A furnace blowing cold air usually means the blower is running while the burners are not. That can be a thermostat fan set to ON instead of AUTO, or a fault where the burners shut down but the blower keeps running to cool the furnace, such as an open limit switch or a failed ignition. Check the thermostat and filter, then read the control board code.
Causes and codes
| What’s happening | Likely reason | Related codes |
|---|---|---|
| Air is cool between heating cycles | Thermostat fan set to ON (continuous) | none |
| Blower runs, no heat | Limit switch open, blower runs to cool the furnace | Goodman EE3, Carrier 33 |
| Burners try, then stop | Ignition or flame sensing failure | Carrier 34, Rheem 11, Trane E2.1 |
| No burner attempt | Pressure switch or lockout | Goodman EE2, Carrier 14 |
The Goodman EE3 entry in its manual notes the circulator blower runs continuously with no heat when the primary limit is open, and Carrier says the blower runs for 4 minutes or until the open switch closes for code 33.
Safe checks
- Thermostat: set to HEAT, fan to AUTO, setpoint above room temperature.
- Replace a dirty filter and open registers.
- Read and look up the control board code.
Safety first: If you smell gas, don’t touch light switches or use a phone inside. Leave the building and call your gas utility from outside. Turn off power to the furnace before opening any panel, and never bypass or jumper a safety switch. Gas, venting, wiring and control-board work is for a licensed HVAC technician.
§FFAQ
Why does my furnace blow cold air at the start?
Furnace controls use a blower on-delay and off-delay (the Carrier manual describes both), so the blower timing does not exactly match the burners. Brief cool air at the end of a cycle can be the normal off-delay. If air stays cold, check the code.
Can a dirty filter make a furnace blow cold air?
A restricted filter can overheat the furnace and open the limit switch, stopping the burners while the blower keeps running.