Heating stays on constantly even when thermostat is turned down or off

Safety first: Isolate power to the heating system before inspecting wiring.

Steps:

• Turn the thermostat to its lowest setting or off and observe whether the call for heat indicator on the thermostat goes off.
• At the thermostat, disconnect the switched live output wire and check if the boiler or zone valve turns off. If heating stops, the thermostat or its relay is the cause.
• Test the relay contacts with a multimeter; if they remain closed when there is no demand, the relay is welded and the thermostat must be replaced.
• If heating continues even with the thermostat output disconnected, investigate wiring at the Heatmiser wiring centre and boiler for permanent live feeds or incorrect links.