Main
Fan Overrun or Stuck Relay
Boiler fan runs continuously even when there is no demand for heating or hot water
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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
PCB relay stuck closed, Faulty temperature sensor causing extended overrun, Incorrect wiring from external controls
How to Fix / Troubleshooting
Safety: Continuous fan operation is usually not dangerous but indicates a control fault. Isolate power before inspection.
- Allow cool-down: Some Main boilers run the fan for a short period after burner shutdown; this should not exceed the specified overrun time (typically a few minutes).
- Check for hidden demand: Ensure no frost protection or hot water preheat function is calling for operation.
- Engineer tasks:
- Test the PCB fan relay and replace the Main control PCB if the relay is stuck.
- Verify sensor readings to ensure the boiler is not falsely detecting high temperatures.
- Check external control wiring for incorrect permanent call signals.
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Repair Difficulty
Hard
4/5
Required Part
Main Control PCB
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