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.
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.
There are four typical reasons heating runs constantly:
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.
These checks involve only the controls and the power supply — never the gas, flue or anything behind the boiler casing. Start at the top and work down.
Turn the thermostat right down (or to its lowest setting). If the boiler and radiators go off within a few minutes, your thermostat was simply set higher than the room could reach. Set it to a comfortable target — around 18–21°C suits most homes — and see 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.
Look at whether the programmer is set to timed rather than constant/on. Make sure the current time and day are correct — a clock that's drifted (for example after a power cut or the change to or from British Summer Time) can run heating at the wrong hours. If you've recently changed the schedule, double-check the on and off periods you entered.
If settings look right but nothing responds, it's worth confirming the boiler and controls have power: check the fused spur or plug and the relevant fuse or breaker. A front-panel reset button can be pressed once if your manufacturer's instructions allow it. If it trips out again, stop and book an engineer rather than repeatedly resetting.
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.
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 valve or a PCB replacement isn't cheap. As a rough, indicative guide for 2026, a motorised valve repair or replacement often lands somewhere around £120–£250, and a combi PCB can run from roughly £300 to £600 or more once parts and labour are included. Prices vary widely by boiler make, model and where you live.
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 |
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.
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.
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.
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.
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.
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.