A furnace is almost always a background player in your home, ensuring you're warm during the cold winter months. It frequently won't be noticed until a malfunction appears. 

One cause might be that your furnace has a cracked heat exchanger. It’s a potentially dangerous issue, so it’s worthwhile to know the symptoms 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 move heat from the combustion chamber of your furnace to the air that moves inside the ventilation. It typically does this via coils or tubes that heat up the air while functioning as a barrier to keep gas produced in the combustion chamber, called flue gasses, from getting out into your home.  

Is a Cracked Heat Exchanger Dangerous? 

Because of its important role, it’s no surprise that a broken heat exchanger can be hazardous. A crack in the heat exchanger can permit dangerous gasses – like carbon monoxide, which can be lethal – to circulate throughout your home. 

For that reason, never turn on your heating if you suspect there's a crack in the heat exchanger, as letting it run could make the entire family sick. Reach out to an HVAC professional right away if you are worried your heating has a cracked heat exchanger that needs to be repaired. 

Four Warning Signs of a Cracked Heat Exchanger: 

  • Furnace turns off: Cracks in the heat exchanger could cause your furnace to switch off. 
  • Unusual Smells: If the air escaping your furnace has a powerful chemical scent, it may be a sign gasses are slipping through cracks in your heat exchanger. These gasses, which can smell like formaldehyde, are a common warning sign. 
  • Carbon monoxide alarm initiates or you recognize health problems: If a cracked heat exchanger is releasing carbon monoxide into your home, your carbon monoxide alarm may go off or household members could struggle with signs of carbon monoxide poisoning. Side effects include headaches, dizziness, weakness, nausea, vomiting or feeling drowsy. If an alarm goes off or you feel unwell, get out of the home right away and then call for help. 
  • Soot: If you notice black sooty buildup around the exterior of your furnace, it’s more evidence something might be seriously wrong.

What You Can Do if Your Furnace Heat Exchanger is Cracked 

If you suspect your furnace has a cracked heat exchanger, contact a professional well versed in furnace installation right away so they can inspect your system and, if required, perform 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 generally protected by the warranty. You’ll want to review the warranty paperwork on your furnace, because while the warranty might 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 most convenient ways to prevent a problem in your furnace overall is via routine furnace maintenance. Furnaces work the best when they operate efficiently. Contacting a skilled professional to inspect your furnace for old parts, dirty filters and other likely problems can help you avoid 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 the entire furnace work harder to accomplish its job. And the harder your furnace needs to run, the more deterioration parts like the heat exchanger will sustain.