How do I know if my furnace heat exchanger is cracked?
You usually can't confirm it yourself: a technician has to see the crack, often with an inspection camera, and back it up with flame behavior and carbon monoxide readings. Warning signs include flames that jump when the blower starts, soot or rust at the burners, a rollout switch that keeps tripping, and CO alarms.
Key points
- The heat exchanger separates the burner flames and exhaust from the air your blower sends through the house.
- Most cracks come from overheating (restricted airflow), years of heat-and-cool cycles, and short cycling from an oversized furnace.
- Signs you might notice: flames that change when the blower starts, soot, a tripped rollout switch, CO alarms. None is proof alone.
- If your furnace is red-tagged, ask to see the crack in a photo or video and where it is.
- A cracked heat exchanger can't be patched. Replace the heat exchanger or the furnace, depending on age and warranty.
You usually can't see a cracked heat exchanger yourself. The signs that point to one are burner flames that lift or flutter the moment the blower starts, soot or rust around the burners, a rollout switch that keeps tripping, and carbon monoxide alarms, and none of them proves a crack alone. A technician confirms it by finding the crack, often with an inspection camera, and backing that up with flame behavior and CO readings. If a CO alarm is sounding right now, stop reading: get everyone outside and call 911 from there.
What does a furnace heat exchanger do, and why does it crack?
Inside a gas furnace, the burners fire into a set of metal chambers called the heat exchanger. Flames and exhaust stay on the inside and leave through the inducer and vent. House air from the blower passes over the outside, picks up heat, and goes out to your rooms. The metal wall between those two streams is all that keeps combustion gases, including carbon monoxide, out of the air you breathe.
Cracks usually form at bends and seams, where the metal flexes most. The common causes:
- Overheating. A clogged filter, closed registers or too little return air keeps heat in the metal. The limit switch is supposed to stop the burners before that happens, but a furnace that trips its limit over and over is cooking its heat exchanger. Our post on the furnace limit switch explains the cycle.
- Years of cycles. Every start heats the metal and every stop cools it. A Minnesota furnace goes through a lot of those in a heating season that can run from October into May.
- Short cycling. An oversized furnace makes many short runs, which adds cycles and keeps the metal from reaching steady temperature. See furnace short cycling.
- Corrosion. Condensation where it doesn't belong, chlorine or solvent fumes pulled in as combustion air (laundry rooms and hobby shops), or a clogged secondary heat exchanger on a condensing furnace.
We see the overheating pattern most in houses where the ductwork never matched the furnace: small returns in 1970s–90s split-levels, or a finished basement where half the registers got closed.
What signs of a cracked heat exchanger can you notice at home?
- Flames that change when the blower starts. If the burner flames lift, lean or flutter the moment the blower kicks on, air may be pushing through a breach. You can sometimes see this through the small window in the burner door.
- Soot, rust or scale. Black soot around the burner openings or on the top of the burner compartment, or rust flakes on the burners, means combustion isn't clean. Soot on the filter can have other causes, and our post on black furnace filters sorts those out.
- A tripped rollout switch. Rollout switches sit near the burners and trip when flame rolls out of the heat exchanger. A rollout that trips more than once isn't something to keep resetting.
- CO alarms, or symptoms that ease when you leave. Headaches, nausea and fatigue that fade after an hour away from the house can point to carbon monoxide.
None of these proves a crack by itself. Each has other causes. They're reasons to have the furnace inspected soon, and some are reasons to shut it off first.
| What you notice | Why it matters | What to do now |
|---|---|---|
| Flames change when the blower starts | Air may be pushing through a breach into the burner side | Turn the furnace off and book an inspection |
| Soot, rust or scale at the burners | Combustion isn't clean, or moisture is getting where it shouldn't | Book an inspection; don't just wipe it off |
| Rollout switch has tripped | Flame left the heat exchanger, from a crack or a blocked vent | Don't reset it again; call |
| Limit code on the status light, or burners that stop while the blower keeps running | The furnace is overheating, the usual cause of cracks | Check the filter and registers, then call if it repeats |
| CO alarm sounds | Carbon monoxide in the house, from the furnace or another appliance | Get everyone out and call 911 from outside |
How does a technician confirm a cracked heat exchanger?
