Why Your Boiler Fires Up Then Turns Off

Why Your Boiler Fires Up Then Turns Off

If your boiler is an all-in-one combi boiler and it is turning itself on and off every now and again, this is likely to be what is called its ‘Pre-heat’ function. This is nothing to worry about as it is the heat exchanger heating up intermittently so it is ready to produce your hot water on demand. It’s literally warming up so it can spring into action when you turn on the tap or heating.

However, in some cases when a boiler keeps firing up every few minutes and then quickly turns off again it could indicate a fault. It is called short cycling and is likely to make your heating bills more expensive and, in the long term, will cause damage to the boiler.

Here we explain what is happening when a boiler is short cycling and some potential causes behind the problem.


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What is boiler short cycling?

When your heating system doesn’t need to be on, the heated water in the heat exchanger will naturally cool down as heat escapes through the flue and around the boiler casing itself.

The boiler has its own internal thermostat. When a boiler short cycles, this internal thermostat detects that the water in the heat exchanger is cooling and, although the temperature in your home has not dropped, fires up the boiler to reheat the water. It does not take the heat exchanger long to reheat this small amount of water back to the thermostat’s temperature setting which is why the boiler quickly turns off again.

If the boiler fires up every few minutes it is using gas but not heating your home. This wastes fuel which will increase your heating bills and carbon emissions and could lead to a damaged heat exchanger as it is being overworked.

If/when the heat exchanger breaks and needs replacing, it typically costs between £300 – £500 (depending on the boiler model) excluding the Gas Safe engineer’s labour costs.

Possible causes of boiler short cycling

Your boiler is oversized, i.e. too powerful for your home

Boilers should be sized to suit the size of your home’s heating system and your likely demand for hot water. If the boiler is too big it will be producing more steam than it can condense which will cause short cycling. In the past engineers would generally fit boilers which were bigger than needed to compensate for the high losses. Today, boilers are much more efficient so there is no need to oversize a boiler.

The pressure is too high

Too much pressure in the boiler will cause the boiler’s automatic safety precautions to kick in and shut the boiler down. Generally, boiler pressure should be between 1 and 2 bar. If lowering the pressure doesn’t make any difference try bleeding the radiators of excess air and water before calling a Gas Safe engineer.

The boiler is the right size, but overfired

When the boiler is being overfired it will produce a lot of steam in a short amount of time which will raise the boiler pressure and cause the boiler to shut down for safety reasons.

The thermostat is faulty or calibrated incorrectly

The thermostat may be faulty or in a cold draft which is affecting its capacity to measure monitor temperature accurately. If the thermostat has a mercury switch and is not level this could also cause a problem.

The air vents are faulty

If air vents are not operating correctly there will be trapped air in the system increasing the pressure until the burner shuts off on the pressuretrol. They may need to be cleaned or replaced.

How to fix a short cycling boiler

NEVER attempt to fix a boiler yourself. Gas boiler repairs and replacements should be carried out by a Gas Safe registered engineer.

By completing one of our simple online forms, you’ll get free quotes from up to 3 trusted engineers in your local area. You’ll then be able to compare the quotes and choose the best one for you, or not, as there’s no obligation to use any of the quotes if they’re not right for you.


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