Boiler not working after a power cut? What to check, safely

Why the boiler stops when the power does

Smell gas, or a carbon monoxide alarm sounding? Skip the rest of this page: leave the house, do not touch light switches or use a flame, open doors and windows on the way out if it is safe, turn the gas off at the meter if you can safely reach it, and call the National Gas Emergency Service on 0800 111 999 (0800 002 001 in Northern Ireland). The gas emergency steps are further down.

Otherwise, the explanation is simple. The gas kept flowing through the cut, but a modern gas boiler cannot use it without mains electricity: GOV.UK's WEEE guidance lists gas boilers among electrical equipment because they rely on electricity to work, and Northern Powergrid tells customers to check whether their gas heating will still function without it. The pump, fan, ignition and control board all run on the mains. Makers' manuals put the running draw of a typical combi at roughly 35 to 123 watts (Vaillant ecoTEC plus, Worcester Bosch Greenstar 4000 and Ideal Logic Max figures, set out in our backup power guide).

So a boiler that is dark during a power cut is not broken. The question is what happens when the power returns, and in most homes it is a few minutes of waiting, a clock to reset and, at worst, one reset done as the manual describes. First, though, confirm the cut is the cause: if the rest of the house has power and only the boiler is dark, check the boiler's own switch, the wall switch that feeds it (often a fused spur) and its breaker in the consumer unit. Citizens Advice says a breaker that keeps tripping should be left alone for a registered electrician.

While the power is off

Report the cut and ask how long it should last by calling 105, which is free in England, Scotland and Wales (in Northern Ireland, NIE Networks is on 03457 643 643). SSEN says heat can drop quickly in a power cut, so dress warmly, keep moving, and leave a light switched on so you know when the power is back. The UK Chief Medical Officers' advice for a national outage, published on GOV.UK in December 2025, is to stay inside, wear layers, gather in one room and close the curtains at dark; the NHS says to heat the rooms you use to at least 18°C if you can.

One shortcut is ruled out however cold it gets. The same GOV.UK advice says: 'Do not use camping stoves and barbecues indoors as there is a risk of carbon monoxide poisoning, which can kill', and do not run petrol or diesel generators or similar fuel-driven equipment indoors; a generator goes outdoors, at least 6 metres away, with its exhaust facing away from windows and doors. Northern Powergrid adds that outdoor heaters stay outdoors. Layers, blankets, a hot water bottle and a hot drink from a flask filled before the cut are the simple options; our guide to keeping warm without heating has the full list.

If anyone at home has reached State Pension age, is pregnant or has young children, is disabled, has a long-term health condition or relies on medical equipment that needs power, join the free Priority Services Register: Ofgem says it brings priority support in an emergency and, wherever possible, advance notice of planned power cuts.

When the power comes back: six checks, in order

1. Wait a few minutes. Many boilers run through a start-up sequence, and Vaillant says a restart can take several minutes or a second attempt. Look at the display before you press anything, and write down any code or flashing light it shows: it is the single most useful thing you can tell an engineer later.

2. Clock, timer and thermostat. Electricity North West says to reset the timer and thermostat as soon as the electricity comes back, along with any clocks that may have been affected. A programmer that has lost its time can run the heating at the wrong hours. Citizens Advice's checklist covers the rest: the heating is not set to 'off' or 'summer', the heating times are right, and the room thermostat is set above the room's temperature, or the boiler has no reason to fire. On a combi with no hot water, Citizens Advice suggests running a hot tap for a minute with the water set to at least 50°C.

3. Smart and wireless controls. A wireless thermostat or app-controlled system has its own recovery to do. Worcester Bosch says that in a power cut its EasyControl has no power and the heating appliance is switched off; when power returns it reboots, takes the time from the internet and restarts its programme. Because it gets the time online, your broadband router needs to be back up too, so give it a few minutes, and check a battery-powered thermostat still has fresh batteries.

