Viessmann
Unwanted heating / valve or control fault
Radiators get hot even when there is no call for heat from the 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
Stuck open diverter valve, Faulty zone valve, Incorrect wiring causing permanent call for heat, Faulty relay in Viessmann control
How to Fix / Troubleshooting
Safety: Components may be hot. Isolate power before working on valves or wiring.
- Check thermostat: Confirm that the thermostat is not calling for heat and that any timer is not in an 'on' period.
- Feel flow pipe: If the boiler flow pipe is hot when there should be no demand, the boiler may be receiving a false call for heat.
- Inspect zone valves: Check motorized valves for stuck or manually latched open levers. Return levers to 'auto' position.
- Wiring check: A professional should verify that external controls are wired correctly to the Viessmann demand terminals and that no permanent live is feeding the call-for-heat input.
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Repair Difficulty
Medium
3/5
Required Part
Diverter Valve / Zone Valve
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