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

One source might 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 to do if you are worried that might be the problem.  

What Is a Heat Exchanger in a Furnace? 

A heat exchanger helps transition heat from the combustion chamber of your furnace to the air that moves inside the ventilation. It usually does this using coils or tubes that heat the air while acting as a barrier to keep byproducts created in the combustion chamber, called flue gasses, from getting out into your home.  

Is a Cracked Heat Exchanger Dangerous? 

Given its important role, it shouldn't come as a surprise that a cracked heat exchanger can be hazardous. A damaged heat exchanger can enable dangerous gasses – like carbon monoxide, which can be lethal – to be distributed through your home. 

For this reason, don't ever turn on your furnace if you suspect it has a cracked heat exchanger, as letting it run could make the whole household sick. Call an HVAC professional right away if you think your heater has a cracked heat exchanger that needs repair. 

Four Signs of a Cracked Heat Exchanger: 

  • Furnace shuts off: A cracked heat exchanger can cause your furnace to turn off. 
  • Strange Smells: If the air leaving your furnace has a powerful chemical odor, it might be a sign gasses are seeping through cracks in your heat exchanger. These gasses, which will often smell like formaldehyde, are a major warning sign. 
  • Carbon monoxide alarm goes off or you recognize symptoms of poisoning: If a cracked heat exchanger is emitting carbon monoxide inside your home, your carbon monoxide alarm could go off or household members may start experiencing signs of carbon monoxide poisoning. Side effects include headaches, dizziness, weakness, nausea, vomiting or feeling tired. If your alarm goes off or you feel unwell, exit the home right away and then call for help. 
  • Soot: If you see black sooty accumulating around the exterior of your furnace, it’s another sign something could be seriously wrong.

What You Should Do if the Furnace Heat Exchanger is Cracked 

If you worry your furnace has a cracked heat exchanger, hire a professional experienced in furnace installation as soon as possible so they can examine your system and, if required, handle a furnace heat exchanger replacement. Costs will vary depending on the situation, but estimates can roughly suggest $1,000 to $3,000. 

However, the good news is that heat exchangers are often included in the warranty. You should confirm the warranty paperwork on your furnace, since while the warranty may not cover the entire cost of repairs, it still may 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 with regular furnace maintenance. Furnaces offer the most benefits when they work efficiently. Hiring a certified professional to inspect your furnace for broken-down parts, clogs in the air 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 ideal some filters be replaced 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 pulling air through a clogged filter makes your entire furnace work more vigorously to do its job. And the harder your furnace has to work, the more wear and tear components like the heat exchanger will endure.