A furnace is usually a background player in your home, helping keep you warm in the cold winter months. It regularly isn't noticed until something breaks down. 

One cause could be that your furnace has a cracked heat exchanger. It can potentially be hazardous, so it’s worthwhile to know the symptoms of a cracked heat exchanger and what you can do if you suspect that might be the problem.  

What Is a Heat Exchanger in a Furnace? 

A heat exchanger helps transition heat from the combustion chamber in your furnace to the air that circulates through the system. It usually handles this via coils or tubes that warm the air while functioning as a barrier to keep byproducts formed in the combustion chamber, called flue gasses, from leaking out into your home.  

Is a Cracked Heat Exchanger Dangerous? 

Given its important role, it isn't surprising that a damaged heat exchanger can be hazardous. A crack in the heat exchanger can permit dangerous gasses – such as carbon monoxide, which can be lethal – to flow throughout your home. 

For this reason, do NOT run your furnace if you think you're dealing with a cracked heat exchanger, as letting it run could make your entire family ill. Reach out to an HVAC professional right away if you think your furnace has a cracked heat exchanger that needs repair. 

Four Warning Signs of a Cracked Heat Exchanger: 

  • Furnace switches off: A crack in your heat exchanger can cause your furnace to shut off. 
  • Unusual Smells: If the air leaving your furnace has an intense chemical odor, it may be an indicator that gasses are slipping through cracks in your heat exchanger. These byproducts, which may smell like formaldehyde, are a common warning sign. 
  • Carbon monoxide alarm is triggered or you recognize poisoning symptoms: If a cracked heat exchanger is releasing carbon monoxide inside your home, your carbon monoxide alarm should go off or family members might experience signs of carbon monoxide poisoning. Side effects include headaches, dizziness, weakness, nausea, vomiting or feeling tired. If an alarm goes off or you feel sick, get out of the home right away and then call for help. 
  • Soot: If you see black sooty accumulating around the exterior of your furnace, it’s more evidence something may be seriously wrong.

What You Should Do if a Furnace Heat Exchanger is Cracked 

If you suspect your furnace has a cracked heat exchanger, call a professional experienced in furnace installation right away so they can take a look at your system and, if necessary, perform a furnace heat exchanger replacement. Costs often differ depending on the situation, but estimates often hover around $1,000 to $3,000. 

Fortunately, 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, because while the warranty might not cover the entire cost of repairs, it can 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 routine furnace maintenance. Furnaces offer the most benefits when they run efficiently. Calling a trained professional to inspect your furnace for old parts, clogged filters and other common problems can help you avoid getting a big bill later on.  

It’s also helpful to take a look at your furnace filters every few months – it’s recommended some filters be changed 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 drawing air through a clogged filter makes the entire furnace work longer to complete its job. And the harder your furnace works, the more strain parts like the heat exchanger will sustain.