Thermostat not working: heating won’t turn on or won’t turn off
Quick summary
Most thermostat problems are configuration (mode/schedule), batteries/pairing, or the boiler not receiving a heat demand signal.
Safety first
- If you smell gas or suspect CO: stop and call 0800 111 999 / seek urgent help.
- If your heating is stuck on and you need it off urgently, you may be able to turn heating off at the programmer/receiver (do not disturb sealed wiring).
- If you’re unsure, avoid opening control housings.
Full checklist: Safety guidance
What to check (in order)
- Check thermostat mode: heat/cool/off, manual vs schedule, and setpoint above room temperature for a ‘turn on’ test.
- If wireless: replace batteries and check the receiver has power (lights/status).
- Check programmer/timer: is heating enabled right now?
- Try a simple demand test: set thermostat high and see if boiler fires for heating.
- If heating won’t turn off: set thermostat low/off and see if boiler continues firing; check whether hot water demand is confusing the observation (combi).
What the result means
- No response to demand: pairing/battery/receiver/controls chain issue likely.
- Heating stuck on: often a control relay, wiring issue, or zone valve end-switch stuck (system-dependent).
- Intermittent behaviour: low batteries, radio range, or loose connections.
What you can safely do
- Replace batteries if the thermostat uses them.
- Turn the thermostat well above room temperature to test demand.
- Check the programmer or receiver is actually calling for heat.
- Move the thermostat away from obvious heat sources before deciding it is faulty.
When to call a professional
- Heating stuck on and you can’t control it from programmer/receiver.
- Repeated failures or you suspect wiring/zone valve issues.
- You’re not confident with controls or access.
Engineer notes
Identify control architecture: on/off vs OpenTherm/eBus, wireless receiver vs wired stat, and zoning. For ‘stuck on’ check relay contacts, receiver output, zone valve end-switch, and boiler control input. For ‘won’t call’ confirm power to stat/receiver, pairing, and call signal presence. Document control chain and voltages (qualified).
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