Furnace Blowing Cold Air: A Diagnostic Order That Finds It Fast
"It's blowing cold air" covers everything from a thermostat setting to a cracked heat exchanger safety trip. Work through it in order and the answer usually shows up in a few minutes.
1. Thermostat and fan setting
- Fan set to ON runs the blower all the time, so it blows room-temperature air between heating cycles. Set it to AUTO.
- Mode and setpoint: confirm it's in HEAT and calling (setpoint above room temperature).
- Heat pump thermostat on a furnace, or the wrong system type configured on a smart thermostat, can send a fan call with no W call. See thermostat wiring.
2. Is it a normal blower run?
After the burners shut off, the blower keeps running for a set time to pull leftover heat out of the heat exchanger. Air cools during that time. That's normal.
3. Are the burners lit?
Look through the sight glass, or open the burner door as the manufacturer allows, and read the LED code first. See how to read furnace flash codes.
| What happens | Likely cause | Guide |
|---|---|---|
| Inducer never starts | Board, pressure switch stuck closed, no call | Inducer motor |
| Inducer runs, nothing else | Pressure switch not closing | Pressure switch |
| Igniter never glows | Igniter or its circuit | Hot surface igniter |
| Burners light, go out in seconds | Flame sensing | Flame sensor |
| Several tries, then stops | Ignition lockout | Ignition lockout |
| Ran, then shut down, blower keeps running | High limit opened (blower runs to cool the furnace) | High limit |
| Burners off, rollout code | Rollout switch tripped | Rollout switch |
A tripped high limit is the classic "blowing cold air" call: the burners shut off to protect the heat exchanger, but the blower keeps running to cool it down, so the customer feels cool air. The cause is usually low airflow: a dirty filter, a blocked return, or a dirty coil above the furnace.
4. Burners lit, but the air is lukewarm
- Measure temperature rise against the rating plate. See furnace temperature rise.
- Check gas pressure: low manifold pressure gives a weak flame. See how to check furnace gas pressure.
- Two-stage furnaces sitting on low fire on a cold day may feel lukewarm; check the staging setup and the thermostat.
- Return duct leaks in an attic or crawlspace pull in cold air.
- Long supply runs through unconditioned space lose heat; check insulation and leaks.
5. No gas
- Check the manual shutoff valve and the gas valve's ON/OFF knob.
- Check other gas appliances, or the propane tank level.
- Check inlet gas pressure.
Common mistakes
- Replacing parts before reading the LED code.
- Resetting the limit or rollout repeatedly without fixing airflow or venting.
- Missing the filter. A clogged filter is the most common cause of a high limit trip.
- Calling normal post-purge blower time a problem.
FAQ
Why does my furnace blow cold air, then warm air?
The furnace may be cycling on the high limit (overheating from low airflow), or the blower may be starting before the heat exchanger warms up. Check the filter and temperature rise.
Can a dirty filter make a furnace blow cold air?
Yes. It causes the high limit to open, which shuts off the burners while the blower keeps running.
Why is my furnace blowing cold air with the burners on?
Usually a very high airflow setting, low gas pressure or a return leak. Measure temperature rise to see which.
Sources
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