A boiler that keeps turning itself off is usually doing one of two things. Either it is short cycling, firing up and switching off every few minutes, or it is locking out, shutting down completely to protect itself and showing a fault light. Both mean something is not right, and the cause is often simple, such as low pressure, a frozen condensate pipe or a thermostat in the wrong place.
Below we explain the difference, the common causes and the checks you can make safely from the outside. Never remove the boiler casing. Anything inside it is a job for a Gas Safe registered engineer.
Short cycling or locking out
Short cycling means the boiler lights, runs for a few minutes, switches off, then starts again, over and over, even though the house is not yet warm. The radiators never quite get hot, and it wastes gas and wears parts out faster.
A lockout is different. The boiler stops altogether, the display shows a fault code or a warning light, and it will not restart on its own. This is a safety feature. The boiler has detected something it does not like, such as low water pressure, a flame that will not stay lit or water that cannot drain away, and it shuts down rather than carry on.
Note which one you are seeing and write down any fault code. It helps an engineer find the problem faster.
Common reasons a boiler keeps switching off
- Low water pressure. Most sealed heating systems sit at around 1 to 1.5 bar on the pressure gauge when cold. Below about 1 bar, many boilers lock out. If the pressure keeps falling, there may be a leak, which our guide to boiler pressure that keeps dropping covers.
- A frozen or blocked condensate pipe. Modern condensing boilers make acidic water as they run. It drains away through a plastic pipe called the condensate pipe. If the pipe freezes or blocks, the water backs up and the boiler shuts down. It is one of the most common faults on frosty mornings.
- The thermostat. A room thermostat in a warm spot, such as near a radiator, a sunny window or the oven, can tell the boiler the house is warm long before it is.
- Radiator valves turned down. If most of the thermostatic radiator valves (TRVs, the numbered heads on the side of each radiator) are turned low or shut, the hot water has nowhere to go. The boiler heats up quickly, reaches temperature and switches off.
- Poor circulation. Sludge in the pipes, air in the radiators or a pump that is starting to fail can stop water moving properly, so the boiler overheats and cuts out.
- A boiler too big for the home. An oversized boiler reaches temperature very quickly, turns off, then fires up again.
- A faulty part. A failing sensor, fan, gas valve or flame sensor can cause repeated lockouts. These are all inside the casing and need an engineer.
Safe checks you can make at home
- Look at the pressure gauge. If it reads below 1 bar, top it up using the filling loop, following your boiler's manual. If you are not sure how, leave it and ask an engineer.
- Check the condensate pipe in cold weather. Find where the plastic pipe leaves the wall outside. If it is frozen, pour warm (not boiling) water along it or hold a hot water bottle against it, then reset the boiler. Our guide to a frozen condensate pipe walks through it step by step.
- Check the thermostat. Make sure it is set above the room temperature and has fresh batteries.
- Open up the radiators. Turn the TRVs up in the main rooms and bleed any radiators that feel cold at the top.
- Check the gas supply. If the hob or other gas appliances are not working either, the problem may be the supply. On a prepayment meter, check there is credit.
- Reset the boiler once. If there is a fault light, press the reset button once, as the manual describes. If it locks out again, stop there.
Why you should not keep pressing reset
It is tempting to keep resetting a boiler on a cold evening, but a lockout is the boiler protecting itself and your home. Resetting it again and again can hide a fault that is getting worse, and some boilers will stop accepting resets altogether until an engineer clears the fault.
If the boiler locks out more than once after your checks, leave it switched off and book a repair. Never take the casing off or try to adjust anything inside. In the UK, only a Gas Safe registered engineer can legally work on a gas boiler.
Signs it could be a safety problem
Turn the boiler off and get help straight away if you notice any of these:
- a smell of gas
- a lazy yellow or orange flame instead of a crisp blue one, on a boiler where you can see the flame
- black or brown sooty marks on or around the boiler or its flue
- your carbon monoxide alarm sounding, or headaches, dizziness or sickness that ease when you leave the house
If you smell gas, open the windows, do not use light switches and call the National Gas Emergency number on 0800 111 999. Carbon monoxide has no smell, so every home with a gas boiler should have a working carbon monoxide alarm.
What we do when your boiler keeps cutting out
When we visit, we find the fault, explain what we have found in plain terms and give you a clear quote for any parts before we fit them. One customer moved into a new home to find the boiler "kept tripping out - not ideal in the freezing conditions we had over the weekend". We went round the afternoon they called, and the replacement parts were fitted the next day.
If the boiler is older and keeps failing, we will talk you through whether a repair still makes sense or whether replacing it is the better option. Many of these faults build up slowly, so a yearly boiler service is the best way to catch them early.
If your boiler keeps turning itself off and the simple checks have not fixed it, Pegasus Plumbers can find out why. Book a visit through our boiler repair service and one of our Gas Safe registered engineers will track down the fault.
