Protherm Control Signal / External Thermostat Fault

Room thermostat calling for heat but boiler does not respond

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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, Incorrect control settings on PCB, Faulty low‑voltage input on PCB

How to Fix / Troubleshooting

Safety: Isolate power before working on control wiring.

Steps:

  • Verify that the thermostat has power (batteries or mains) and is set above room temperature.
  • Check programmer or timer settings to ensure heating is enabled.
  • Test continuity of the thermostat circuit at the boiler terminals; bridge the terminals temporarily (if allowed by Protherm) to simulate a call for heat. If the boiler starts, the thermostat or wiring is faulty.
  • Replace the thermostat or repair wiring as needed.
  • If the boiler does not respond even when terminals are bridged, the PCB input may be faulty and require replacement.
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Repair Difficulty

Medium 3/5

Required Part

Room Thermostat
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