Protherm
Stuck Fan Relay or Incorrect Fan Control Setting
Furnace blower runs continuously even when there is no heat demand
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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
Stuck fan relay on Protherm PCB, Fan set to continuous mode on thermostat, Shorted fan control wiring, Faulty fan limit switch (on some models)
How to Fix / Troubleshooting
Safety first: Turn off power before accessing wiring.
- Step 1 – Thermostat fan setting: Check that the thermostat fan mode is set to 'Auto' rather than 'On'.
- Step 2 – Control settings: On Protherm integrated controls, verify that continuous fan mode is not enabled in the menu.
- Step 3 – PCB relay check: If the fan runs even with thermostat disconnected, the fan relay on the PCB may be stuck closed and the board will need replacement.
- Step 4 – Limit switch (if fitted): Some furnaces use a fan/limit control. If it is misadjusted or stuck, a technician should inspect and replace it.
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Repair Difficulty
Medium
3/5
Required Part
Main Control PCB
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