Heating runs continuously and does not turn off even when the set temperature is reached or the thermostat is turned off.

Safety first: Immediately switch off the heating circuit breaker to prevent overheating or damage to the system.

Steps:

• Confirm thermostat command: Set the thermostat to OFF or a very low setpoint. If the boiler or heater continues to run, the issue is in wiring or hardware.
• Check wiring: With power off, verify that the live feed to the boiler or heater passes only through the thermostat's designated output terminals. Remove any unauthorized bypass connections.
• Test relay contacts: Disconnect the output wires from the thermostat and restore power. If the heater stops, the thermostat was driving it. If the heater still runs, the fault is in external wiring or controls.
• Inspect thermostat output: With the thermostat removed and power off, measure continuity across the relay contacts (e.g., COM and NO). If continuity is present even when the thermostat is off, the relay is welded closed.

Resolution: Replace the thermostat if the relay or triac is stuck. Do not attempt to pry open or file relay contacts.