A furnace is almost always a background player in your home, ensuring you're warm in the cold winter months. It frequently isn't noticed until something goes wrong. 

One cause might be that your furnace has a cracked heat exchanger. It can be a safety risk, so it’s worthwhile to familiarize yourself with the signs of a cracked heat exchanger and what you should do if you suspect that may be the problem.  

What Is a Heat Exchanger in a Furnace? 

A heat exchanger helps transition heat from the combustion chamber inside your furnace to the air that moves through the air ducts. It generally handles this using coils or tubes that heat the air while functioning as a barrier to keep the gasses created in the combustion chamber, called flue gasses, from escaping out into your home.  

Is a Cracked Heat Exchanger Dangerous? 

Given its important role, it isn't surprising that a broken heat exchanger can pose a risk. Cracks in the heat exchanger can allow dangerous gasses – like carbon monoxide, which can be lethal – to flow throughout your home. 

For obvious reasons, never turn on your furnace if you think you're dealing with a cracked heat exchanger, as letting it run could make the whole household sick. Call an HVAC professional immediately if you believe your furnace has a cracked heat exchanger that needs repair. 

Four Signs of a Cracked Heat Exchanger: 

  • Furnace switches off: A crack in your heat exchanger can cause your furnace to shut off. 
  • Strange Smells: If the air coming out of your furnace has a strong chemical odor, it could be a sign gas is slipping through cracks in your heat exchanger. These byproducts, which will often smell like formaldehyde, are a significant warning sign. 
  • Carbon monoxide alarm initiates or you notice symptoms of poisoning: If a cracked heat exchanger is emitting carbon monoxide into your home, your carbon monoxide alarm should go off or family members may start experiencing signs of carbon monoxide poisoning. Complications include headaches, dizziness, weakness, nausea, vomiting or feeling sleepy. If the alarm goes off or you feel unwell, leave the home as soon as you can and then call for help. 
  • Soot: If you notice black sooty collecting around the exterior of your furnace, it’s an indication something could 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 pro experienced in furnace installation right away so they can take a look at your system and, if needed, perform a furnace heat exchanger replacement. Costs will fluctuate depending on the situation, but estimates often hover around $1,000 to $3,000. 

Estimates aside, the good news is that heat exchangers are regularly included in the warranty. You should check the warranty paperwork on your furnace, because while the warranty may not cover the entire cost of repairs, it could significantly reduce your bill.  

How to Prevent a Cracked Heat Exchanger in Your Home  

One of the best ways to minimize the risk of problems in your furnace overall is with consistent furnace maintenance. Furnaces offer the most benefits when they work efficiently. Calling a trained professional to examine your furnace for broken-down parts, clogged filters and other common 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 encouraged 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 dragging air through a clogged filter makes your entire furnace work harder to complete its job. And the harder your furnace works, the more wear and tear parts like the heat exchanger will experience.