The boiler does not respond to the room thermostat; heating stays off.

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 time clock 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 the continuity of control wiring and the thermostat contacts or OpenTherm communication.