A furnace is usually a background player at home, ensuring you're warm in the cold winter months. It frequently isn't noticed until something breaks down. 

One cause could be that your furnace has a cracked heat exchanger. It’s a potentially dangerous issue, so it’s important to learn the signs of a cracked heat exchanger and what to do if you believe 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 flows inside the air ducts. It typically handles this through coils or tubes that heat up the air while acting as a barrier to keep the gasses formed in the combustion chamber, called flue gasses, from escaping out into your home.  

Is a Cracked Heat Exchanger Dangerous? 

Because of its central role, it isn't surprising that a damaged heat exchanger can pose a risk. A crack in the heat exchanger can enable dangerous gasses – including carbon monoxide, which can be lethal – to circulate across your home. 

For obvious reasons, do NOT use your heating if you think you're dealing with a cracked heat exchanger, as letting it run could make your entire household sick. Call an HVAC professional immediately if you think your heater has a cracked heat exchanger that should be repaired. 

Four Symptoms of a Cracked Heat Exchanger: 

  • Furnace switches off: Cracks in the heat exchanger may cause your furnace to shut off. 
  • Odd Smells: If the air escaping your furnace has a powerful chemical odor, it may be an indicator that gasses are seeping through cracks in your heat exchanger. These byproducts, which will often smell like formaldehyde, are a major warning sign. 
  • Carbon monoxide alarm initiates or you notice symptoms of poisoning: If a cracked heat exchanger is emitting carbon monoxide in your home, your carbon monoxide alarm should go off or household members may start experiencing signs of carbon monoxide poisoning. Complications include headaches, dizziness, weakness, nausea, vomiting or feeling drowsy. If an alarm goes off or you feel unwell, get out of the home immediately and then call for help. 
  • Soot: If you spot black sooty accumulating on the exterior of your furnace, it’s an indication something could be seriously wrong.

What You Should Do if Your Furnace Heat Exchanger is Cracked 

If you worry your furnace has a cracked heat exchanger, contact a professional well versed in furnace installation as soon as possible so they can examine your system and, if needed, perform a furnace heat exchanger replacement. Costs will vary depending on the situation, but estimates can roughly suggest $1,000 to $3,000. 

Fortunately, the good news is that heat exchangers are generally covered by the warranty. You should confirm the warranty paperwork on your furnace, because while the warranty may not cover the entire cost of repairs, it could significantly shrink 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 offer the most benefits when they work efficiently. Hiring a certified professional to check your furnace for worn-out parts, dirty filters and other potential problems can help you avoid getting a big bill later on.  

It’s also beneficial to review 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 aren't a part of the heat exchanger itself, the strain of dragging 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 endure.