Thermostat not working: what to check before you call
When a room thermostat stops working, the cause is usually one of four things: flat batteries or no power, a wireless thermostat that has lost its link to the receiver by the boiler, a setting such as holiday mode or a schedule that is keeping the heating off, or a boiler that is not responding even though the thermostat is asking for heat. The quickest test is to turn the thermostat up to about five degrees above the room temperature and wait two or three minutes. If you hear a click, see a flame or heating symbol, or the receiver light changes, the thermostat is doing its job and the problem is further along: the boiler, the wiring or a valve. If nothing happens at all, the thermostat or its power supply is the place to start.
Loyal Plumbers · Updated
Discuss your plumbing workWhat does 'not working' look like in your home?
What you can see on the thermostat and the boiler narrows the fault down quickly. Note what the display shows before pressing buttons, because some thermostats lose their settings when the batteries are changed.
| What you notice | Most likely cause | What to do first |
|---|---|---|
| Screen is blank or very faint | Flat batteries, or no power to a wired thermostat | Fit fresh batteries; for a wired model, check the boiler and heating controls have power |
| Screen works but the heating never comes on | Schedule, holiday or frost mode, a set point below room temperature, or the boiler not responding | Turn it up well above room temperature and watch the boiler |
| Wireless thermostat shows a no-signal symbol, or the receiver light is red or flashing | The thermostat and receiver have lost their pairing | Move the thermostat closer, then follow the re-pairing steps in its manual |
| Heating will not switch off | Stuck relay, a zone valve jammed open or a wiring fault | Switch the heating off at the programmer or boiler and book a repair |
| Reading is several degrees out | Thermostat near a radiator, in sun or in a draught, or an old dial that has drifted | Compare with a separate thermometer and check where it is mounted |
| Old dial thermostat clicks but nothing starts | The thermostat is switching; the fault is in the boiler, valve or wiring | Check the boiler display and pressure, then call an engineer |
Blank screen: batteries and power
Most wireless and many wired digital thermostats run on AA or AAA batteries. A low-battery symbol often appears for weeks before the screen goes blank, and a weak battery can keep the display alive but not the radio, so the thermostat looks fine while the boiler never hears it. Fit a fresh set of good-quality alkaline batteries the right way round, rather than mixing old and new.
Some thermostats are powered from the heating wiring instead. If a wired thermostat is blank, check whether the boiler and programmer have power too. A tripped circuit at the consumer unit or a switched-off fused spur next to the boiler will kill the whole set of controls. After a power cut, the programmer's clock and schedule may also have reset, which our guide to heating controls after a power cut covers step by step.
A wired room thermostat in a UK heating system is often connected at mains voltage. Do not unclip one from its wall plate or touch the terminals unless the power to the heating controls is isolated, and leave any wiring changes to an engineer or electrician.
Wireless thermostats that lose their connection
A wireless thermostat talks to a receiver, usually a small box on the wall near the boiler or wired into it. If the pairing between the two drops, the thermostat can show the right temperature and look perfectly normal while the boiler never gets the call for heat. Most receivers have a light that shows whether they are linked and whether they are calling for heat. The colours differ between makes, so check the manual for yours.
Common reasons for a lost link are flat batteries, a power cut at the receiver, the thermostat being moved further away or behind something metal, or a new receiver fitted during boiler work. Try the thermostat in the same room as the receiver. If it reconnects there, the old spot is out of range.
Every manufacturer has its own re-pairing, or binding, procedure, usually a sequence of button presses on the receiver and then on the thermostat. Honeywell Home and Drayton both publish model-specific manuals. Follow the one for your model rather than a general video, because the wrong sequence can clear other settings too.
With a smart thermostat, an app showing offline often means a Wi-Fi or hub problem rather than a heating fault. Check whether the heating responds to the thermostat itself before assuming the system has failed.
