Boiler Won't Turn Off? Why Your Heating Is Running Constantly
Radiators stay hot, the boiler keeps firing and your gas bill climbs. Here's what causes a boiler that won't switch off, what you can safely check, and when it's a job for a Gas Safe registered engineer.
Quick answer
A boiler that won't turn off is usually caused by a control setting you can fix yourself: a room thermostat set too high (or with a flat battery), or a programmer left on "constant" rather than timed.
Turn the thermostat right down and check the programmer is set to timed with the correct time and day — if the heating then cycles off, that was the cause.
If the controls and power all check out and the heating still won't switch off, the likely culprit is a stuck motorised or diverter valve or a PCB/wiring fault, which needs a Gas Safe registered engineer.
A boiler that won't turn off is the opposite of the usual complaint — but it's just as frustrating, and it can be expensive. If your radiators stay warm long after the heating "should" be off, or the boiler keeps cycling on through the night, something is telling it to keep producing heat.
Often the cause is a simple control setting you can correct yourself in a couple of minutes. Sometimes it points to a component inside the boiler or heating system that needs an engineer.
Below we walk through the common causes, the checks that are safe for a homeowner to do, and the signs that mean it's time to book a professional.
Why won't my boiler turn off?
There are four typical reasons heating runs constantly:
- The room thermostat is set too high or is faulty. If the thermostat is set to, say, 28°C, your home will rarely reach that temperature, so the boiler never gets the signal to stop. A faulty thermostat — or a dead battery in a wireless model — can also fail to send the "off" signal.
- The programmer or timer is set wrong, or has failed. A programmer left on "constant" (sometimes shown as "On" or "24hr") will run heating around the clock. A faulty timer can stick in the on position even when the schedule says off.
- A stuck motorised valve or diverter valve. These valves direct hot water around your system. If a motorised (zone) valve or the diverter valve inside a combi sticks open, hot water keeps circulating to the radiators regardless of what the controls ask for.
- A wiring or PCB fault. The printed circuit board (PCB) is the boiler's electronic "brain". A wiring fault between the controls and the boiler, or a fault on the PCB itself, can leave the boiler firing when it shouldn't.
Quick distinction: a combi that briefly fires up on its own now and then is often just keeping itself warm for fast hot water (sometimes called pre-heat or eco mode) — that's normal. The problem we're solving here is heating and radiators staying on more or less continuously.
How your heating controls work together
It helps to picture the chain of controls that tells your boiler to fire. Each link can be the reason the heating won't stop, so it's worth knowing the order before you start checking:
The heating-control chain: room thermostat → programmer/timer → motorised (zone) or diverter valve → PCB → boiler fires.
The thermostat and programmer decide whether heat is called for; the valve physically directs hot water to the radiators; the PCB is the electronic brain that ties it all together and ignites the burner. If any one link gets stuck "on", the boiler keeps producing heat.
The common causes, and the fix for each
Below we take each cause in turn. For every one there's a short note on what's safe for you to check and what is Gas Safe engineer only. The golden rule never changes: nothing behind the boiler casing, and nothing involving gas, the flue, combustion or internal wiring, is a DIY job.
1. Room thermostat set too high or faulty
If the thermostat is set higher than the room can realistically reach — say 28°C — your home never hits the target, so the boiler never gets the "stop" signal and keeps firing. A faulty thermostat, or a flat battery in a wireless or smart model, can also fail to send the off signal.
Safe for you to check: turn the thermostat right down (or to its lowest setting). If the boiler and radiators go off within a few minutes, the setting was the cause. Set a comfortable target instead — around 18–21°C suits most homes — and watch whether the heating now cycles off normally.
If you have a wireless or smart thermostat, replace the batteries; a fading battery is a very common cause of a thermostat that stops "talking" to the boiler.
Engineer only: a thermostat that has genuinely failed and needs replacing or rewiring into the boiler. Swapping a like-for-like wired thermostat is electrical work best left to a qualified engineer; any work that involves the boiler's own wiring is Gas Safe territory.
2. Programmer or timer set wrong, or failed
A programmer left on "constant" (sometimes shown as "On" or "24hr") runs heating around the clock. A clock that has drifted — for example after a power cut, or the change to or from British Summer Time — can run heating at the wrong hours. A timer can also stick in the on position even when the schedule says off.
Safe for you to check: make sure the programmer is set to timed rather than constant/on, and that the current time and day are correct. If you've recently changed the schedule, double-check the on and off periods you entered.
