Safety first: Isolate power before altering wiring.

Check thermostat settings: Ensure the room thermostat or ACV controller is set to heating mode and above room temperature.
Check programmer: Verify that the timeclock or programmer is set to ON or a timed period that is active.
Override test: If the controller has a manual override or boost, activate it and see if the boiler starts.
Professional testing: A technician should test continuity of control wiring and the thermostat contacts or OpenTherm communication.

Boiler does not respond to room thermostat, heating stays off

Possible Causes