Settings that look like a fault
A surprising number of 'broken' thermostats are doing exactly what they have been told. Before booking a repair, check these:
- The set temperature is below the current room temperature, so it never calls for heat
- The thermostat is in holiday, away, eco or frost-protection mode
- The schedule is in an off period, or the clock is wrong after a power cut or the clocks changing
- On a system with a separate programmer, the heating channel is set to off or to hot water only
- On a combi boiler, the heating dial or setting on the boiler itself has been turned off or right down
- The thermostatic radiator valves in the rooms are turned down, so radiators stay cool even though the boiler runs
Timer and thermostat working against each other
On many systems the programmer decides when the heating is allowed to run and the room thermostat decides whether it needs to. Both have to agree before the boiler fires. Our guide on how to set a heating timer explains the channels and on/off periods, and the page on thermostatic radiator valves explains the numbers on each radiator valve. Keep the radiator valve in the thermostat's own room fully open, or the room never reaches the setting and the boiler keeps running.
Our guide on how to turn on central heating shows how the boiler, programmer and room thermostat each need to be set for the radiators to come on.
Wrong temperature on the display
A thermostat measures the air right around it. Mounted above a radiator, in afternoon sun, next to a kitchen, on a cold outside wall or in a draughty hall, it will read high or low and switch the heating at the wrong time. The Energy Saving Trust advises keeping a room thermostat where air can circulate freely, away from heat sources and draughts, and not blocked by curtains or furniture.
Put a separate thermometer next to it for an hour. If the two agree, the thermostat is simply in the wrong place. Older dial thermostats can drift and are usually replaced rather than repaired. Moving a wired thermostat means running new cable, which is a job for an engineer.
When the thermostat is calling but the boiler does nothing
If the thermostat shows it is calling for heat and the receiver light agrees, but the boiler does not fire, the thermostat is probably fine. Look at the boiler. A fault code or flashing light, a pressure gauge below about 1 bar on a sealed system, or a frozen condensate pipe in cold weather will all stop it responding. Our guide to a boiler not working walks through those checks in order.
On systems with a hot water cylinder, the heating call from the thermostat usually goes through a motorised zone valve, which has to open before the boiler will run for the heating. If the valve motor has failed, you often still have hot water but no heating. The pages on hot water but no heating and on zone-valve faults describe what to look for.
If the heating will not switch off even with the thermostat turned right down, read our guide for when the heating stays on when the thermostat is off.
Repair, re-pair or replace?
Once batteries, settings and pairing have been ruled out, an engineer will confirm whether the thermostat is actually switching, then test the receiver, wiring and any zone valve, so you only pay for the part that has failed. The options are then to repair the wiring or receiver, or to replace the thermostat. A like-for-like wireless set is often the simplest fix. If you are replacing it anyway, our guide on upgrading heating controls explains what to ask for. Any new thermostat has to be compatible with the boiler: some modern boilers use their own low-voltage communication rather than a simple on/off switch, and the wrong type can stop the boiler modulating properly.
Get the heating back on
Loyal Plumbers covers London and everywhere inside the M25, 24 hours a day, with 10 vans on the road. Our Gas Safe registered engineers repair thermostats, receivers, zone valves and boilers. Call 020 4652 8585 and tell us the make of thermostat, what the display and the boiler show, and whether you still have hot water.
Our call-out charge is £60, and the engineer explains what they have found and what the repair involves before starting. You can read more about how we handle central heating repairs, or about boiler repair and servicing if the fault turns out to be in the boiler.
Common questions
- Why is my thermostat on but the heating not coming on?
- Check the set temperature is above the room temperature, the schedule is in an on period and the programmer allows heating. If the thermostat shows it is calling for heat and the boiler still does not fire, the fault is in the boiler, a zone valve or the wiring.
- How do I reset a room thermostat?
- It depends on the model. Many digital thermostats have a reset option in the menu, and some restart when the batteries are taken out for a minute. Wireless ones may then need re-pairing to the receiver. Use the manual for your model so you do not clear the schedule by mistake.
- Can a faulty thermostat stop my hot water?
- On a combi boiler, no: the room thermostat only controls the heating. On a system with a hot water cylinder, the cylinder has its own thermostat, but a fault in the shared programmer or wiring centre can affect both heating and hot water.
- Why is my Honeywell or Drayton thermostat not working?
- The same checks apply to both: batteries, the link to the receiver, the mode and schedule, and whether the boiler responds. Both makes publish manuals with model-specific re-pairing steps, so find the model number on the back or inside the battery cover.
- Where should a room thermostat be fitted?
- In a room you use regularly, often the hall or living room, on an inside wall where air can circulate freely. Keep it away from radiators, direct sun, draughts and cooking heat, and do not hide it behind curtains or furniture.
Sources and further reading
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