Signs Your Heat Exchanger is Cracked
Signs Your Heat Exchanger is Cracked
If your carbon monoxide alarm is sounding, or anyone in the house has a headache, dizziness, or nausea that eases when they leave, go outside now and call 911 or your local emergency number. Deal with the furnace afterward.
A cracked heat exchanger lets combustion gases, including carbon monoxide, mix into the air your furnace blows through your home. Carbon monoxide has no color and no smell, so your senses won’t warn you. A working CO detector and your own symptoms are the warning.
The most telling physical sign of a crack is a burner flame that flickers, leans, or changes shape the moment the blower fan starts. If you see that, or any of the signs below, stop using the furnace until a licensed technician inspects it.
Signs of a Cracked Heat Exchanger, Strongest to Weakest
| Sign | How much it tells you | What it means |
| CO alarm sounds, or headache, dizziness, nausea, or flu-like symptoms that ease when you leave the house | Strongest. Treat as an emergency | Get everyone outside and call 911 first, then the HVAC company |
| Burner flame moves, leans, or flickers when the blower starts | Strong | A crack lets blower air disturb the flame. A technician can test this. If your furnace has a viewing window, you may see it yourself |
| Furnace keeps shutting off, or the flame rollout switch trips | Strong | A disturbed flame can roll out of the combustion chamber. The safety switch shutting the furnace down is it doing its job |
| Yellow or orange flame instead of steady blue | Moderate | Dirty burners cause this too, so it needs inspection either way |
| Soot around the furnace or flue | Moderate | Incomplete combustion. A crack is one possible cause |
| Sharp, formaldehyde-like odor while the furnace runs | Moderate | A combustion byproduct smell. Carbon monoxide itself has no odor |
| Rattling, popping, or banging when the burners start | Weak | Heating metal expands, and other furnace problems cause the same sounds |
No single sign proves a crack. Yellow flames, soot, and noises all have other causes, which is why a technician’s test matters.
A rotten-egg smell is a different emergency. That’s the additive in natural gas, and it means a gas leak, not a cracked heat exchanger. Leave the house immediately and call your gas utility or 911 from outside.
What to Do Right Now
- CO alarm or symptoms: get everyone outside into fresh air and call 911 before anything else.
- Suspect a crack but no alarm: turn the furnace off at the thermostat, then at the furnace switch or breaker. Open windows to ventilate.
- Don’t run the furnace again until a licensed technician has inspected it.
- Call an HVAC company and tell them you suspect a cracked heat exchanger. Describe which signs you saw.
- Check your CO detectors. You need one on every level and outside sleeping areas. Test them monthly and replace them by the date printed on the unit.
How Technicians Check, and How to Verify the Diagnosis
Cracks are often too small to see, so a proper inspection combines several checks: a visual inspection, often with an inspection camera; watching the burner flame when the blower starts; and combustion analysis that measures carbon monoxide in the exhaust and airflow.
Because a condemned heat exchanger can mean a four-figure repair, it’s reasonable to verify the finding before you pay:
- Ask which test found the crack, and ask to be shown the crack, a photo, or the flame change on the blower start.
- Get a second opinion from another licensed company before approving a replacement. Keep the furnace off in the meantime if the first technician shut it down.
Trane’s own guidance says the same: ask to see the cracked part before deciding.
Repair or Replace
A cracked heat exchanger can’t be safely repaired. Patching or welding isn’t accepted practice. You have two options: replace the heat exchanger itself, or replace the whole furnace.
| Your situation | Lean toward |
| Furnace under about 10 years old and the heat exchanger is under warranty | Replace the heat exchanger. Check your warranty first, since terms vary by manufacturer and model |
| Furnace 15 years or older | Replace the furnace |
| Heat exchanger quote approaches half the cost of a new furnace | Replace the furnace |
Trane puts heat exchanger replacement at roughly $1,000 to $3,000, depending on the furnace model, your location, and local labor rates.
Why Heat Exchangers Crack
The most common preventable cause is overheating from restricted airflow. When the blower can’t move enough air across the heat exchanger, the metal runs too hot. The constant expanding and contracting eventually cracks it, usually at bends and welds. Airflow restrictions include a clogged filter, closed or blocked registers, and undersized or leaky ductwork. Age and corrosion cause the rest.
Preventing It
- Replace the furnace filter every 1-3 months during heating season (how often to replace a furnace filter)
- Keep every register open and unblocked
- Have the furnace inspected once a year before winter, and ask that the heat exchanger be checked
- If your furnace shows repeated overheating or limit-switch codes, here’s what the blinking light patterns mean
- Keep working CO detectors on every level
If you’re in the Philadelphia area and want a furnace inspection or a second opinion on a heat exchanger diagnosis, Cool Aid Co. can take a look.