How to check and replace a cracked furnace heat exchanger
Steven E / Thursday March 20, 2025
With winter looming, a faulty furnace is probably the last thing on your wish list. A cracked heat exchanger isn’t just a minor glitch in your heating system—it’s a serious concern that could impact the safety and coziness of your home. Spotting the warning signs and knowing how to handle them can be crucial for keeping your family safe and snug.
The tips in this article might not fit your exact appliance model. We aim to help with all sorts of DIY appliance fixes, but different models and setups can mean different troubleshooting steps, error codes, parts, and so on. For the best advice, it’s a good idea to check the manufacturer’s documentation for your specific appliance, or feel free to reach out to us if you’ve got questions.
Understand your furnace’s heat exchanger
The heat exchanger is kinda like the beating heart of your furnace. It moves heat from the combustion gases over to the air that keeps your place cozy. It also makes sure those gases stay sealed off from the air you’re breathing, keeping things safe and running smoothly. Getting a grasp on how it functions can help you notice when something’s off.
Warning signs of a cracked heat exchanger
- You can see damage. One of the easiest ways to spot a cracked heat exchanger is just by taking a look. If you’re okay with getting into your furnace (just be sure it’s switched off and cooled down first), check for any cracks or rust on the heat exchanger. The constant heating up and cooling down makes the metal expand and contract, which can cause cracks as time goes by.
- Soot’s showing up. If you notice soot inside your furnace or around the heat exchanger, that’s definitely a red flag. Soot means the fuel isn’t burning completely, often because of a bad air-fuel mix or clogged burners. A cracked heat exchanger might not be the direct cause of soot, but finding soot could point to other issues that need fixing. Plus, it can make your furnace less efficient and might even be dangerous.
- Flame acts weird. Have a look at the furnace flame. Normally, it should be steady and blue. If you see it’s yellow or flickering, that could mean the combustion isn’t right—maybe not enough oxygen or a messed-up air-fuel mix. A cracked heat exchanger doesn’t usually mess with the flame directly, but any weird flame behavior means it’s time to call in a pro. Don’t brush this sign off.
- Funky smells. Is there a strong, nasty smell, kinda like formaldehyde, when your furnace is running? That could be gases leaking out from a cracked heat exchanger. These fumes can give you headaches and make you feel sick, so you need to deal with this ASAP.
- Everyone’s feeling off. If people in your home are getting headaches, feeling nauseous, dizzy, or just plain exhausted for no good reason, it could be from carbon monoxide leaking due to a cracked heat exchanger. Carbon monoxide doesn’t have any smell or color, so these symptoms might be the only hint you’ve got.
- Water’s gathering. Noticing water puddling up around the bottom of your furnace? Sure, some furnaces produce condensation (especially the high-efficiency ones), but if you’ve got moisture that doesn’t make sense, it might mean there’s a crack in the heat exchanger letting water vapor leak out.
- Hearing strange noises. Banging, popping, or whistling sounds coming from your furnace? That’s not normal. These noises might happen because cracks are making parts of the heat exchanger expand or contract in weird ways.
- Bills are climbing. If your heating bills have suddenly shot up but you haven’t changed how much you’re using the furnace, it might be because your furnace is overworking to heat your place. A cracked heat exchanger can make it less efficient.
- Finding dust or soot around vents. Seeing a lot of dust or soot building up near your furnace vents? That might be stuff escaping through a crack and getting spread around your house.
- Bad smells coming from the furnace. A cracked heat exchanger might make your furnace give off nasty odors. Trust your nose—if something smells weird, it’s time to check it out.
What to do if you notice these signs
Spotting any of these signs can be pretty scary, but taking action fast can help keep your home safe.
- First things first, turn off your furnace. This stops any more harmful gases from circulating. Find the power switch or the circuit breaker for your furnace and flip it off.
- Clogged filters or blocked vents can make furnace problems worse. Look at your furnace filter and swap it out if it’s dirty. Make sure all vents and returns are open and not blocked.
- Even if the heat exchanger might be the issue, other parts can cause similar signs. Check for loose connections, damaged ducts, or other visible problems you can fix safely.
- If you don’t have them yet, put carbon monoxide detectors on every floor of your house. They’ll give you an extra line of defense and warn you about dangerous gas levels.
- Write down the signs you’ve spotted. Snap pictures of any damage you see and jot down notes about weird smells or noises. It really helps to know your furnace’s model number. If you’re not sure where to find it, take a look at our guide on how to find your appliance’s model number.
Stay safe and warm
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With nearly a decade of experience in providing top-notch customer service regarding appliance parts and repair, Steven enjoys sharing practical advice, troubleshooting tips, and interesting information to help readers stay informed.