Boiler keeps turning off? Causes and what to do

This guide is for a boiler that starts, then stops, locks out or keeps turning on and off. Boiler won't start at all? Then 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 does my boiler keep turning off?

A boiler turns itself off when one of its safety sensors spots a problem. Low pressure, a frozen condensate pipe, no gas and overheating are the most common reasons. If the boiler can't sort the problem out itself, it locks out and shows a fault code or a red light.

A lockout is the boiler's safety shutdown. It stays off until someone presses reset. Short cycling is different. That is when the boiler keeps turning on and off by itself, every few minutes. Both are covered below. If a switch at your fuse box flips off instead, see our guide to a boiler tripping the electrics.

What should I check first?

Check the pressure, the condensate pipe, the gas and the thermostat. You can do all of these from the outside, without opening the boiler. Fix anything you find before you press reset, or the boiler will just lock out again.

1. Pressure. Look at the gauge on the front of the boiler. With the heating off, it should usually read about 1 to 1.5 bar. If it's low, see how to top it up in our guide to a boiler losing pressure.
2. Condensate pipe. This is the small plastic pipe that takes waste water from the boiler to a drain. On a freezing day, check the part outside for ice.
3. Gas. Does your gas hob light? If you have a prepayment meter, check it has credit. A boiler stops when the gas runs out.
4. Thermostat. Check where it is. If it's near a radiator, a lamp or a sunny window, it can think the room is warm and switch the heating off too soon.
5. Radiator valves. If most of your TRVs are turned right down, the boiler has little to heat and may cycle on and off. A TRV is a thermostatic radiator valve, the numbered dial on a radiator.
6. Fault code. Write down any code or flashing light, and the boiler make and model. Your manual explains what it means. Then try the reset button once.

What do the signs tell you?

How the boiler stops often points to the cause. A boiler that locks out with a code has a fault. A boiler that keeps firing up and stopping is usually short cycling. This table sums up the common signs.

Why does my boiler keep locking out?

A boiler locks out when a safety check fails and it can't fix the problem itself. The lockout stops the boiler running while something is wrong. It shows a fault code or a red light until you reset it.

These are the usual reasons a boiler locks out:

• The flame won't stay lit. A flame sensor checks the gas has lit. If it can't see a flame, the boiler shuts off the gas. A dirty sensor, a gas supply problem or an ignition fault can cause this.
• Low pressure. Most boilers have a sensor that stops them if the pressure falls too low.
• A blocked or frozen condensate pipe. The waste water backs up, and the boiler stops to stay safe.
• Overheating. A safety sensor turns the boiler off if the water inside gets too hot.
• A fan or flue problem. The flue is the pipe that carries fumes outside. If the fan can't push the fumes out safely, the boiler won't run. Keep the flue outside clear of plants, washing and anything stacked against it.

If the gas hob won't light either, the problem is the gas supply, not the boiler. Your gas supplier can help with meter problems.

What is short cycling?

Short cycling is when a boiler fires up, runs for a few minutes, turns off, then starts again. It wastes gas and wears the boiler's parts faster. You may hear it click and fire up many times an hour, even though the house never feels very warm.

Common causes of short cycling:

• The thermostat is in the wrong spot, such as near a radiator, a cooker or a sunny window.
• Most of the TRVs are turned down, so very little water flows round the radiators.
• Low pressure or air in the system.
• Sludge in the pipes and radiators. Sludge is dirt and rust that builds up in the water over the years.
• A weak or faulty pump, the part that pushes hot water round your radiators.
• A boiler that is too powerful for the size of the home, so it heats the water too quickly.

The first three you can often sort yourself. Sludge, the pump and the boiler size need an engineer. Sludge is usually cleared with a power flush, a deep clean of the pipes and radiators. Our power flush page explains it.

Why does my boiler cut out before reaching temperature?

A boiler that cuts out early usually can't get rid of its heat fast enough. The water inside gets too hot, so a safety sensor turns the boiler off. This often happens when water isn't moving round the radiators freely.

• A weak pump or a stuck valve. The hot water stays near the boiler, and it overheats. See our guide to hot water but no heating for more on pumps and valves.
• Sludge. It slows the water down. Radiators cold at the bottom are a common sign. See our guide to cold radiators.
• Closed radiator valves. Open the TRVs in a few rooms and try again.
• The thermostat. If it sits near a heat source, it switches off before the rest of the house is warm.

Why does my boiler keep turning off in cold weather?

In freezing weather, the most likely cause is a frozen condensate pipe. Ice blocks the pipe, the waste water backs up and the boiler shuts down. You may hear gurgling before it stops. You can often thaw the pipe yourself with warm water, never boiling. See our guide to a frozen condensate pipe for the steps.

Is it normal for a boiler to turn on and off?

Yes, some of it is normal. A boiler turns down or stops once the water is hot enough, then fires again when it cools. That on and off pattern is how it keeps the house at the right heat. A few other things can look like a fault, but aren't.

• Many combi boilers fire for a moment now and then to keep some water warm. This means hot water comes through faster.
• Many boilers have frost protection. It turns the boiler on in very cold weather to stop the pipes freezing.
• The pump or fan may keep running for a few minutes after the burner stops. This cools the boiler down safely.
• Your thermostat may click as it switches the heating on and off. That's normal too.

It isn't normal if the boiler locks out, shows a code, or fires up every few minutes while the house stays cold.

How do I reset my boiler?

Most boilers have a reset button, or a dial you turn to a reset setting. Fix the cause first, such as low pressure or a frozen pipe, then reset once. Your manual shows exactly how, and how long to hold the button.

1. Write down the fault code and the boiler make and model.
2. Check the pressure and, on a freezing day, the condensate pipe.
3. Press and hold the reset button, or turn the dial, as the manual shows.
4. Give the boiler a few minutes to start up again.

Is it safe to keep resetting my boiler?

No. One reset is fine. If the boiler locks out again, it is shutting down for a safety reason. Pressing reset again and again won't fix that. Each time, the boiler tries to start and then stops, which can make some faults worse. Leave it off and call a Gas Safe engineer.

Do you need a new boiler if it keeps turning off?

Not usually. Low pressure, a frozen condensate pipe or a dirty flame sensor can all be fixed without a new boiler. Short cycling caused by a badly placed thermostat or sludge doesn't need one either.

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

• Repeated lockouts on an old boiler whose spare parts have stopped being made, or are hard to track down.
• A boiler far too powerful for the home. It can keep short cycling, however well it's repaired.

A new boiler is sized to your home after a survey, so it runs in longer, steadier bursts. If repairing it is the cheaper way forward, we'll say so plainly. If not, our boiler installation page explains how we pick the right size.

When to call a Gas Safe engineer

Call a Gas Safe engineer if the boiler locks out again after one reset, or keeps turning on and off.A boiler that keeps shutting down after these checks usually has a fault inside the case. By law, paid gas work must be done by someone on the Gas Safe Register.

• the pressure, gas and condensate pipe are all fine, but it still turns off
• it locks out again after a reset, or the same fault code keeps coming back
• it lights, then the flame goes out
• it fires up and stops every few minutes, and the house stays cold
• you need to top up the pressure often
• 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 helps stop many lockouts. The engineer checks the boiler 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 losing pressure: what the gauge should read and how to top it up.
• 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.
• Cold radiators: what cold at the top or bottom means.
• Boiler making noise: what banging, gurgling and kettling mean.
• Leaking boiler: what to do first, and where the water is coming from.
• Boiler not working after a power cut: how to reset it, and set the timer clock.
• 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 keeps turning off? Book a boiler repair and we'll find the cause.