Boiler not working after a power cut? What to do

This guide is for a boiler that stopped when the power went off and came back on. Boiler won't start, and there was no power cut? See our guide to a boiler that won't turn on. If the checks below don't help, you'll need a boiler repair from a Gas Safe engineer. This guide is part of our common boiler problems series.

Why has my boiler stopped working after a power cut?

A boiler needs electricity to run, so it stops the moment the power goes. When the power comes back, most boilers restart, but some lock out and wait for you to press reset. The timer may also have lost the time, so the heating stays off when you expect it on.

Other common causes are a tripped switch at the fuse box and a smart thermostat that has lost its link. In freezing weather, the condensate pipe may have frozen while the boiler was off. Less often, a power surge has damaged a part inside the boiler. That needs a Gas Safe engineer.

What should I check first?

Check that power reaches the boiler, then reset it once, then check the timer and thermostat. Work through these steps in order, and try the boiler after each one. None of them need you to open the boiler or the fuse box.

1. Is the power back in your home? Try a light. If the street lights and your neighbours are off too, the power cut isn't over yet. Call 105, the free national power cut number, for updates.
2. Is the boiler's display on? If not, check its fused spur. This is the switched plate on the wall that the boiler is wired into. Make sure it's switched on. Some spurs have a small light that glows when they're on.
3. Has anything tripped at the fuse box? Your fuse box is also called a consumer unit. Look for a switch that has flipped down, including the RCD. An RCD is a safety switch that cuts the power if electricity leaks. Push it back up once. Never take the cover off.
4. Give it a few minutes. Many boilers run a short start-up routine when the power comes back. Wait before you press anything.
5. Reset it once. If the boiler shows a fault code or a flashing light, write it down with the boiler make and model. Then press reset once, as your manual shows.
6. Check the timer and thermostat. Set the timer clock to the right time. Then turn the thermostat a few degrees above the room temperature.
7. Check the pressure. A power cut doesn't usually change it, but it's a quick check. With the heating off, the gauge should usually read about 1 to 1.5 bar. If it's low, see our guide to a boiler losing pressure.

What do the signs tell you?

What the boiler does once the power is back points to the cause. A blank display means it has no power. A fault code means it has locked out. This table sums up the common signs after a power cut.

How do I reset my boiler after a power cut?

Most boilers have a reset button, or a dial you turn to a reset setting. Make sure the display is back on, then reset once and give the boiler a few minutes to start. Your manual shows exactly how.

Write down any fault code and the boiler make and model first. The model is usually on a sticker on the front or underneath. Then run a hot tap or turn the heating on to check it's working. Only reset it once. If it locks out again, the boiler is shutting down for a safety reason. Pressing reset again and again won't fix that. Our guide to a boiler that keeps turning off explains lockouts in more detail.

Why is my central heating timer not working after a power cut?

Most timers lose the time, their settings or both when the power goes off. Set the clock to the right time, then check the on and off times are still there. The boiler may be fine. It just isn't being told to come on.

It depends on the type of timer you have:

• A dial timer has a clock face with small pins or tabs round the edge. It stops turning while the power is off. So after a three-hour power cut, it runs three hours late. Turn the dial to the right time.
• A digital timer, or programmer, has a screen. Some keep their settings in a power cut, but lose the time. Others go back to their factory settings, so you need to set your on and off times again.
• A clock built into the boiler works the same way. Your boiler manual shows how to set it.

Also check it's set to "auto" or "timed", not "off". If the screen stays blank, the timer may have failed. Replacing one means working on the wiring, so that's a job for an electrician or heating engineer. We're electricians as well as Gas Safe engineers, so one visit can cover the timer and the boiler. Our guide to hot water but no heating has more timer and thermostat checks.

Does a smart thermostat need resetting after a power cut?

Usually not. Most smart and wireless thermostats reconnect by themselves once the power and broadband are back. This can take a few minutes, as your internet router needs to start up first.

If it still shows "offline" or "no signal", check any batteries first. Then follow the maker's steps to link it up again, in the app or the manual. Check your schedule is still set, as some go back to a default.

Fuse box tripping when the boiler restarts after a power cut?

If a switch at your fuse box trips as soon as the boiler starts again after a power cut, there is an electrical fault. Leave the switch off and get it checked. Don't keep pushing it back up, as the switch is cutting the power to keep you safe.

