Vaillant
Heating Stuck On / Runaway Heating
Heating remains on even when the Vaillant smart thermostat is set to OFF or a low temperature.
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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 closed contacts in thermostat relay, Permanently open Vaillant zone valve microswitch, Incorrect wiring bypassing thermostat, Faulty wiring center linking call for heat permanently
How to Fix / Troubleshooting
Safety: This can cause overheating. Isolate the boiler and heating system at the main switch before inspection.
- 1. Confirm thermostat status: Ensure the thermostat actually shows heating OFF and the setpoint is below room temperature.
- 2. Check boiler demand input: At the boiler, see if the room stat or eBUS demand input indicates a constant call for heat. If so, suspect wiring or valve issues.
- 3. Inspect zone valves: For systems with motorised valves, check if any valve is stuck in the open position. A stuck valve can keep the boiler running via its end‑switch.
- 4. Test thermostat output: Disconnect the thermostat control wires at the wiring center or boiler. If heating stops, the thermostat or its relay is likely faulty and should be replaced.
- 5. Correct wiring faults: Compare wiring to the Vaillant diagram and remove any unauthorised links that bypass the thermostat.
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Repair Difficulty
Hard
4/5
Required Part
Vaillant smart thermostat relay output / zone valve
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