If the controls look right but nothing responds, try a safe mains power-cycle: switch the boiler off at its fused spur or plug, wait about 30 seconds, then switch it back on — this clears a hung programmer or controller in the same way restarting a frozen appliance does, without touching anything inside the boiler.
You can also press the front-panel reset button once if your manufacturer's manual allows it.
Engineer only: a programmer or timer that has failed and needs replacing, or that doesn't respond after a power-cycle and one reset. If a reset or power-cycle trips the boiler out again, stop and book an engineer rather than repeatedly resetting.
3. Stuck motorised valve or diverter valve
Motorised (zone) valves and the diverter valve inside a combi direct hot water around your system. If a valve sticks in the open position, hot water keeps circulating to the radiators regardless of what the controls ask for — so the heating stays on even with the thermostat turned down and the programmer set to off.
Safe for you to check: nothing beyond confirming the symptom — if the controls are all correct and the radiators still won't cool, a stuck valve is a strong suspect. A valve usually sits on the pipework near the cylinder or boiler, but diagnosing or freeing it is not a homeowner job.
Engineer only: testing the valve's motor and head, freeing or replacing a seized valve or its actuator, and confirming it seats correctly. Replacing the full valve body may need the system drained, so it's an engineer's job.
4. PCB or wiring fault
The printed circuit board (PCB) is the boiler's electronic brain. A wiring fault between the controls and the boiler, or a fault on the PCB itself, can leave the boiler firing when nothing is calling for heat — the controls are effectively ignored.
Safe for you to check: confirm the boiler and controls have power (check the fused spur or plug and the relevant fuse or breaker), and try the single front-panel reset and mains power-cycle described above. That is the limit of safe homeowner action.
Engineer only: everything else. Diagnosing and replacing a PCB, or tracing and repairing a wiring fault behind the casing, needs the right test equipment and Gas Safe qualifications. Never open the casing to inspect the board or wiring yourself.
Safety first. Do not remove the boiler casing, touch the gas valve, gas pipework, the flue or any internal wiring. Anything inside the boiler is gas and electrical work that must be carried out by a Gas Safe registered engineer. If you ever smell gas, call the National Gas Emergency line on 0800 111 999.
Safe for you vs Gas Safe engineer only — at a glance
If you only remember one thing, make it this split. The left-hand list is the full extent of safe homeowner action; everything on the right needs a registered professional.
What happens if the heating runs constantly?
Beyond the discomfort of an overheated home, a boiler that won't switch off is worth fixing promptly for three reasons:
- Wasted gas and higher bills. Heating that runs around the clock burns gas you don't need, and a boiler forced to keep firing typically consumes more fuel than one cycling normally — so the cost shows up directly on your next bill.
- Premature component wear. Constant running means the burner, pump, valves and other parts clock up far more hours than intended, which can bring forward breakdowns and shorten the boiler's life.
- Overheating and rising pressure. A system that can't stop heating can run hotter than it should and push the pressure up. If you see the gauge climbing well above normal (typically around 1–1.5 bar cold, and not far above that when hot), turn the heating off at the controls and call an engineer — persistently high pressure should be checked professionally.
None of this is usually an immediate emergency, but it's exactly why you shouldn't just live with it: the longer it runs, the more it costs and the harder it works the system. The gas-safety boundary still applies — investigate the controls yourself, but leave anything inside the boiler to a Gas Safe engineer.
When it's a job for an engineer
If the thermostat, programmer and power all check out and the heating still won't switch off, the cause is most likely inside the system — and that's where a homeowner's safe checks end. Typical engineer-only faults include:
- A stuck motorised or diverter valve. An engineer can test the valve's motor and head, free or replace it, and confirm it's seating correctly.
- A PCB or wiring fault. Diagnosing and replacing a printed circuit board, or tracing a wiring fault between the controls and boiler, needs the right test equipment and qualifications.
- A faulty thermostat or programmer that needs replacing rather than just resetting.