The fault could be in the boiler, its wiring or something else on that circuit. Never open the fuse box or touch any wiring. If you have no power to part or all of your home, see our emergency electrician page. Our article on why RCDs matter explains what that switch does. For the safe checks to try, see our guide to a boiler tripping the electrics.

Why is there no hot water from my combi boiler after a power cut?

A combi boiler heats water as you use it, so it needs power for hot water as well as heating. Check the display is back on, reset once if it shows a fault, then run a hot tap. The steps at the top of this page cover most cases.

If the heating works but the taps stay cold, the fault is likely inside the boiler, such as the diverter valve. See our guide to heating but no hot water. Have a hot water cylinder instead? It has its own timer, so check that clock too. An immersion heater, an electric heater inside the cylinder, can heat water while you wait.

Was the power cut in freezing weather?

If the power was off during a frost, check the condensate pipe. Many boilers have frost protection, but it can't run without power. So a long, cold power cut can let pipes freeze.

The condensate pipe is the small plastic pipe that takes waste water from the boiler to a drain. If it freezes, the boiler shows a fault and stops. You can often thaw it with warm water, never boiling. See our guide to a frozen condensate pipe for the steps.

Can a power cut damage a boiler?

It can. When the power comes back, a short spike in the voltage, called a power surge, can damage electronics. Lightning near your home can cause a surge too. In a boiler, the part most at risk is the PCB.

The PCB is the printed circuit board, the boiler's main control board. It tells every other part what to do. Some PCBs also have a small fuse on them, which can blow in a surge. A blank display when the spur is on is one sign. A new fault code, or a lockout straight after a reset, is another. These parts are inside the case, so they need a Gas Safe engineer. If you smell burning, switch the boiler off at the fused spur and don't use it.

What should I do with my boiler during a power cut?

You don't need to do much. The boiler stops by itself and should restart once the power is back. A few simple steps help.

• Call 105 to find out when the power should be back.
• Switch off appliances you don't need, and leave one light switched on. Then you'll know when the power returns.
• If you like, switch the boiler off at its fused spur. Turn it back on once the power has been steady for a few minutes.
• Never use a gas cooker or oven to heat your home. It can build up carbon monoxide.

Do you need a new boiler after a power cut?

Almost never. Most boilers that stop in a power cut only need a reset, a new clock time or a tripped switch put back on. Even if a surge damaged the PCB, a new PCB is a normal repair, and the boiler can carry on.

There are a few cases where a replacement is worth talking about:

• the boiler is old, and the maker no longer makes its PCB, so a repair may not be possible
• the PCB is a costly part, and the boiler was already breaking down often before the power cut
• it's an old non-condensing boiler. These don't reuse heat from their fumes, so they waste more gas

Some home insurance policies cover damage from a power surge, so it's worth checking yours. If a repair is the better way forward, we'll say so. If not, our boiler installation page explains how a replacement works. It starts with a free survey, and you get a fixed price.

When to call a Gas Safe engineer

Call a Gas Safe engineer if the boiler has power but still won't work after one reset. A boiler that won't start after these checks may have a part damaged inside the case. By law, paid gas work must be done by someone on the Gas Safe Register.

• the display is blank, but the fused spur is on and the house has power
• it locks out again after a reset, or shows a code it has never shown before
• the timer, thermostat, pressure and condensate pipe are all fine, but it still won't fire up
• you smell burning, or see soot or black marks round the boiler

The fault code is only a starting point. The engineer first runs a thorough check of the boiler. That finds the fault and the parts needed. See our boiler repair page for the faults we fix. We repair all makes.

A yearly boiler service won't stop a power cut, but it keeps the boiler in good shape. The engineer checks it is safe, cleans key parts and spots wear before it causes a breakdown.

Other boiler guides

• Boiler won't turn on: the checks to try first, and what a pilot light is.
• Boiler keeps turning off: lockouts, short cycling and when to reset.
• Frozen condensate pipe: how to thaw it safely.
• Hot water but no heating: thermostat, timer and valve checks.
• Heating works but no hot water: checks to try, and what a diverter valve does.
• Boiler losing pressure: what the gauge should read and how to top it up.
• Cold radiators: what cold at the top or bottom means.
• Leaking boiler: what to do first, and where the water is coming from.
• Boiler pressure too high: what is too high, and how to lower it safely.
• Boiler tripping the electrics: why it trips the fuse box, and who to call.
• All common boiler problems.

Boiler still not working after the power cut? Book a boiler repair and we'll find the cause.