Boiler Not Working After a Power Cut? What to Check

Why a gas boiler stops in a power cut at all

People are often surprised that a gas boiler goes off when the electricity does, even though the gas is still on. Every modern boiler needs power for its control board, its fan, its ignition and usually its pump. Without electricity it cannot light safely, so it stops. When the power returns, some boilers pick up exactly where they left off. Others need a nudge.

The checks, in order

Work through these once the power has been back for a few minutes.

• Power to the boiler. Is there any light or display on it? If not, check the fused switch or plug next to the boiler is on, then check your fuse board for a tripped switch. A surge when the supply returns can trip a circuit.
• The programmer or timer. Many lose their clock in an outage and either sit at midnight or reset to a factory programme. Set the correct time and check the heating and hot water are set to on or to your usual times. Our guide on how to set a heating timer covers dial and digital types.
• A lockout. If the display shows a fault code, a flashing light or a flame symbol with a cross, the boiler has locked out for safety. Press the reset button once and wait a couple of minutes.
• The pressure gauge. On a combi or system boiler it should normally read somewhere around 1 to 1.5 bar when cold. If it is near zero the boiler will not fire, and that is a separate fault from the power cut.
• The thermostat. Turn it up above the room temperature so the boiler has a reason to fire. Wireless and smart thermostats may need a minute to reconnect to their receiver, and some need the batteries or the hub checking.
• Gas. Check a gas hob lights, if you have one. If the hob is dead too, the problem is the gas supply, not the boiler.

Press reset once, maybe twice with a pause between. If the boiler locks out again every time it tries to light, stop resetting it. Repeated resets do not fix an ignition or flue fault.

Common fault codes after an outage

The code that appears most often after a power cut is an ignition lockout, where the boiler tried to light and did not detect a flame. On an Ideal boiler that is usually L2, and on a Vaillant it is F28. A single ignition lockout after an outage is common and one reset usually clears it. The same code coming back on every attempt points to something that genuinely needs attention, which our pages on the Ideal boiler L2 fault code and the Vaillant F28 fault explain.

If the code is one you do not recognise, write it down along with the boiler make and model before you do anything else. It saves a lot of time when you call. Our guide on how to reset a boiler shows where the reset control sits on the common makes and when to stop pressing it.

Heating works but no hot water after a power cut

If you have a boiler and a hot water cylinder, the heating and the hot water are controlled separately. It is common for the heating to come back and the hot water not to, simply because the programmer has reset with the hot water channel set to off. Check that first.

If the programmer is right and the cylinder still stays cold, the next suspects are the immersion heater switch, which may have been turned off or tripped, and the motorised valve that sends boiler water to the cylinder. Our guide on no hot water but the heating works walks through how to tell which one it is.

Your hot water during the power cut itself

If you have a cylinder, the water already in it stays hot for many hours even with everything off, so you can still wash while you wait. A tank fed cylinder keeps supplying water by gravity. An unvented cylinder runs on mains pressure, so it keeps supplying hot water too, until the stored water cools. Use it sparingly and it will last.

A combi boiler has no stored water, so with the power off there is no hot water at all until it returns. Electric showers stop too, of course. If the cut is long, our guide on how to get hot water without a boiler covers the safe stopgaps.

Signs the power cut damaged something

Occasionally a surge when the supply comes back, or a lightning strike nearby, damages the boiler control board or a component on it. It is not common, but these signs point that way:

• No lights or display at all, even though the fused spur is on and other sockets in the house work.
• The boiler display is on but it will not respond to any controls.
• A burning smell from the boiler or its wiring.
• The fuse at the boiler spur blows again as soon as you replace it.
• The boiler runs but the pump does not, so radiators stay cold while the boiler overheats and locks out.

If you smell burning, switch the boiler off at the spur and leave it off. If you smell gas at any point, do not operate any switches. Turn the gas off at the meter, open windows and call the national gas emergency service on 0800 111 999.

Reporting the power cut itself

If the power is still off, or keeps going off, call 105 from any phone. It is the free national number that puts you through to your local electricity network operator, who can tell you whether it is a local fault and when it should be fixed. If only your home has no power and the neighbours do, check your fuse board first, because a single tripped switch is far more likely than a network fault.

When to call us

If you have been through the checks and the heating or hot water is still off, or the boiler keeps locking out, it needs someone who can test it properly. We diagnose boilers and repair cylinders, immersion heaters and the valves between them across London, with 20 engineers who carry the common parts. Call 020 4652 5111 or send an enquiry with the make, model and any fault code, and we will tell you what we think it is before we come. Our rate is on the pricing page and agreed before we attend.