Viessmann
Boiler Demand Signal Not Recognized
Thermostat clicks (relay sound) but boiler does not start; no error code on thermostat.
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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 wired to wrong boiler terminals, Boiler configured for KM‑BUS but thermostat wired as on/off, Open circuit in demand wiring, Boiler internal fault ignoring demand
How to Fix / Troubleshooting
Safety first: Turn off power to the boiler and thermostat before working on wiring. Only work on low‑voltage terminals if you are competent.
- Check wiring method: Determine whether your system should use KM‑BUS or simple on/off control. Follow the Viessmann manual for the correct wiring scheme.
- Verify terminal connections: Ensure the thermostat output is connected to the correct demand or bus terminals on the boiler.
- Test boiler input: If safe, temporarily bridge the boiler’s demand terminals to see if it starts. If it does, the issue is between thermostat and boiler; if not, the boiler has an internal fault.
- Reconfigure control type: In the boiler and thermostat settings, ensure both are set to the same control type (bus vs on/off).
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Repair Difficulty
Hard
4/5
Required Part
Thermostat‑to‑boiler control wiring
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