A proper heat exchanger inspection isn't a glance with a flashlight. Depending on the furnace, it includes:
- Watching the flames at startup and again when the blower comes on
- Looking inside the heat exchanger, often with an inspection camera through the burner openings, and removing burners or the inducer where the design allows
- Checking the temperature rise across the furnace against the range printed on its rating plate, since overheating is the usual cause
- Carbon monoxide readings in the flue and in the supply air, compared with the room, before and after the blower starts
A confirmed crack is something the technician can show you. If your furnace is red-tagged, ask for a photo or video of the crack, where it is, and what the readings were. Write down the model and serial number from the rating plate for the warranty check. A second opinion is reasonable before a replacement, as long as the furnace stays off in the meantime.
What should you do if your CO alarm goes off?
Get everyone out of the house, then call 911 from outside. Don't go back in to find the source. If you smell gas (a rotten-egg odor), leave and call your gas utility from outside. Minnesota law requires working CO alarms within 10 feet of every sleeping room, and our CO alarm placement guide covers where else they belong.
A red tag in a Minnesota cold snap
A red tag in October is an inconvenience. A red tag on a January night below zero means planning for tonight:
- Leave the furnace off. Don't relight it or work around the tag to get through the night.
- Heat safely. Electric space heaters go straight into a wall outlet, not an extension cord, and at least three feet from bedding, curtains and anything else that burns. Never heat the house with a gas oven, a grill or a generator.
- Protect the pipes. Open the sink cabinets on outside walls, and let a faucet drip if the house is cooling fast.
- Get both prices in writing with the model and serial numbers, so the repair-or-replace decision happens in daylight.
No-heat calls get priority with us, and our guide to what to do when you have no heat covers the rest of that night.
Can a cracked heat exchanger be repaired?
A cracked heat exchanger can't be patched. You have two options:
- Replace the heat exchanger. National cost guides such as HomeGuide put this at $1,000–$3,000. It makes the most sense on a younger furnace where the manufacturer's warranty still covers the part, so you're paying mainly for labor.
- Replace the furnace. On a furnace past about 15 years old, replacement usually makes more sense than tearing it down for a new heat exchanger. Our installed furnace range is $4,500–$9,500: roughly $4,500–$6,000 for an 80% AFUE furnace and $6,500–$9,500 for a 96–98% model. Our furnace replacement cost guide explains where a house lands, and our furnace installation page covers what the job includes, with a 2-year labor warranty on top of the manufacturer's parts warranty.
Either way, fix whatever caused the crack. A new furnace on the same starved return will overheat too. We size replacements with a Manual J load calculation, and we check return air as part of the job.
Prevention is mostly airflow. Change the filter on schedule, and sooner with pets or during a remodel that makes dust. Keep supply registers and return grilles open and clear of furniture and rugs. Have the furnace checked each fall: a maintenance visit includes a look at the burners and heat exchanger, a temperature-rise check and a carbon monoxide test. The best time to find a crack is on a mild October afternoon, not the night a cold snap arrives.
Getting it checked
If you've seen any of the signs above, turn the furnace off if you suspect a CO problem, and book an inspection. A furnace repair visit is a $79 trip charge, waived when you approve the repair, and includes a carbon monoxide check around the furnace and flue. The same flat pricing covers furnace repair in Minneapolis, furnace repair in Saint Paul and the towns around them. If we find a crack, we'll show you what we found and give you both a repair and a replacement price in writing. Call 612-429-3465, open 7 days, 7am–9pm.
Questions
Will a furnace still run with a cracked heat exchanger?
Often, yes, which is what makes it dangerous. The burners light, the house warms, and nothing seems wrong. Depending on where the crack is and how the furnace drafts, combustion gases can reach the air stream, especially once the blower pressurizes the cabinet. That's why the diagnosis relies on inspection and readings, not on whether the furnace still heats.
Can a cracked heat exchanger be welded or sealed?
No. The metal expands and contracts with every heating cycle, so a weld or a sealant won't hold, and neither is an accepted repair. The fix is a new heat exchanger or a new furnace. Until one of those happens, the furnace should stay off, even on a cold night, and the gas to it should stay shut.
Is it normal for a technician to red-tag a furnace?
It's normal when there's a real hazard: a confirmed crack, flue gas spilling into the house or a gas leak. The technician shuts off the gas to the appliance and tags it so nobody restarts it. You're entitled to see what was found, and getting a second opinion before you spend thousands on a replacement is reasonable.
Does the manufacturer's warranty cover a cracked heat exchanger?
Often the part is covered for a long time, especially if the furnace was registered after installation, but labor usually isn't covered past the first years. Find your model and serial number on the rating plate and check the manufacturer's registration record before you decide between a new heat exchanger and a new furnace.
Sources
- HomeGuide cost guides (2026 editions) homeguide.com