How the thermostat talks to the boiler
A room thermostat is a switch. When the room is colder than the set temperature, it closes a circuit that tells the boiler to fire for heating. On a wired thermostat, that signal travels through a cable. On a wireless or smart thermostat, it goes by radio to a receiver, usually a small box on the wall near the boiler or built into the boiler itself.
Along the way, the signal may also pass through a timer or programmer, a wiring centre and, on system boilers, a zone valve. A fault anywhere in that chain looks the same from the living room: the thermostat says it is heating and the boiler does nothing.
Quick checks first
- Turn the thermostat up by at least 3 or 4 degrees above the room temperature. A small change may not trigger it.
- Check the boiler is in heating mode. Many combis have a summer mode that disables the radiators, and some have a separate heating on or off button.
- Check the boiler's heating temperature is not turned right down.
- Check the timer or programmer. The heating channel must be on, in a timed on period or on boost. A thermostat cannot turn the boiler on during an off period.
- Look for a fault code on the boiler. If the boiler is locked out, it will not respond to anything until the fault is cleared. Codes such as Ideal L2 or Worcester Bosch EA 227 mean the boiler failed to light and needs attention, not the thermostat. Our guide on how to reset a boiler covers that.
- Check the pressure is 1 to 1.5 bar with the system cold. If it is low, follow our repressurising guide.
Wireless thermostats: batteries and pairing
Batteries
Flat batteries are the single most common cause. Many wireless thermostats keep showing a display on weak batteries but no longer transmit reliably. Replace all the batteries with new ones of the right type, not ones from a drawer.
Pairing or binding
The thermostat and receiver are paired so the receiver only listens to your thermostat. A battery change, power cut or new receiver can break that link. The re-pairing process is different for every model, so check the manual. In general terms, you put the receiver into learn or pairing mode (often by holding a button until a light flashes), then trigger pairing from the thermostat. Manufacturers such as Honeywell Home, Drayton, Salus, Worcester Bosch, Hive and Google Nest all publish these steps online.
Range and interference
Wireless thermostats have a limited range, and thick stone walls, foil-backed plasterboard, mirrors, large metal objects and even a new fridge freezer can block the signal. If the problem started after you moved the thermostat or rearranged a room, move it closer to the receiver and see whether the connection returns. Some systems let you check signal strength on the thermostat display or in the app.
The receiver
Check the receiver near the boiler. Most have lights showing power, signal and whether it is calling for heat. No lights at all can mean it has lost power, perhaps because the fused spur or plug feeding the boiler and controls is off. A red or flashing light usually means no signal from the thermostat. Some receivers have a manual override button that makes the boiler fire for heating. If that works, the receiver and boiler are fine and the problem is the signal from the thermostat.
| What you see | Likely cause | What to do |
|---|---|---|
| Thermostat display blank or faint | Flat batteries | Fit new batteries |
| Receiver has no lights | No power to receiver | Check the fused spur and plug |
| Receiver shows no signal | Lost pairing or out of range | Re-pair, move thermostat closer |
| Receiver says heating on, boiler silent | Wiring, zone valve or boiler fault | Call an engineer |
| Smart app shows offline | Wi-Fi or hub problem | Restart router and hub |
Smart thermostats
With a smart thermostat such as Hive, Nest, tado° or Drayton Wiser, check the app first. Schedules, away modes, eco settings and geofencing can all hold the heating off without it being obvious. Make sure the thermostat is not in frost protection or holiday mode. If the app shows the hub or thermostat as offline, restart the internet router and the hub, then give them a few minutes to reconnect.
Some smart thermostats are wired to the boiler through its own timer. If the boiler has a built-in timer or clock, it usually needs to be set to constant on so the smart thermostat is in charge. If the boiler's timer is in an off period, the thermostat's calls for heat will be ignored.
After a power cut, check both the smart thermostat app and any clock on the boiler. Mechanical timers keep turning while the power is off and can end up hours out.
Wiring and receiver faults: engineer only
If the receiver shows it is calling for heat and the boiler still does not fire, the problem is in the wiring, the wiring centre, a zone valve or the boiler's control board. These are connected at mains voltage. Do not open the receiver backplate, wiring centre or boiler casing.
Wiring between thermostats, receivers and the boiler should be checked by a Gas Safe registered engineer or a qualified electrician. Anything inside the boiler casing must only be worked on by a Gas Safe registered engineer.
On a system boiler with a hot water cylinder, a failed heating zone valve can also stop the call for heat reaching the boiler. Our guide to heating not working when the hot water is explains how that shows up.
Typical costs in the UK
Prices depend on the thermostat model, whether the existing wiring can be reused and your area. London and the South East are usually at the top of these ranges.
| Job | Typical UK cost |
|---|---|
| Diagnostic call-out | Typically £60 to £120 |
| Replacement receiver (same brand) | Typically £80 to £200 fitted |
| New wired or wireless room thermostat | Typically £100 to £250 fitted |
| Smart thermostat supplied and fitted | Typically £200 to £350 |
A replacement receiver from the same manufacturer is usually the cheapest fix, because the existing wiring and thermostat can stay. Switching brands or moving to a smart system costs more, especially if the wiring centre needs changing. An engineer can also check the controls at your annual service; see our guide to boiler service costs.