F10 error and thermostat cannot control hot water schedule

Safety first: Isolate power to the heating system at the fused spur or breaker.

Steps:

• 1. Check HW wiring at thermostat: Remove the thermostat and inspect the HW ON/CALL terminal. Ensure the wire is secure and not broken. Re‑terminate if necessary.
• 2. Verify wiring at wiring centre: At the wiring centre, locate the HW ON from the thermostat and ensure it is correctly connected to the cylinder valve or boiler HW input.
• 3. Continuity test: With power off, test continuity of the HW call wire between thermostat and wiring centre. Repair or replace the cable if open.
• 4. Manual override test: Manually operate the HW zone valve lever (if fitted). If the boiler fires and cylinder heats, the valve and boiler are functional and the fault is likely at the thermostat or wiring.
• 5. Replace thermostat head if relay faulty: If wiring is sound but F10 persists and the HW relay does not click when HW is demanded, replace the thermostat head.