Vaillant Control Signal / Demand Fault

Boiler display on but no response to heating or hot water 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

Faulty room thermostat or programmer, Broken control wiring, eBUS communication fault, Stuck diverter valve microswitch

How to Fix / Troubleshooting

Safety: Isolate power before altering control wiring.

  • Check programmer: Ensure heating and hot water programs are set to ON or timed correctly.
  • Override thermostat: Turn the room thermostat up to maximum to force a demand.
  • Check boiler display: Many Vaillant models show a radiator or tap symbol when there is demand. If none appears, the boiler is not seeing a call.
  • Test with link (if safe/competent): A competent person can temporarily link the room stat terminals at the boiler to see if it fires. If it does, the external control or wiring is faulty.
  • Engineer help: For eBUS controls, a Vaillant-trained engineer can diagnose communication issues.
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Repair Difficulty

Medium 3/5

Required Part

Room Thermostat / Control Interface
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