When the radiators stay cold, the cause is often something simple you can check in a few minutes. Here are the seven most common reasons UK central heating stops working, the safe checks you can do yourself, and when it's a Gas Safe engineer's job.
A heating system that won't fire up is frustrating, especially in winter. The good news is that many causes are minor and fixable without a tool in sight. The list below runs from the easiest checks to the ones that need a professional. Work through them in order before you call anyone out.
It sounds obvious, but the most common reason heating "stops working" is a thermostat that has been knocked, run flat or set too low. Check that your room thermostat is set higher than the current room temperature — if the room is already 20°C and the dial is on 18°C, the boiler has no reason to fire.
Safe to check yourself: turn the thermostat up well above room temperature and listen for the boiler to respond. If it's a battery-powered or wireless stat, replace the batteries — a fading battery often shows a blank or dim display. Smart thermostats can lose their connection to the hub after a router reset, so check the app is showing "online".
Engineer job: if a new thermostat or fresh batteries make no difference and other zones are also dead, the wiring or the boiler's control board may be at fault. That's an engineer's diagnosis.
Your programmer (or timer) tells the system when to heat. After a power cut, a clock change or a flat backup battery, programmers often reset to a default schedule — or to the wrong time entirely — so the heating runs at hours you don't expect.
Safe to check yourself: confirm the current time and day are correct on the programmer, and switch it to "Constant" or "On" to test. If the radiators warm up on constant but not on the timed schedule, you simply need to reprogram the on/off periods.
Engineer job: if the programmer display is dead with the power on, or buttons don't respond, the unit itself may have failed and need replacing.
A combi or system boiler needs water pressure to circulate heat around the house. If the pressure drops too low, the boiler often shuts down as a safety measure and may show a fault code or warning light. Look at the pressure gauge: a normal cold reading is around 1 to 1.5 bar, rising to roughly 2 bar when the system is hot. Below about 1 bar is too low.
Safe to check yourself: you can top the system back up yourself using the filling loop — usually a silver braided hose with one or two valves under the boiler. Our step-by-step guide on what to do when boiler pressure is too low walks you through it safely.
Engineer job: if the pressure keeps dropping again and again, there's a leak somewhere in the system or a failed expansion vessel. Repeated top-ups are a warning sign, not a fix — book an engineer to find the cause.
Modern condensing boilers produce a small amount of acidic water that drains away through a condensate pipe. If that pipe runs outside, it can freeze solid in cold weather, blocking the drain and causing the boiler to lock out — often with a gurgling sound and a fault code. This is one of the most common winter call-outs, and frequently one you can clear yourself.
Safe to check yourself: if the blockage is in an accessible external pipe, you can gently thaw it by pouring warm (not boiling) water over the frozen section, then reset the boiler once. Our guide on a frozen condensate pipe explains exactly how. Boiling water can crack the pipe, so use warm water only.
Engineer job: if the pipe is high up, hard to reach, or refreezes repeatedly, or if the boiler won't restart after thawing, call an engineer. They can also fit a wider or better-lagged pipe to stop it happening again.
If the boiler is firing but some radiators are cold at the top while warm at the bottom, air is trapped inside them. Trapped air stops hot water reaching the whole surface, so rooms never quite warm up even though the system is "working".
Safe to check yourself: bleeding a radiator is a simple, homeowner-friendly job that needs only a radiator key and a cloth. See our guide on how to bleed a radiator. Afterwards, recheck the boiler pressure, as bleeding can lower it slightly and you may need a small top-up.
Engineer job: if radiators are cold at the bottom rather than the top, or you're bleeding the same radiator constantly, the system may be full of sludge or need a power flush — that's a professional job.
A motorised valve directs hot water between your radiators and your hot water cylinder; in a combi, an internal diverter valve switches between heating and hot taps. When these seize up, you can get classic symptoms like plenty of hot water at the taps but no heating, or vice versa.
Safe to check yourself: there's very little a homeowner can safely do here beyond confirming the symptom — for example, noting that the taps run hot but no radiators warm up. That observation helps the engineer diagnose the fault faster.
Engineer job: diverter and motorised valves involve the sealed heating circuit and, in a combi, the boiler's internals. Replacing or freeing a stuck valve is a job for a qualified heating engineer, not a DIY task.
The circulation pump pushes hot water around your radiators. If it fails or jams, the boiler may light but the heat never moves, so the boiler can overheat and cut out, or the radiators simply stay cold despite everything else being correct.
Safe to check yourself: if you can locate the pump (often a chunky unit near the boiler or in an airing cupboard), you may be able to tell whether it's running by a faint hum or warmth — but do not attempt to open, bleed or restart it yourself.
Engineer job: a seized or failed pump needs testing and replacing by a heating engineer. It's a common wear item, especially on older systems, and not something to tackle at home.
| Symptom | Likely cause | Your first move |
|---|---|---|
| Nothing fires at all | Thermostat, programmer, power | Check stat, timer, fuse and power |
| Boiler off, low gauge reading | Low pressure | Top up via the filling loop |
| Gurgling + lockout in cold weather | Frozen condensate | Thaw external pipe with warm water |
| Some radiators cold at the top | Trapped air | Bleed the radiator |
| Hot water fine, no heating | Valve or pump | Note the symptom, call an engineer |
Most of the seven causes above eventually need a professional: a failed pump, a stuck valve, a persistent leak or a faulty control board are all repair jobs that can run into hundreds of pounds. Boiler cover spreads that cost into a monthly payment and gives you a number to call when the heating goes down. If you're weighing it up, our guide on whether boiler cover is worth it and our overview of what boiler cover includes will help you decide what level of protection fits your home.
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Compare boiler coverIf the boiler fires but no heat reaches the radiators, the most likely culprits are trapped air, a stuck motorised or diverter valve, or a failed circulation pump. Try bleeding the radiators first — if that doesn't help, a valve or pump is probably to blame and you'll need a Gas Safe registered engineer.
For most UK combi and system boilers, normal pressure is around 1 to 1.5 bar when the system is cold, rising to roughly 2 bar when hot. Below about 1 bar is too low and the boiler may shut down until you top it back up via the filling loop.
Some checks are perfectly safe: adjusting the thermostat or timer, replacing stat batteries, topping up pressure, bleeding radiators, thawing an external condensate pipe and pressing the reset once. Anything involving the gas supply, the flue, the pump, internal valves or the boiler's sealed parts must be left to a Gas Safe registered engineer.
An overnight failure in winter very often points to a frozen condensate pipe, which freezes in cold weather and locks the boiler out. A flat thermostat battery, a programmer reset after a power cut, or a slow pressure drop are also common overnight causes. Work through the list above before calling anyone out.
This article is general information, not personalised advice. We compare a selected panel of providers, not the whole market, and may earn a commission if you buy through our links. Always use a Gas Safe registered engineer for any gas or boiler work.