Thermostat calls for heat but boiler or heat pump does not start.

Safety first: Isolate mains power to the boiler and thermostat before touching any wiring.

• Step 1 – Confirm call for heat: Raise the setpoint above room temperature and verify that the thermostat indicates a heat demand (flame icon or similar).
• Step 2 – Check boiler controls: Ensure the boiler’s own programmer or master switch is set to accept external thermostat control and is not in Off or Summer mode.
• Step 3 – Verify wiring: At the thermostat backplate, confirm that the heating output terminals (e.g. NO/COM or OT bus) are wired correctly to the boiler’s thermostat input. Tighten all screws.
• Step 4 – Continuity test: With power off, test continuity of the call‑for‑heat wire between thermostat and boiler. Repair or replace if open.
• Step 5 – Bypass test (advanced): At the boiler, briefly bridge the thermostat input terminals. If the boiler starts, the issue is with the thermostat or its wiring. If not, the boiler control is at fault.
• Step 6 – Replace thermostat: If wiring is correct and the boiler responds to a manual bridge but not to the thermostat, replace the thermostat or its base.