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

One cause may be that your furnace has a cracked heat exchanger. It can be a safety risk, so it’s critical to familiarize yourself with the signs of a cracked heat exchanger and what you should do if you suspect that is 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 through the system. It generally handles this via coils or tubes that heat up the air while functioning as a barrier to keep the gasses produced in the combustion chamber, called flue gasses, from escaping 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. Cracks in the heat exchanger can allow dangerous gasses – like carbon monoxide, which can be lethal – to circulate throughout your home. 

For this reason, never turn on your heater if you believe it has a cracked heat exchanger, as this could make the whole family ill. Reach out to an HVAC professional as soon as possible if you believe your heater has a cracked heat exchanger that should be repaired. 

Four Signs of a Cracked Heat Exchanger: 

  • Furnace switches off: A cracked heat exchanger could cause your furnace to switch off. 
  • Odd Smells: If the air leaving your furnace has a strong chemical odor, it might be an indicator that gas is slipping through cracks in your heat exchanger. These byproducts, which will often smell like formaldehyde, are a significant warning sign. 
  • Carbon monoxide alarm initiates or you recognize health problems: If a cracked heat exchanger is relieving carbon monoxide inside your home, your carbon monoxide alarm should go off or household members may experience signs of carbon monoxide poisoning. Complications include headaches, dizziness, weakness, nausea, vomiting or feeling tired. If an alarm goes off or you feel unusually tired, exit the home as soon as you can and then call for help. 
  • Soot: If you notice black sooty accumulating near the exterior of your furnace, it’s another sign something could be seriously wrong.

What You Can Do if a Furnace Heat Exchanger is Cracked 

If you worry your furnace has a cracked heat exchanger, contact a pro well versed in furnace installation as soon as possible so they can take a look at your system and, if needed, perform a furnace heat exchanger replacement. Costs will fluctuate depending on the situation, but estimates often hover around $1,000 to $3,000. 

Estimates aside, the good news is that heat exchangers are generally protected by the warranty. You’ll want to review the warranty paperwork on your furnace, since while the warranty might not 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 best ways to avoid problems in your furnace overall is via routine furnace maintenance. Furnaces work the best when they operate efficiently. Calling a trained professional to inspect your furnace for old parts, dirty filters and other likely problems can keep you from getting a big bill later on.  

It’s also beneficial to review 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 pulling air through a clogged filter makes the entire furnace work longer to complete its job. And the harder your furnace works, the more wear and tear parts like the heat exchanger will endure.