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

One root cause may be that your furnace has a cracked heat exchanger. It can potentially be hazardous, so it’s worthwhile to learn the symptoms of a cracked heat exchanger and what you should do if you are worried that is the problem.  

What Is a Heat Exchanger in a Furnace? 

A heat exchanger helps move heat from the combustion chamber inside your furnace to the air that moves inside the air ducts. It usually does this via coils or tubes that warm the air while serving as a barrier to keep byproducts created in the combustion chamber, called flue gasses, from leaking out into your home.  

Is a Cracked Heat Exchanger Dangerous? 

Because of its key role, it shouldn't come as a surprise that a cracked heat exchanger can pose a risk. 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 turn on your furnace if you believe there's a crack in the heat exchanger, as doing so could make the entire household ill. Reach out to 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 turns off: Cracks in the heat exchanger can cause your furnace to switch off. 
  • Unusual Smells: If the air coming out of your furnace has an intense chemical odor, it might be an indicator that gas is seeping through cracks in your heat exchanger. These byproducts, which can smell like formaldehyde, are a significant warning sign. 
  • Carbon monoxide alarm initiates or you feel health problems: If a cracked heat exchanger is releasing carbon monoxide in your home, your carbon monoxide alarm should go off or household members may struggle with signs of carbon monoxide poisoning. Complications include headaches, dizziness, weakness, nausea, vomiting or feeling sleepy. If an alarm goes off or you feel unusually tired, get out of the home right away and then call for help. 
  • Soot: If you see black sooty collecting around the exterior of your furnace, it’s an indication something may be seriously wrong.

What to Do if the Furnace Heat Exchanger is Cracked 

If you suspect your furnace has a cracked heat exchanger, call a professional experienced in furnace installation right away so they can take a look at your system and, if necessary, perform a furnace heat exchanger replacement. Costs will vary depending on the situation, but estimates run in the neighborhood of $1,000 to $3,000. 

Estimates aside, the good news is that heat exchangers are regularly 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 shrink your bill.  

How to Avoid a Cracked Heat Exchanger in Your Home  

One of the best ways to prevent a problem in your furnace overall is with consistent furnace maintenance. Furnaces provide the best possible return on investment when they work efficiently. Hiring a skilled professional to examine your furnace for old parts, clogs in the air filters and other potential problems can keep you from getting a big bill later on.  

It’s also helpful to review your furnace filters every few months – it’s ideal some filters be swapped out 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 more vigorously to complete its job. And the harder your furnace needs to run, the more wear and tear parts like the heat exchanger will experience.