A furnace is often a background player in your home, keeping you warm across the cold winter months. It often doesn’t get noticed until something goes wrong. 

One root cause may be that your furnace has a cracked heat exchanger. It’s a potentially dangerous issue, so it’s worthwhile to familiarize yourself with the symptoms of a cracked heat exchanger and what you can do if you believe that is 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 moves through the ventilation. It typically accomplishes this via coils or tubes that warm the air while serving as a barrier to keep gas 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’s no surprise that a broken heat exchanger can be very dangerous. A damaged heat exchanger can permit dangerous gasses – such as carbon monoxide, which can be lethal – to circulate throughout your home. 

For that reason, never turn on your heating if you think it has a cracked heat exchanger, as doing so could make your entire household sick. Contact an HVAC professional immediately if you believe your furnace has a cracked heat exchanger that needs to be repaired. 

Four Warning Signs of a Cracked Heat Exchanger: 

  • Furnace turns off: A crack in your heat exchanger can cause your furnace to turn off. 
  • Unusual Smells: If the air escaping your furnace has a strong chemical odor, it could be an indicator that gasses are leaking through cracks in your heat exchanger. These byproducts, which may smell like formaldehyde, are a major warning sign. 
  • Carbon monoxide alarm initiates or you feel health problems: If a cracked heat exchanger is releasing carbon monoxide inside your home, your carbon monoxide alarm could go off or household members might struggle with signs of carbon monoxide poisoning. Complications include headaches, dizziness, weakness, nausea, vomiting or feeling sleepy. If your alarm goes off or you feel unusually tired, exit the home right away and then call for help. 
  • Soot: If you notice black sooty buildup on the exterior of your furnace, it’s more evidence something might be seriously wrong.

What to Do if Your Furnace Heat Exchanger is Cracked 

If you worry your furnace has a cracked heat exchanger, contact a pro experienced in furnace installation as soon as possible so they can take a look at your system and, if required, start a furnace heat exchanger replacement. Costs should vary depending on the situation, but estimates can roughly suggest $1,000 to $3,000. 

However, the good news is that heat exchangers are generally included in the warranty. It's a good idea to confirm the warranty paperwork on your furnace, as while the warranty may not cover the entire cost of repairs, it can significantly lower your bill.  

How to Avoid a Cracked Heat Exchanger in Your Home  

One of the easiest ways to minimize the risk of problems in your furnace overall is through routine furnace maintenance. Furnaces work the best when they run efficiently. Hiring a skilled professional to examine your furnace for broken-down 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 encouraged some filters be replaced 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 dragging air through a clogged filter makes the entire furnace work longer to do its job. And the harder your furnace works, the more strain pieces like the heat exchanger will endure.