A furnace is almost always a background player in your home, helping keep you warm across the cold winter months. It often won't be noticed until something breaks down. 

One cause may be that your furnace has a cracked heat exchanger. It can be a safety risk, so it’s important to learn the signs of a cracked heat exchanger and what you should do if you are worried that might be the problem.  

What Is a Heat Exchanger in a Furnace? 

A heat exchanger transfers heat from the combustion chamber in your furnace to the air that circulates inside the ventilation. It typically accomplishes this via coils or tubes that warm 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 central role, it isn't surprising that a damaged heat exchanger can be hazardous. Cracks in the heat exchanger can allow dangerous gasses – like carbon monoxide, which can be lethal – to flow through your home. 

For obvious reasons, never use your heating if you believe you're dealing with a cracked heat exchanger, as this could make the whole family ill. Contact an HVAC professional as soon as possible if you believe your heater has a cracked heat exchanger that needs repair. 

Four Signs of a Cracked Heat Exchanger: 

  • Furnace switches off: A crack in your heat exchanger could cause your furnace to turn off. 
  • Strange Smells: If the air escaping your furnace has an intense chemical odor, it might be an indicator that gasses are leaking through cracks in your heat exchanger. These gasses, which will often smell like formaldehyde, are a common warning sign. 
  • Carbon monoxide alarm initiates or you recognize health problems: If a cracked heat exchanger is relieving carbon monoxide in your home, your carbon monoxide alarm should go off or family members may struggle with signs of carbon monoxide poisoning. Symptoms include headaches, dizziness, weakness, nausea, vomiting or feeling drowsy. If the alarm goes off or you feel unusually tired, exit the home right away and then call for help. 
  • Soot: If you find black sooty collecting on the exterior of your furnace, it’s an indication something may be seriously wrong.

What to Do if Your Furnace Heat Exchanger is Cracked 

If you suspect your furnace has a cracked heat exchanger, call a pro well versed in furnace installation right away so they can examine your system and, if necessary, perform a furnace heat exchanger replacement. Costs will differ 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 covered by the warranty. You should confirm the warranty paperwork on your furnace, since while the warranty won't always cover the entire cost of repairs, it still may significantly shrink your bill.  

How to Prevent 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. Calling a skilled professional to check your furnace for old parts, clogs in the air filters and other common problems can keep you from getting a big bill later on.  

It’s also a good idea to review your furnace filters every few months – it’s encouraged 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 longer to accomplish its job. And the harder your furnace needs to run, the more deterioration parts like the heat exchanger will sustain.