Furnace Runs but No Heat from Vents (Trane & American Standard)
If your gas furnace turns on initially but only blows cool or room-temperature air (if at all), the blower is running but the heating process is not completing correctly.
What should normally happen
A gas furnace should ignite, heat the exchanger, then deliver warm air shortly after the blower starts.
Common causes (most likely first)
- Ignition failure or failed flame sensor
- Pressure switch not closing
- Limit switch tripped from overheating or airflow restriction
- Gas supply issue
- Control board lockout
Safe homeowner checks
- Replace the air filter
- Make sure gas supply valves are on, tank is not empty if using propane, meter is turned on if natural gas
- Check the furnace door switch is fully closed
- Look for flashing error codes on the control board
- Reset furnace breaker (caution this will clear diagnostic codes if present, but may reset and clear the issue. Recommended to record the codes first)
When this becomes a technician issue
If the furnace repeatedly attempts ignition or shuts down after lighting, testing of flame signal, pressure switch operation, and safety circuits is required.
Parts commonly involved
- Flame sensor
- Igniter
- Pressure switch
- Limit switch
- Furnace control board