Protherm
Stuck Heating Demand / Relay Welded Closed
Boiler runs continuously and does not turn off even when the set temperature is reached.
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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
Welded relay contacts in thermostat, Boiler configured to ignore external control, Shorted control wiring between thermostat and boiler
How to Fix / Troubleshooting
Danger: Continuous boiler operation can overheat the system. Isolate power before inspection.
- Step 1 – Turn off heating mode:
- Set thermostat to OFF or a very low setpoint.
- If boiler continues to run, the issue is likely wiring or boiler-side.
- Step 2 – Disconnect thermostat control wires at boiler:
- With power off, remove the thermostat control wires from the boiler terminals.
- Restore power; if boiler stops, the thermostat or wiring is at fault.
- Step 3 – Check for shorted cable:
- Inspect the cable between thermostat and boiler for damage.
- Replace if insulation is damaged or conductors are shorted.
- Step 4 – Test thermostat relay:
- With thermostat removed, check continuity across relay contacts when there is no heat demand.
- If contacts remain closed, the relay is welded.
- Step 5 – Replace thermostat:
- Install a new Protherm smart thermostat to restore proper control.
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Repair Difficulty
Hard
4/5
Required Part
Protherm Thermostat Relay Output
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