Boiler Not Working After a Power Cut? How to Restart It

Why a power cut stops your boiler

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.

What to check	What it is	Safe to DIY?
Fused spur or switch	The wall switch (often marked "Boiler") that may have been knocked or turned off	Yes
Breaker / RCD in the consumer unit	A switch tripped by the cut or the surge when supply returns — flip it firmly back on	Yes (call an electrician if it trips again)
Fuse in the spur or plug	A small cartridge fuse (commonly 3A) that can blow — swap like-for-like	Yes (ask an electrician if unsure)
Front-panel reset	Clears a lockout after the outage — reset once only	Yes
Clock, timer and thermostat	Settings wiped by the outage, so the schedule is wrong — re-set the time and turn the thermostat up	Yes
Pressure gauge	About 1–1.5 bar cold; top up via the filling loop if below 1 bar	Yes
PCB (control board)	Boiler stays dead despite confirmed power — likely surge-damaged main board	No — Gas Safe engineer only

Step 1: check the power is actually back at the boiler

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.

• Check the fused spur or switch. Most boilers are wired to a switched fused spur on the wall nearby (often marked "Boiler"). Make sure it's switched on — it's easy to knock off by accident, and some people switch it off out of habit during a power cut.
• Check the consumer unit (fuse box). A power cut, or the surge when supply returns, can trip a breaker or the RCD. Open your consumer unit and look for a switch that has flicked to "off" or the middle position. Flip it firmly back on. If it trips again straight away, leave it off and call an electrician.
• Check the fuse in the spur or plug. If the spur has a small cartridge fuse (commonly 3A), it can blow. A like-for-like replacement fuse is a homeowner-safe swap; if you're not confident, ask an electrician.
• Confirm the rest of the house has power. If only the boiler is dead but lights and sockets work, the problem is on the boiler's own circuit — start with the spur and its fuse.

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.

Step 2: reset the boiler — once

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.

1. Note any fault code or warning light first; it's useful if you end up needing an engineer.
2. Press and hold the reset button (or turn the control dial to the reset position) for the time stated in your manual — usually around 3 to 10 seconds.
3. Release, and let the boiler run through its ignition sequence. You may hear the fan spin up and a click as it fires.
4. Wait a few minutes and check the heating or hot water comes back on and the fault has cleared.

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.

Step 3: re-set the clock, timer and thermostat

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.

• Reset the clock on the boiler or wall programmer to the correct time and day.
• Check the heating programme — your on/off periods may have reverted to factory defaults.
• Turn up the room thermostat above the current room temperature so it calls for heat, and confirm the timer is set to "on".
• Battery-powered thermostats usually keep their settings, but a low battery can show up oddly after an outage — replace it if the display is blank or faint.

Once the clock and schedule are right and the thermostat is calling for heat, a healthy boiler should fire normally.

Step 4: a quick pressure check

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.

Power's on but the boiler is still dead — the PCB

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.

When to stop and call a Gas Safe engineer

Get a Gas Safe registered engineer out if any of these apply after the power returns:

• The boiler is dead despite confirmed power at the spur and a sound fuse.
• It locks out again straight after a single successful reset.
• The same fault code keeps coming back.
• The RCD or breaker trips again as soon as you switch the boiler on (call an electrician for the circuit; an engineer for the boiler).
• You notice water leaking from the boiler, a burning smell, or scorch marks.

Where boiler cover fits in

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.

Frequently asked questions