Why is my thermostat not turning the heating on?

A thermostat that has stopped calling for heat is one of the more frustrating heating faults, because the boiler is fine and the house is cold. It is also one where the first two checks are free and resolve most of them.

Safety

Step by step

1. Check the display is on.A blank or faint display on a wireless thermostat is almost always flat batteries. They last a year or two and fail in cold weather, which is when you notice.
2. Change the batteries anyway.Even if the display looks fine. A thermostat can have enough power to show a display and not enough to transmit reliably to the receiver, which produces exactly this intermittent behaviour.
3. Turn it well above room temperature and listen.Set it several degrees higher than the room. A mechanical thermostat clicks audibly; a digital one shows a flame or heating symbol. That tells you whether the thermostat itself is calling for heat.
4. Check the receiver, not just the thermostat.Wireless systems have a receiver box near the boiler with its own lights and often a manual override. If the thermostat calls for heat and the receiver shows nothing, the pairing has been lost.
5. Re-pair a wireless thermostat.Every wireless system has a pairing procedure, usually in the manual and often available online by model. Pairings are lost after battery changes, power cuts and interference, and re-pairing is a householder task.
6. Check the programmer or timer separately.A thermostat only calls for heat during a period the timer allows. A schedule that drifted after a clock change, or a holiday mode left on, looks exactly like a broken thermostat.
7. Check the boiler is otherwise working.Run the hot water. If hot water works and heating does not, the boiler is fine and the fault is in the heating control chain rather than the appliance.
8. Use the receiver's manual override to test.Most receivers have an on-off-auto switch. Setting it to on should fire the boiler. If it does, the boiler and wiring are fine and the problem is the thermostat or the pairing. Set it back to auto afterwards.
9. Consider the thermostat's position.A thermostat above a radiator, in direct sun, in a draught, or near a lamp reads the wrong temperature and stops calling for heat while the rest of the house is cold. It is working correctly and being misled.
10. Check for a smart thermostat losing its connection.Smart models depend on wifi and sometimes on a hub. A router change, a new network password or a firmware update can break the link. Most keep a local schedule, but not all.
11. Check the thermostatic radiator valves too.If the boiler fires but rooms stay cold, the room thermostat is working and the valves are the problem — commonly a seized pin after summer.
12. Call an engineer if the receiver override does nothing.That points at the wiring, the wiring centre, a zone valve or the boiler itself. None of those is a householder repair.

Tips

• Photograph the thermostat's model number before searching for a pairing procedure. They differ between models from the same manufacturer.
• Change thermostat batteries every autumn as routine, in the same way as smoke alarms. It removes the most common cause entirely.
• If the heating works when the thermostat is turned right up but not at a normal setting, the thermostat is reading the room wrongly rather than failing.

Common mistakes

• Assuming the boiler has failed — If hot water still works, the boiler is fine. The fault is in the control chain — thermostat, pairing, timer or valve — which is where to look.
• Leaving the receiver on manual override — It runs the heating continuously with no control at all. Use it to test, then set it back to auto and fix the actual fault.

If it doesn't work

Questions people ask