Furnace Blowing Cold Air
Quick Answer
When your furnace blows cold air, first check your thermostat - make sure it's set to HEAT with fan on AUTO. Next, check your air filter. A clogged filter causes overheating, triggering safety shutdowns while the fan keeps running.
Safe Checks You Can Do
- 1Verify thermostat settings - Set to HEAT mode, fan on AUTO, temperature above current room temp.
- 2Replace air filter - A dirty filter is the #1 cause of furnace problems including cold air.
- 3Check pilot light - On older furnaces, ensure the pilot is lit. Relight following instructions on the unit.
- 4Reset the system - Turn off at thermostat for 5 minutes, then restart to reset safety switches.
Likely Causes
- Thermostat set incorrectly
Fan set to ON instead of AUTO will blow air continuously, including when burners are off.
- Dirty air filter
Restricted airflow causes overheating. The limit switch shuts off burners but fan continues.
- Pilot light out
No pilot means no ignition. The fan runs but burners never light.
- Flame sensor dirty
A dirty sensor can't detect the flame, causing the gas valve to shut off after ignition.
- Gas supply issue
Check that the gas valve is open and other gas appliances are working.
When to Call a Pro
- You smell gas near the furnace
- The pilot won't stay lit after relighting
- Filter replacement and reset don't help
- You see error codes flashing on the furnace
- The furnace is cycling on and off repeatedly
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