Danger: The thermostat relay terminals may carry mains voltage. Isolate power at the main breaker before opening the unit or touching any wiring.

Step 1 – Verify wiring:
- Confirm that the boiler control input is correctly wired to the thermostat’s COM and NO (or appropriate) relay terminals.
- Ensure no high-current loads (e.g. pumps directly) are connected beyond the relay’s rating.
Step 2 – Listen for relay click:
- With power restored and cover on, raise the set temperature.
- Listen for a distinct click from the thermostat.
- No click suggests relay coil or driver failure.
Step 3 – Electrical test (qualified persons only):
- With power off, remove thermostat and inspect PCB for burn marks around the relay.
- Check continuity across relay contacts when manually energized (if possible) or during a call for heat.
Step 4 – Replace thermostat:
- Relay replacement on the PCB is not typically user-serviceable.
- Install a new Protherm smart thermostat and reconfigure settings.

FD error and thermostat cannot drive heating relay or boiler demand output.

Possible Causes