A furnace is almost always a background player at home, keeping you warm across the cold winter months. It regularly isn't noticed until something goes wrong. 

One source may be that your furnace has a cracked heat exchanger. It can be a safety risk, so it’s important to know the symptoms of a cracked heat exchanger and what to do if you believe that is 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 circulates throughout the air ducts. It typically does this using coils or tubes that heat the air while acting as a barrier to keep gas produced in the combustion chamber, called flue gasses, from escaping out into your home.  

Is a Cracked Heat Exchanger Dangerous? 

Thanks to its central role, it’s no surprise that a broken heat exchanger can be hazardous. Cracks in the heat exchanger can permit dangerous gasses – like carbon monoxide, which can be lethal – to flow through your home. 

For this reason, do NOT use your furnace if you believe you're dealing with a cracked heat exchanger, as doing so could make the entire household sick. Call an HVAC professional immediately if you are worried your heating has a cracked heat exchanger that needs repair. 

Four Signs of a Cracked Heat Exchanger: 

  • Furnace turns off: Cracks in the heat exchanger may cause your furnace to shut off. 
  • Unusual Smells: If the air escaping your furnace has a powerful chemical smell, it could be an indicator that gas is slipping 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 symptoms of poisoning: If a cracked heat exchanger is relieving carbon monoxide into your home, your carbon monoxide alarm could go off or household members could experience signs of carbon monoxide poisoning. Side effects include headaches, dizziness, weakness, nausea, vomiting or feeling drowsy. If the alarm goes off or you feel unwell, exit the home as soon as you can and then call for help. 
  • Soot: If you find black sooty accumulating around the exterior of your furnace, it’s another sign something may be seriously wrong.

What to Do if the Furnace Heat Exchanger is Cracked 

If you suspect your furnace has a cracked heat exchanger, hire a professional experienced in furnace installation right away so they can inspect your system and, if required, start a furnace heat exchanger replacement. Costs will fluctuate depending on the situation, but estimates often hover around $1,000 to $3,000. 

However, the good news is that heat exchangers are often covered by the warranty. It's a good idea to confirm the warranty paperwork on your furnace, since 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 easiest ways to avoid problems in your furnace overall is through regular furnace maintenance. Furnaces work the best when they run efficiently. Hiring a trained professional to check your furnace for broken-down parts, clogs in the air filters and other common problems can help you avoid getting a big bill later on.  

It’s also helpful 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 are not part of the heat exchanger itself, the strain of pulling air through a clogged filter makes the entire furnace work longer to accomplish its job. And the harder your furnace has to work, the more deterioration parts like the heat exchanger will experience.