Thermia
Continuous Fan Operation
Furnace blower runs continuously even when there is no call for heat or fan from the thermostat.
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Thermostat fan set to ON, Stuck blower relay on Thermia control board, High limit switch open, Miswired thermostat
How to Fix / Troubleshooting
Safety first: Turn off power before accessing the control board.
- 1. Check thermostat fan setting: Ensure the fan is set to AUTO, not ON.
- 2. Verify thermostat wiring: Confirm that the G (fan) wire is not shorted to R at the thermostat or furnace terminal strip.
- 3. Inspect high limit switch: If the furnace overheats and the high limit is open, some Thermia models will run the blower continuously for safety. Replace dirty filters and clear airflow restrictions.
- 4. Check control board relay: If the blower runs even with the G wire disconnected, the blower relay on the Thermia control board may be stuck closed and the board will need replacement.
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Repair Difficulty
Medium
3/5
Required Part
Main Control Board
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