Skip to the content
Trustpilot logo 0800 61 22 302

Why does my boiler turn on and off?

Discover what is causing your boiler to fire up then cut off and how to fix this to stop costs escalating.

Get a Fixed Price Boiler repair

What is boiler short cycling?

If your boiler is short cycling, there is an issue with its internal thermostat (the heated water in the heat exchanger is firing up and reheating when it isn’t necessary). In a well-working boiler, this water would naturally cool down as heat escapes the unit rather than keep reheating.

This unnecessary reheating of water uses up your gas supply. But instead of using it to heat your home effectively, it is instead a waste of fuel and sends your bills through the roof.

Additionally, over time, this short cycling could lead to a damaged heat exchanger - which costs around £300-£550 to replace.

What can cause a boiler to short cycle?

The most common cause of this short cycling is an oversized boiler, but there can be a host of things making your unit cut out. 

Oversized Boiler

We aren’t talking about a giant boiler here, just one that is more powerful than it needs to be. If the unit is too powerful, the increase in steam and thus condensation is what triggers the short cycling. 

This problem is far less common when you have an energy-efficient boiler (that isn’t bigger than required). Choosing the right boiler for your home can be overwhelming, consult our guide to buying a new boiler and make the price of a new unit manageable with one of our payment plans.

The table below shows the ideal power output needed for the boiler type and number of radiators in your home.

Boiler Type

Up to 10 Radiators

10-15 Radiators

15 - 20 Radiators

Combi 24-27 kW 28 - 34kW 35 - 42 Kw
System 9-18 kW 18 - 26 kW 27 - 40 kW
Regular 9-18 kW 18 - 26 kW 27 - 40 kW


Faulty thermostat

If your thermostat is faulty, it may cause the boiler to fire up and turn off again, whereas a working thermostat that measures temperature accurately would not evoke this short cycling.

Ensure that your thermostat isn’t in an unusually cold spot as this also may be affecting its ability to read the temperature effectively. A smart thermostat shouldn't have this issue; so it’s a good idea to have one fitted.

Incorrect Boiler pressure

Your boiler pressure being correct is vital to keep your unit working effectively. It should sit at around 1.5 bar for optimum efficiency.

Make sure it is at the correct level, by taking a reading then fixing the boiler pressure to the correct level.

If it is too low, you might be suffering from a leak. Visually inspect all elements of the central heating system (pipes, radiators and the boiler itself) for a leak. If you do uncover one, you should contact an engineer for a repair.

Alternatively, boiler pressure that is too high is likely a result of air trapped somewhere within the central heating system. If this is the case, you will need to bleed the radiators.

A Faulty Water Pump

Your water pump should be in excellent working condition for your central heating system to be efficient. A faulty pump means that water isn’t being pumped around effectively. As it isn’t being pumped away from the boiler, it causes the unit to overheat and then cut out after firing up (short cycling).

If you do find this to be the issue, a water pump replacement is probably necessary, typically costing in the region of £300.

Take away the pains of central heating maintenance and repair costs by opting for one of our boiler cover plans.

Frozen condensate pipe

Water in your condensate pipe can often freeze in the colder months, thus causing a blockage. In turn this can cause the short cycling you’re experiencing. 

After locating your condensate pipe (check out your manual if you’re unsure) you can thaw it safely at home if you find it to be frozen. 

Read our full guide on thawing a frozen condensate pipe.

Problems with your fuel supply

Anything that gets in the way of your boiler receiving continuous fuel, will cause the unit to cut out or short cycle. To see if you are having issues with your fuel supply check that all other gas appliances in your home are still working effectively or get in touch with your energy supplier.

Boiler need some TLC?

Let our experts take care of it with an emergency repair

If your boiler is constantly firing up then cutting out, you may need to seek help from an engineer.

Our Boiler Plan Gas Safe experts will point out any work that needs to be carried out, they'll also provide a fixed price repair quote.

With our 24 hour emergency repair and 95% first time fix rate, we’ll have your heating up and running again in no time.

Book a Boiler Repair

What can happen if you ignore a boiler short cycling?

You should never ignore a short cycling boiler as it could lead to further problems. So, it’s important that you call a professional engineer if you aren’t able to rectify the problem yourself.

It left unchecked, short cycling boiler can result in increased heating bills, more frequent faults and constant heating engineer call outs, and will ultimately lead to you replacing your boiler far sooner than necessary.

How do I stop my boiler short cycling?

To get a diagnosis quickly and stop your boiler short-cycling, the best action to take is calling in a Gas Safe engineer for a boiler repair. Whether your boiler cuts out when running hot water or simply when turning on, the engineer will provide you with an accurate answer then deal with the problem efficiently.

This will prevent further damage and put a stop to escalating repair and gas bills.

Get a Boiler repair