How do I know if my furnace heat exchanger is cracked?
A technician, not the homeowner, confirms a cracked furnace heat exchanger with a camera view, flame behavior and carbon monoxide readings. Warning signs are jumping flames, soot, rollout trips and CO alarms. HomeGuide puts a replacement heat exchanger at $1,000–$3,000 nationally.
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 crack is something you suspect from indirect clues and a technician proves with a camera and instruments, and the sections below go in that order. 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.
Overheating is most likely 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. Under Minn. Stat. 299F.51, working CO alarms must be within 10 feet of every sleeping room. 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 (our red tag guide covers what the tag means):
- 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. Prop open the cabinet doors under sinks on outside walls, and keep one faucet dripping if the house is cooling quickly.
- Get both prices in writing with the model and serial numbers, so the repair-or-replace decision happens in daylight.
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, a new furnace usually beats tearing the old one down for a heat exchanger. We install furnaces at $4,500–$9,500, with 80% AFUE models near the bottom of that range and 96–98% condensing models near the top. Our furnace replacement cost guide explains where a house lands, and our furnace installation page covers what the job includes, and our 2-year labor warranty on installs adds to 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 the return air should be part of that review.
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 heat exchanger inspection and a combustion and carbon monoxide reading. 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, book an inspection. If a CO alarm sounds, get everyone outside and call 911 from there; keep the furnace off until it's checked. On a furnace repair visit the $79 trip charge is waived when you approve the repair; ask for a carbon monoxide reading around the furnace and flue. We do furnace repair in Minneapolis, furnace repair in Saint Paul and the towns around them. If a crack is confirmed, ask for the photo or video and for both a repair and a replacement price in writing. Call 612-429-3465, open daily 7am to 9pm.
Common questions about heating
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.
Will a carbon monoxide alarm catch a cracked heat exchanger?
Not reliably. A crack doesn't always leak carbon monoxide into the living space, and an alarm sounds only when CO builds up where the alarm is mounted. Keep working CO alarms outside every sleeping area, as Minnesota law requires, and still have the heat exchanger inspected each fall.
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.
How much does it cost to fix a cracked heat exchanger?
National cost guides such as HomeGuide put a heat exchanger replacement at $1,000–$3,000. That makes sense mainly on a younger furnace where the manufacturer's warranty still covers the part. On a furnace past about 15 years old, replacement usually makes more sense, and our installed furnace range is $4,500–$9,500. A crack cannot be patched.
Sources
- Minnesota Statutes 299F.51 (carbon monoxide alarms) revisor.mn.gov
- HomeGuide cost guides (2026 editions) homeguide.com