Thermostat Not Working? How to Tell If It's the Stat, the Boiler or the Radiator
If your thermostat is not working, the first job is to find out whether it is actually the thermostat at all. Most of the time on our vans, “the thermostat has packed up” turns out to be one of three separate things: a room stat that has stopped calling for heat, a radiator valve that has seized, or a boiler receiving the demand perfectly well and failing to act on it.
They feel identical from the hallway. They are completely different repairs. Here is the order our engineers work through, and the parts of it you can safely do yourself first.
First, work out which thermostat you mean
People use the word for two different things, and the fix has nothing in common between them.
The room thermostat is the one on the hall or living room wall - a dial, a small screen, or an app-controlled unit. Its only job is to switch the whole heating system on and off based on the temperature of that one room.
The radiator thermostat, or TRV, is the numbered valve on the side of an individual radiator. It does not switch the boiler on. It only throttles hot water into that one radiator once the boiler is already running.
So if every radiator is stone cold, look at the room stat. If one radiator is cold and the rest are hot, the room stat is doing its job and the problem is at that radiator - our guide to finding the cause when one radiator will not heat up covers that fault instead.
Prove whether the stat is calling for heat
This is the single most useful test, and it takes a minute.
Turn the room thermostat well past the current room temperature - to 25 or 30 degrees. On most stats you will hear a soft click, see a flame symbol or a “heating on” indicator appear, and within a few seconds hear the boiler fire.
- Click, indicator, boiler fires: the thermostat is working. Your fault is downstream - the boiler, a valve, or the radiators.
- Click and indicator, but silence from the boiler: the stat is calling for heat and nothing is responding. That is usually a wiring, motorised valve or boiler fault.
- No display, no click, nothing: the stat itself has lost power or failed.
Note which of those three you got. It is the first thing we will ask, and it often saves a visit.
Thermostat not working but the boiler is on
This one confuses people, and it is often not a fault at all.
If the boiler is running and the house still will not warm up, the stat may be doing exactly what it was told. Check what the programmer or schedule actually says - a stat set to 18 degrees in a room already at 19 will not call for heat no matter how cold the back bedroom is. Smart stats are particularly good at this: a holiday mode, or a schedule someone set in March, will quietly override anything you do at the dial.
The other version is a boiler that fires briefly, heats the hot water and stops. If your hot water is fine and the radiators are not, that is a controls or valve fault rather than a boiler fault - and worth reading alongside what it means when the heating works but there is no hot water, which is the same problem in reverse.
The five causes we find most often
1. Flat batteries
Far and away the most common. Wireless and battery-backed stats do not always warn you clearly - some go blank, some show a faint display that looks alive but has no power to transmit. Fit fresh alkaline batteries, not rechargeables, and give it a couple of minutes to re-establish the link.
2. Lost pairing on a wireless stat
Wireless stats talk to a receiver box, usually beside the boiler or in an airing cupboard. If that pairing drops - after a power cut, a battery change, or a new router installed near it - the stat looks perfectly normal and the boiler simply never hears it. Most makes have a re-pairing sequence in the manual. If the receiver has a green light and the stat has a display but nothing happens between them, that is your fault.
3. A schedule nobody remembers setting
Programmable and smart stats hold their schedule through a power cut. Every autumn we attend jobs where the heating is “broken” and the real answer is that it is set to come on for an hour at 6am, because that is what suited the house in April.
4. A seized motorised valve
In a system with a hot water cylinder, a motorised valve directs hot water to the radiators or to the cylinder. When one seizes - and after a summer sitting still, they do - the stat calls for heat, the boiler is willing, and the water never reaches the radiators. You can sometimes hear the actuator humming without moving. This is an engineer’s job.
5. The stat has genuinely failed
Contacts wear out, and older mechanical stats drift so far out of calibration that a setting of 20 delivers something closer to 15. Replacement is straightforward and usually a good moment to reposition it - on an internal wall, away from radiators, draughts, lamps and direct sun, all of which make a stat read the room wrongly.
Radiator thermostat not working
A TRV that has stopped controlling its radiator is almost always a stuck pin rather than a broken valve. The pin sits under the plastic head and is pushed down to close the valve; a summer spent fully open leaves it corroded in place.
Take the head off - it usually unscrews by hand or with a small nut - and you will see a small metal pin. If it is free, it springs back when pressed. If it is solid, that is your fault, and gently working it loose often restores it. Do not force it: snapping a pin turns a five-minute job into a drained system.
Two things worth knowing. TRVs have a service life, and after fifteen or twenty years in hard water they are consumables rather than repairs. And the radiator in the same room as the room stat should have a lockshield or fully open valve rather than a TRV, otherwise the two controls fight each other.
Smart thermostats
We fit and work alongside Hive, Nest, tado and Honeywell controls across East Kent, and the failure modes differ from a traditional stat. A smart stat that has lost wi-fi usually keeps running its last known schedule, so the heating carries on while the app shows nothing. A hub unplugged or moved during decorating breaks the chain between app and boiler even though every individual part looks healthy.
Work through it in order: does the stat have power, does the hub have power and a connection, and does the receiver at the boiler respond to its manual boost button? That button bypasses the app entirely. If the boiler fires from the receiver but not from the app, the fault is in the controls chain, not the boiler.
What you can safely do yourself, and what you cannot
Batteries, schedules, re-pairing a wireless stat and freeing a TRV pin are all reasonable homeowner jobs. So is checking that nobody has knocked the isolation switch by the boiler.
Anything beyond that is not. Wiring at the boiler or receiver, motorised valves, PCBs and anything inside the boiler casing must be handled by a Gas Safe registered engineer - we are registration number 2836. It is not only a legal point: a stat miswired into a modern combi can leave the appliance running in a way it was never designed to.
If the boiler is showing a fault code rather than simply ignoring the stat, that is a different diagnosis. Our boiler fault code guides explain what the common Glow-worm codes mean, and a boiler losing pressure will lock out and look like a controls fault when it is nothing of the kind.
What a controls fault costs to put right
Controls diagnosis is charged at our hourly rate rather than a fixed price, because until we have tested the chain nobody knows whether it is a receiver setting or a failed valve. Our labour rate is £90 plus VAT for the first hour, which is always billable, then £45 plus VAT per half hour after that, with parts on top. A straightforward stat swap or TRV replacement is usually well inside that first hour.
Most controls faults are also things a proper annual service catches early - a sticking motorised valve announces itself long before it strands you. Members on a BOYLA HomeCare plan pay no call-out fees, and the plan includes the annual service. Our rates are set out in full on the pricing page.
Get it sorted
If you have run through the checks above and the heating still will not come on, ring us on 01304 381999. Tell us which of the three results you got when you turned the stat up, and we can usually arrive with the right part on the van. We are Deal-based, covering Deal, Dover, Sandwich, Canterbury, Thanet and the rest of East Kent, and we book same-day breakdown repairs within working hours wherever the diary allows.