Indicative repair costs (2026)
The figures below are indicative, last checked 2026, and include parts and typical labour unless stated. Real prices vary by make, model, region and how much work is involved, so treat these as a guide for budgeting, not a quote.
| Job / component | Indicative 2026 cost | Notes |
|---|---|---|
| Room thermostat replacement (standard wired) | ~£75–£200 | Unit plus labour; simple like-for-like swap |
| Smart thermostat (supply & install) | ~£160–£370 | Brand-dependent; extra if new wiring is needed |
| Programmer / timer replacement | from ~£105 | Unit plus a couple of hours' labour |
| Motorised / diverter valve (actuator or full valve) | ~£120–£300 | Actuator-only is cheaper; full valve may need a drain-down |
| Wiring fault diagnosis | ~£40–£90+ per hour | Charged at labour rate; cost depends on time to trace |
| PCB (printed circuit board) replacement | ~£300–£700 | Parts ~£180–£400 plus labour; weigh against a new boiler if old |
| Typical heating engineer labour | ~£40–£60/hr (more in London) | Day rates roughly £220–£300; call-out may be extra |
Indicative, last checked 2026. Prices vary by make/model/region and are not a quote.
Why does my combi keep firing up when no one is using hot water?
This one is usually not a fault. Many combi boilers have a pre-heat or "eco/keep-warm" feature that briefly fires the burner now and then to keep the heat exchanger warm, so hot water arrives at the tap faster.
You'll notice short bursts every so often even when no tap is open and the heating is off — that's by design, and most boilers let you turn pre-heat off in the settings if you'd rather save the small amount of gas it uses.
The problem this page is about is different: heating and radiators staying on more or less continuously. If your combi is only firing in brief, occasional bursts and the radiators are cold, it's almost certainly normal pre-heat, not a boiler that won't turn off.
Where boiler cover fits in
Faults like a stuck valve or a failed PCB are exactly the kind of unpredictable, mid-range repair bills that boiler cover is designed to smooth out.
A policy typically gives you an annual service plus repairs to the boiler and, depending on the tier, parts of the wider heating system — for a fixed monthly cost instead of a one-off bill. Whether that maths works for you depends on your boiler's age and the plan's excess and limits.
If you're weighing it up, our guide to whether boiler cover is worth it and our what is boiler cover explainer cover the basics. You can also compare boiler cover across our selected panel of providers to see what's included before you commit.
| Symptom | Likely cause | Safe to check? |
|---|---|---|
| Radiators hot all the time, thermostat set very high | Thermostat setting | Yes — turn it down |
| Heating runs at the wrong times | Programmer/timer set wrong or clock drifted | Yes — check schedule |
| Smart/wireless thermostat unresponsive | Flat batteries | Yes — replace batteries |
| Settings correct but heating still on | Stuck motorised/diverter valve | No — engineer |
| Controls ignored entirely; boiler keeps firing | PCB or wiring fault | No — engineer |
Is it dangerous if my boiler won't turn off?
In most cases it's a controls or component fault rather than an immediate danger, but a boiler that can't be switched off normally still needs investigating — both to avoid wasted gas and because it could signal a failing part.
If you smell gas, hear unusual banging, or see the pressure climbing well above normal (around 2 bar when hot), turn the heating off at the controls and call a Gas Safe registered engineer. For a suspected gas leak, ring 0800 111 999.
Why do my radiators stay hot even when the heating is off?
The most common reasons are a programmer left on "constant", a thermostat set higher than the room reaches, or a motorised/diverter valve stuck open so hot water keeps circulating. Check the controls first; if they're correct and radiators still won't cool, a stuck valve is the likely culprit and needs an engineer.
Can a thermostat cause the heating to stay on?
Yes. If it's set too high the boiler never gets the "target reached" signal, so it keeps firing. A faulty thermostat or a flat battery in a wireless one can also fail to send the off signal. Turning it down or replacing the batteries is a safe first test.
Should I keep resetting the boiler?
Press the front-panel reset button only once, and only if your manufacturer's manual says it's appropriate. Repeatedly resetting a boiler that keeps faulting can mask a real problem and, in some cases, make things worse. If one reset doesn't fix it, book a Gas Safe registered engineer.
Will boiler cover pay for a stuck valve or PCB?
Many policies cover repairs to the boiler and heating controls, which can include motorised valves and the PCB — but cover, excesses and exclusions vary between providers and tiers. Always read the policy terms, and note that faults present before you took out the cover are usually excluded. Comparing plans helps you see exactly what's included.
Use our tool to compare cover across our panel and find a plan that fits your boiler and your budget.
Compare boiler cover before the next breakdown
See what's covered, the excess and the monthly cost across our selected panel of UK providers — so a stuck valve or a failed PCB doesn't land as a surprise bill.
Compare boiler coverThis article is general information, not advice, and covers a selected panel of providers rather than the whole market. Boiler Cover UK is an independent comparison site and may earn a commission when you take out a policy through our links. Gas appliance work must always be carried out by a Gas Safe registered engineer.