A furnace is almost always a background player for your home, keeping you warm during the cold winter months. It frequently won't be noticed until something breaks down. 

One cause may be that your furnace has a cracked heat exchanger. It can potentially be hazardous, so it’s worthwhile to learn the symptoms of a cracked heat exchanger and what you should do if you are worried that may be the problem.  

What Is a Heat Exchanger in a Furnace? 

A heat exchanger transfers heat from the combustion chamber of your furnace to the air that moves throughout the system. It generally does this with coils or tubes that heat the air while acting as a barrier to keep the gasses produced in the combustion chamber, called flue gasses, from getting out into your home.  

Is a Cracked Heat Exchanger Dangerous? 

Given its key role, it’s no surprise that a cracked heat exchanger can be hazardous. A damaged heat exchanger can enable dangerous gasses – such as carbon monoxide, which can be lethal – to circulate throughout your home. 

For obvious reasons, never use your heating if you suspect there's a crack in the heat exchanger, as this could make the entire family sick. Call an HVAC professional immediately if you believe your heater has a cracked heat exchanger that needs repair. 

Four Symptoms of a Cracked Heat Exchanger: 

  • Furnace shuts off: Cracks in the heat exchanger can cause your furnace to turn off. 
  • Strange Smells: If the air leaving your furnace has an intense chemical smell, it may be a sign gas is seeping through cracks in your heat exchanger. These gasses, which will often smell like formaldehyde, are a significant warning sign. 
  • Carbon monoxide alarm is triggered or you feel symptoms of poisoning: If a cracked heat exchanger is emitting carbon monoxide in your home, your carbon monoxide alarm could go off or household members could struggle with signs of carbon monoxide poisoning. Side effects include headaches, dizziness, weakness, nausea, vomiting or feeling tired. If an alarm goes off or you feel unwell, exit the home immediately 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 Your Furnace Heat Exchanger is Cracked 

If you believe your furnace has a cracked heat exchanger, contact a pro experienced in furnace installation as soon as possible so they can take a look at your system and, if necessary, start a furnace heat exchanger replacement. Costs often fluctuate depending on the situation, but estimates can roughly suggest $1,000 to $3,000. 

Estimates aside, the good news is that heat exchangers are regularly covered by the warranty. You should check the warranty paperwork on your furnace, because while the warranty may not cover the entire cost of repairs, it still may significantly reduce your bill.  

How to Prevent a Cracked Heat Exchanger in Your Home  

One of the best ways to prevent a problem in your furnace overall is through regular furnace maintenance. Furnaces provide the best possible return on investment when they operate efficiently. Contacting a trained professional to inspect your furnace for worn-out parts, dirty filters and other potential problems can keep you from getting a big bill later on.  

It’s also helpful to review your furnace filters every few months – it’s recommended 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 drawing air through a clogged filter makes the entire furnace work harder to do its job. And the harder your furnace works, the more strain parts like the heat exchanger will experience.