BoilerFaultCodesThermostat not working? Try the batteries first. Honestly.
Flat batteries in a wireless thermostat are one of the most common heating callouts of the autumn, and the screen often still looks perfectly alive when it happens. It costs two pounds to rule out, so start there before you assume anything worse.
The five-minute check
Change the batteries
Even if the display works. A thermostat can have enough power to light the screen but not enough to hold the radio link to the boiler. Use fresh alkalines rather than rechargeables, which sit at a lower voltage.
Turn it well above room temperature
Wind it up to 25 degrees or so. A thermostat set to 18 in an 19 degree room is working perfectly by doing nothing. You want to force a demand for heat.
Listen for the click
Most thermostats make a soft click when they call for heat. Hear the click but the boiler stays silent and the fault is between the thermostat and the boiler. No click at all and the thermostat itself is the suspect.
Check the programmer is not overriding it
The thermostat only gets a say when the timer says heating is allowed on. If the programmer is between scheduled periods, nothing will happen no matter what you set. Put it to constant to test.
Check the boiler has no fault code
If the boiler is locked out on a fault, it will ignore the thermostat entirely. Look at the display before blaming the controls.
Look up a fault code →Reading the symptom
| What you see | Usual cause |
|---|---|
| Blank screen | Flat batteries, or the fused spur has tripped on a mains-wired unit |
| Screen on, boiler ignores it | Lost wireless pairing, or a failed relay in the receiver |
| Heating runs constantly | Stuck contacts, or the thermostat is sensing cold air from a draught |
| Heating never runs | Set below room temperature, or the programmer is off |
| Room never reaches the set temperature | Thermostat sited badly, near a draught, a radiator or in direct sun |
| Works sometimes | Weak batteries or a marginal wireless signal |
By brand
Most makes have one quirk that accounts for a large share of their faults.
Honeywell
The wireless models pair to a receiver box near the boiler, and losing that pairing is the classic fault. The receiver has a binding or reset button behind its cover to re-pair the two. On the older dial models, the fault is usually the batteries or the receiver relay rather than the dial.
Hive
Three things have to be talking: thermostat, hub and receiver. If the app works but the heating does not, the break is between hub and receiver rather than anything to do with your broadband. Check the receiver by the boiler has power and a steady light, and try a manual boost at the thermostat itself to bypass the app.
Nest
The battery charges from the heat link, and a thermostat that keeps dying is usually a wiring or charging problem rather than a faulty unit. Nest also runs its own schedule learning, so check it has not simply learned a pattern that does not match how you actually live.
Salus
Pairing between the thermostat and its receiver is the common failure, same as Honeywell. The receiver usually has an LED that tells you whether it is receiving a call for heat, which is the quickest way to split the fault between control and boiler.
Worcester Bosch
Worcester controls such as the Comfort and Wave range talk to the boiler over a two-wire connection rather than a simple on-off switch, so a fault can show as a code on the boiler display rather than a dead thermostat. Check the boiler for a code first.
Vaillant
Vaillant controls including the vSMART and calorMATIC range use eBUS. As with Worcester, a communication fault often shows as a code on the boiler rather than a blank thermostat, so read the boiler display before replacing anything.
Where the thermostat is matters
A thermostat in a draughty hallway reads colder than the rooms you actually use, so the heating runs longer than it needs to. One above a radiator or in direct sun reads warmer, and the heating shuts off too early while you sit there cold. If the heating behaves oddly but the thermostat tests fine, look at where it is fitted before assuming it is broken.
What a replacement costs
| Job | Typical cost |
|---|---|
| Basic wired room thermostat, fitted | £120 to £200 |
| Wireless thermostat and receiver, fitted | £180 to £300 |
| Smart thermostat, fitted | £250 to £450 |
| Diagnostic callout only | £60 to £100 |
Safety
Changing batteries, re-pairing a wireless thermostat and adjusting settings are all fine. Anything behind the faceplate is mains wiring and belongs to a qualified electrician or heating engineer. If you smell gas, leave the house and ring 0800 111 999.
Avoid the next call-out bill
Monthly cover means no unexpected bills when your boiler breaks down.
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