A furnace is often a background player in your home, keeping you warm in the cold winter months. It regularly isn't noticed until a malfunction appears. 

One root cause could be that your furnace has a cracked heat exchanger. It’s a potentially dangerous issue, so it’s critical to familiarize yourself with the signs of a cracked heat exchanger and what you can do if you are worried that might be the problem.  

What Is a Heat Exchanger in a Furnace? 

A heat exchanger helps transition heat from the combustion chamber of your furnace to the air that circulates inside the air ducts. It usually accomplishes this via coils or tubes that warm the air while serving 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? 

Because of its central role, it’s no surprise that a damaged heat exchanger can pose a risk. A crack in the heat exchanger can allow dangerous gasses – including carbon monoxide, which can be lethal – to circulate through your home. 

For this reason, don't ever use your heater if you think it has a cracked heat exchanger, as this could make the whole household sick. Call an HVAC professional immediately if you think your heater has a cracked heat exchanger that should be repaired. 

Four Signs of a Cracked Heat Exchanger: 

• Furnace shuts off: A crack in your heat exchanger could cause your furnace to shut off. 
• Unusual Smells: If the air escaping your furnace has a powerful chemical smell, it may be an indicator that gasses are leaking through cracks in your heat exchanger. These byproducts, which may smell like formaldehyde, are a common warning sign. 
• Carbon monoxide alarm goes off or you notice symptoms of poisoning: If a cracked heat exchanger is emitting carbon monoxide in your home, your carbon monoxide alarm may go off or household members may experience signs of carbon monoxide poisoning. Symptoms include headaches, dizziness, weakness, nausea, vomiting or feeling tired. If the alarm goes off or you feel unusually tired, leave the home immediately and then call for help. 
• Soot: If you find black sooty accumulating near the exterior of your furnace, it’s another sign something might be seriously wrong.

What You Should Do if the Furnace Heat Exchanger is Cracked 

If you believe your furnace has a cracked heat exchanger, contact a pro well versed in furnace installation as soon as possible so they can take a look at your system and, if needed, handle a furnace heat exchanger replacement. Costs often fluctuate depending on the situation, but estimates run in the neighborhood of $1,000 to $3,000. 

However, the good news is that heat exchangers are often protected by the warranty. You should review the warranty paperwork on your furnace, because while the warranty won't always cover the entire cost of repairs, it could significantly reduce your bill.  

How to Avoid a Cracked Heat Exchanger in Your Home  

One of the most convenient ways to prevent a problem in your furnace overall is with routine furnace maintenance. Furnaces offer the most benefits when they work efficiently. Calling a trained professional to inspect your furnace for worn-out parts, dirty filters and other common problems can help you avoid getting a big bill later on.  

It’s also a good idea 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 the entire furnace work more vigorously to accomplish its job. And the harder your furnace needs to run, the more strain components like the heat exchanger will sustain.