4. The pressure gauge. Citizens Advice says that if the gauge reads below 1 bar you can repressurise following the manufacturer's guide: heating off and the boiler cool, aim for 1 to 1.5 bar, and close the filling-loop valves properly afterwards. Vaillant's, Glow-worm's and Ideal's fault-code pages all give 1 to 1.5 bar as the normal range, though Vaillant's ecoTEC plus manual accepts up to 2 bar when cold, so your own manual has the final word. If your manual does not show how to top up, or you cannot find the filling loop, leave it for an engineer.

5. One reset, the way the manual describes. Your manual says which faults you may reset and how; Citizens Advice notes that after thawing a frozen condensate pipe you might need to switch the boiler's power off and on again. Deal with the cause first, then reset once. If the same code returns or the boiler locks out again, stop: Vaillant says errors beyond the minor fixes should be looked at by a Gas Safe registered engineer, and some faults are not for the user at all (Baxi's 800 Combi manual marks those with a spanner symbol). Do not keep pressing reset.

6. In freezing weather, the condensate pipe. If the cut came with a cold snap, the plastic pipe that drains a condensing boiler may have frozen while the boiler stood cold. Worcester Bosch's manual says a frozen or blocked condensate pipe shuts the boiler down; Citizens Advice lists the signs as a fault code, gurgling or dripping. Thaw it only if it is outside, at ground level and easy to reach, using a hot water bottle, a microwaveable heat pack or a cloth soaked in warm water; Citizens Advice says not to use boiling water because it might melt the pipe. Our frozen condensate pipe guide shows the steps.

What the fault codes usually mean (and why none says 'power cut')

No maker we could read publishes a code that means 'there has been a power cut'. What people see after a cut are the ordinary pressure, ignition and lockout codes, which is why noting the exact code matters more than guessing from the brand. Below are codes you may meet, and what the maker says causes them.

Vaillant's fault-code table gives F.22 as low water pressure (below 1 to 1.5 bar; increase it slowly to that range), F.28 as no flame signal during ignition, caused by 'not enough gas reaching the boiler (for example, low gas pressure or insufficient credit on a pre-payment meter)', F.29 as a failed re-ignition after a flame loss (interrupted gas supply or low gas pressure), and F.54 as a gas supply fault seen with F.28 or F.29. Ideal's page gives a flashing F and 1 as low pressure (top up to 1 to 1.5 bar, reset; if that fails, contact Ideal) and a flashing L and 3 as no water flow.

Two practical points follow. First, an ignition code after a cut is a prompt to check the gas side, not the electrics: does the hob light, and on a prepayment meter is there credit? If the gas is off at the meter and you do not know why, leave it off and ask a Gas Safe registered engineer before it goes back on. Our winter boiler guide covers the meter and temporary credit. Second, a code that clears with one reset and does not return was most likely the restart itself; a code that comes straight back is the end of the resident's part of the job.

Surges and a damaged control board

Could the power coming back have damaged the boiler? It is possible, and there is no safe check a resident can make. The IET's consumer guidance on surge protection describes surges as transient overvoltages that can cause instant failure or damage that only shows over time, and it names boiler controls among the equipment worth weighing when deciding on protection. National Grid's distribution arm (NGED) warns that electrical surges during power cuts can damage digital devices and suggests turning sensitive electronics off or unplugging them. Neither source says how often a boiler's board is affected, and we will not guess.

Signs to describe to an engineer, not diagnose yourself: a display that stays blank with the house power, spur and breaker on; no attempt at ignition after one proper reset; repeat lockouts; any burning smell or sparking. With a burning smell, switch the boiler off at its switch and treat it as unsafe: the HSE says it is illegal to use a gas appliance you suspect is unsafe, and not to touch it until a Gas Safe registered engineer has checked it. A hard-wired boiler cannot be moved to a surge-protected extension lead; whole-house protection is an electrician's job, covered in our guide to power cuts and appliance damage.

