Vaillant Fan Overrun / Control Fault

Boiler fan runs continuously even when there is no demand

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.

Possible Causes

Stuck relay on PCB, Faulty temperature sensor causing extended overrun, Incorrect wiring from external controls, Software glitch

Safety: Continuous fan running is usually not immediately dangerous but indicates a control issue.

Check for demand: Ensure all heating and hot water demands are off at the programmer and thermostats.
Power cycle: Turn off the boiler at the fused spur for 5 minutes, then restore power. If the fan starts immediately with no demand, suspect PCB.
Engineer diagnosis: A technician can test PCB outputs and replace the board if the fan relay is stuck.

Hard 4/5

Main Control PCB

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