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A power cut can leave your boiler dead, flashing, or simply ignoring the timer. In most cases it just needs the power restored, a single reset and the clock putting right. Here's how to get it going again safely — and the one thing that usually means you need an engineer.
Every modern combi, system and heat-only boiler is run by electronics. The control board needs a steady mains supply to power the fan, the ignition, the pump and the display. When the electricity drops out — or comes back with a surge as the network reconnects — the boiler simply loses power along with everything else in the house.
Once power returns, a boiler will often restart on its own. But two things commonly go wrong. First, the sudden loss or surge can leave the boiler in a lockout (a safety shutdown) that needs a manual reset. Second, the boiler's clock and timer settings can be wiped, so it thinks it's the middle of the night and won't fire when you expect heat. Neither is necessarily a fault — both are usually a five-minute fix you can do yourself.
Smell gas at any point? Stop. Don't touch the boiler, light switches or the reset button. Open windows, leave the property and call the National Gas Emergency line on 0800 111 999. A power cut doesn't cause a gas leak, but if you ever smell gas this always comes first.
Before you assume the boiler is broken, confirm it's receiving electricity. A boiler with a completely blank, dark display has no power — that's a supply problem, not a boiler fault.
If the display now lights up, great — move on. If the spur is on, the breaker is up and the fuse is sound but the boiler is still completely dead, skip to the section on the PCB below.
If the boiler has power but is showing a fault code, a warning light, or just won't fire, it has probably locked out and needs a reset. This is a normal, homeowner-safe action done entirely from the front panel — you never remove the casing.
Our full guide to resetting your boiler walks through where the button is on different models. The golden rule: reset once. If it locks out again, don't keep pressing — that points to a genuine fault, not just power-cut hangover.
The reset-once rule. A single lockout after a power cut is normal. If the boiler fires up and stays running, you're done. If it locks out again within minutes, stop and book a Gas Safe registered engineer — repeated lockouts are the safety system telling you something is wrong.
This is the step people most often miss. After a power cut, many boilers and programmers lose the time, so the heating runs on the wrong schedule — or not at all. The boiler itself is fine; it just doesn't know when it's meant to come on.
Once the clock and schedule are right and the thermostat is calling for heat, a healthy boiler should fire normally.
While you're there, glance at the pressure gauge. Cold, it should sit at roughly 1 to 1.5 bar (rising towards 2 bar when hot). Below about 1 bar is low, and many boilers lock out on low pressure — which can coincide with a power cut if the timing is unlucky. Topping up via the filling loop is a homeowner-safe job; see our guide to low boiler pressure for how.
If you've confirmed the spur is on, the breaker is up and the fuse is good, but the boiler stays completely lifeless — no display, no fan, no response to the reset button — the problem is likely inside the boiler rather than your wiring. The most common culprit is the PCB (printed circuit board), the boiler's main control board.
A power surge as the supply returns can damage the PCB or its components. There is no homeowner DIY here: diagnosing or replacing a control board means opening the sealed casing and working on live electrics and the gas-side controls, which is a job for a Gas Safe registered engineer only. A new PCB is one of the pricier boiler repairs, so this is exactly the kind of fault where having cover pays off.
A note on terminology: any engineer working on your boiler's gas and combustion side must be on the Gas Safe Register (gassaferegister.co.uk) and able to show their ID card. "CORGI" registration was replaced by Gas Safe in 2009, so anyone still trading on the CORGI name is out of date.
Get a Gas Safe registered engineer out if any of these apply after the power returns:
A reset and a clock reset cost nothing, but a surge-damaged control board can run to a sizeable bill. That's where a policy earns its place: a good boiler cover plan covers the engineer's call-out and the repair, so an unlucky power cut doesn't turn into an unexpected expense. If you're weighing it up, our guides to the best boiler cover and cheaper options break down what's typically included, and our piece on whether boiler cover is worth it helps you decide.
Compare boiler cover plans side by side — breakdown repair, annual servicing and call-out limits — and find a policy that fits your boiler and budget.
Compare boiler coverUsually one of three things: the fused spur or a breaker tripped and needs switching back on, the boiler locked out and needs a single reset, or the clock and timer were wiped so it's running on the wrong schedule. Work through those before assuming a fault. If it's still completely dead with confirmed power, the control board (PCB) may be damaged — that needs a Gas Safe registered engineer.
It can. The surge when the electricity supply reconnects sometimes damages the PCB (the boiler's main control board) or its fuse. More often, though, the boiler is fine and simply needs a reset and the clock re-setting. If it's truly dead despite having power, book an engineer.
Sometimes. Many boilers restart automatically when power returns. If yours is showing a fault code or warning light, press the front-panel reset button once. If it then runs normally you're fine; if it locks out again, stop and call a Gas Safe engineer.
The most common reason after an outage is that the clock and heating programme were reset, so the timer isn't calling for heat. Set the correct time, check your on/off schedule and turn the room thermostat up. If the programme is right and it still won't fire, it may have locked out — try a single reset.