On cost, two dated figures. Which?, which is independent, found in its June 2025 survey of 10,064 homeowners that the average paid for a one-off repair following a boiler service was £156. Selectra, a commercial comparison site that also refers readers to installers, lists a replacement printed circuit board at £250 to £380 fitted (May 2026 figures). Ring any home emergency or boiler cover insurer before booking: the Financial Ombudsman says most policies will not let you claim for a tradesperson you booked yourself. A boiler that fails after a power cut usually needs a repair or a reset, so get a repair quote before anyone talks about replacement.

When to call a Gas Safe engineer

The boundary is set by law. The Gas Safety (Installation and Use) Regulations 1998 say no one may work on a gas fitting unless competent to do so, and the HSE says anyone employed to work on gas appliances in a home must be a Gas Safe registered engineer competent for that work, shown on the back of their ID card. The Heating and Hotwater Industry Council says not to fit, fix or move a boiler yourself, and Worcester Bosch's manual says only a competent engineer can remove the casing. A resident's part happens with the casing on: switches, settings, the gauge, one reset, an external condensate pipe.

Call an engineer when: a code returns after one proper reset; an ignition or flame code persists once you have confirmed the gas is on at the meter and the hob lights; the boiler locks out repeatedly; the display stays blank with the house power, spur and breaker all on; the pressure keeps dropping (Citizens Advice says that if it changes over the next week, ask a Gas Safe engineer to look); or there is any burning smell or sparking. If a breaker keeps tripping, Citizens Advice says a registered electrician comes first. For oil, nidirect advises an OFTEC registered technician.

Check the engineer's ID card and licence number on the Gas Safe Register or by calling 0800 408 5500. If the boiler is still under the maker's guarantee, ring the maker first: Worcester Bosch's manual says it will not accept invoices for repairs by another firm. If you rent, the repair is normally your landlord's to arrange; report it in writing tonight and see the landlord sections of our winter boiler guide for how fast they must act. And make sure there is a working carbon monoxide alarm: the HSE strongly recommends one as a useful precaution, though not as a replacement for regular maintenance and safety checks.

If you smell gas or a carbon monoxide alarm sounds

Get everyone out. National Gas's steps: do not smoke or light matches, do not turn electrical switches on or off, open doors and windows if it is safe to, turn off the meter at the control handle unless the meter is in the cellar, and call the National Gas Emergency Service on 0800 111 999, free, day or night, making the call from outside. In Northern Ireland, call the NI Gas Emergency Service on 0800 002 001. For LPG, the Gas Safe Register says to call the number on the bulk tank or meter. Do not turn the supply back on until a Gas Safe registered engineer has checked it.

Carbon monoxide has no smell. If a CO alarm sounds, or people at home have headaches, dizziness or sickness that ease outdoors, National Gas says to stop using all appliances and switch them off, open doors and windows, get everyone out, call 0800 111 999 and not go back in until the emergency services advise you. The NHS says call 999 or go to A&E if someone exposed is finding it hard to breathe, is suddenly confused, has lost consciousness, feels weak or has chest or muscle pain, and not to drive there; call NHS 111 if you think you have symptoms.

Compensation for the cut, and backup power for next time

A long cut may be worth a payment, though it is for the disruption, not for the boiler. Ofgem's 2026/27 standards give £100 to a domestic customer if fewer than 5,000 properties were off for more than 12 hours (or 5,000 or more for more than 24 hours), with £45 for each further 12 hours, and separate storm rates; most payments are automatic. The amounts change each April, so use the calculator in our power cut compensation guide. If the boiler's board did fail, Citizens Advice says network operators do not have to pay for damaged devices but might make a goodwill payment: keep photos and the engineer's report.

If you want the boiler to keep running through the next cut, read our backup power for a gas boiler guide before buying anything. A combi draws little power, but it is hard-wired through a fused double-pole isolator, so a plug-in power station does not simply run it; a qualified electrician has to fit a suitable changeover arrangement, and the guide covers pure-sine-wave output, runtimes and thermostat receivers. Never wire a boiler to an extension lead from a power station or generator yourself, and never run a generator indoors. For heat without the boiler, see our electric heaters comparison and keeping warm in a power cut kit.