A furnace is usually a background player for your home, ensuring you're warm across the cold winter months. It frequently isn't noticed until a malfunction appears. 

One root cause could be that your furnace has a cracked heat exchanger. It can be a safety risk, so it’s worthwhile to know the symptoms of a cracked heat exchanger and what to 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 inside your furnace to the air that flows through the ventilation. It typically handles this through coils or tubes that warm the air while serving as a barrier to keep gas produced in the combustion chamber, called flue gasses, from leaking out into your home.  

Is a Cracked Heat Exchanger Dangerous? 

Given its important role, it’s no surprise that a cracked heat exchanger can pose a risk. A damaged heat exchanger can enable dangerous gasses – such as carbon monoxide, which can be lethal – to be distributed across your home. 

For this reason, never use your heater if you believe there's a crack in the heat exchanger, as this could make the entire family sick. Call an HVAC professional as soon as possible if you think your furnace has a cracked heat exchanger that needs to be repaired. 

Four Signs of a Cracked Heat Exchanger: 

  • Furnace turns off: A crack in your heat exchanger could cause your furnace to shut off. 
  • Strange Smells: If the air escaping your furnace has a strong chemical smell, it may be evidence gasses are slipping through cracks in your heat exchanger. These gasses, which will often smell like formaldehyde, are a significant warning sign. 
  • Carbon monoxide alarm goes off or you recognize poisoning symptoms: If a cracked heat exchanger is relieving carbon monoxide into your home, your carbon monoxide alarm could go off or household members might struggle with signs of carbon monoxide poisoning. Side effects include headaches, dizziness, weakness, nausea, vomiting or feeling sleepy. If the alarm goes off or you feel sick, get out of the home as soon as you can and then call for help. 
  • Soot: If you notice black sooty collecting near the exterior of your furnace, it’s more evidence something may be seriously wrong.

What to Do if Your Furnace Heat Exchanger is Cracked 

If you believe your furnace has a cracked heat exchanger, hire a professional with extensive experience in furnace installation right away so they can examine your system and, if necessary, perform a furnace heat exchanger replacement. Costs should vary depending on the situation, but estimates often hover around $1,000 to $3,000. 

However, the good news is that heat exchangers are often protected by the warranty. You’ll want to review the warranty paperwork on your furnace, because while the warranty may not cover the entire cost of repairs, it still may significantly lower your bill.  

How to Prevent a Cracked Heat Exchanger in Your Home  

One of the best ways to avoid problems in your furnace overall is through consistent furnace maintenance. Furnaces offer the most benefits when they operate efficiently. Calling a skilled professional to examine your furnace for old 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 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 more vigorously to accomplish its job. And the harder your furnace has to work, the more strain pieces like the heat exchanger will experience.