vomiting or feeling sleepy. If the alarm goes off or you feel unwell, exit the home as soon as you can and then call for help. 
  • Soot: If you spot black sooty buildup on the exterior of your furnace, it’s more evidence something may be seriously wrong.

What You Can Do if the Furnace Heat Exchanger is Cracked 

If you believe your furnace has a cracked heat exchanger, hire a professional well versed in furnace installation right away so they can inspect your system and, if needed, perform a furnace heat exchanger replacement. Costs often fluctuate depending on the situation, but estimates often hover around $1,000 to $3,000. 

Fortunately, the good news is that heat exchangers are generally covered by the warranty. It's a good idea to 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 Avoid a Cracked Heat Exchanger in Your Home  

One of the most convenient ways to minimize the risk of problems in your furnace overall is with routine furnace maintenance. Furnaces offer the most benefits when they operate efficiently. Contacting a trained professional to inspect your furnace for worn-out parts, clogged filters and other potential problems can keep you from getting a big bill later on.  

It’s also a good idea to take a look at 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 are not part of the heat exchanger itself, the strain of drawing air through a clogged filter makes your entire furnace work harder to accomplish its job. And the harder your furnace has to work, the more wear and tear pieces like the heat exchanger will endure.