Danfoss
Blower Stuck in Manual/On Mode or Fan Relay Welded
Furnace blower runs continuously even when there is no call for heat
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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 switch set to ON, stuck Danfoss fan relay on control board, shorted blower wiring, high limit switch open forcing fan operation
How to Fix / Troubleshooting
Safety first: Turn off power before accessing the control board or blower wiring.
Steps:
- Check thermostat fan setting: Ensure the thermostat fan switch is set to AUTO, not ON.
- Inspect high limit switch: If the high limit is open, some systems run the blower continuously to cool the heat exchanger. Investigate and correct overheating causes (filter, vents, blower).
- Check control board relay: Listen for a click from the Danfoss control when the fan should turn off. If the blower continues without relay action, the relay contacts may be welded closed.
- Call a technician for board replacement: Replacing the control board or relay should be done by a professional to ensure correct wiring and safety.
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Repair Difficulty
Medium
3/5
Required Part
Danfoss Furnace Control Board
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