Does Your Boiler Fire Up, Then Turn Off? Here’s Why

If you boiler is firing up and then turning off it can make you wonder and no one wants to have doubts about such an essential appliance. But it isn’t necessarily an issue — if your combi boiler is going through this routine, it could simply be ‘preheating’ so that there is always hot water available when you turn on the hot tap.

If it continues to fire up and then shut down rapidly, it may be an issue. If this happens, it’s possible that your boiler is ‘short cycling’. If this is the case, you may notice an increase in your heating costs, as well as the boiler becoming damaged in the long run.


What is Boiler Short Cycling?

When your heating system isn’t in use, the hot water in the heat exchanger cools down naturally as heat departs the device.

All units include an integrated thermostat to keep track of the temperature inside the boiler. A short cycling boiler’s internal thermostat detects that the water in the heat exchanger is cooling and, despite the fact that the temperature in your home has not dropped, starts the boiler to reheat the water.

The heat exchanger does not take long to reheat this small volume of water to the thermostat’s temperature setting, which is why the boiler shuts down rapidly.

This process of switching on and off the boiler  wastes fuel, resulting in higher heating costs, as well as the possibility of a broken heat exchanger.  If the heat exchanger does indeed break and needs to be replaced, the cost can be somewhere between £300 and £500.

Nobody likes unwanted bills, so it’s best to stay on the safe side and check that it isn’t an issue.

Possible reasons For Combi Boiler Short-Cycling

Let us take a brief and informed look at what causes Short-Cycling.

Cause For Boiler Shirt CyclingSigns
Water Pressure IssueLeaks, Cold Patches On Radiators, Low/High Pressure Gauge Reading
Heat ExchangerError Code On Boiler
ThermostatDifficult To Spot
Water PumpPump Overheating, Noises From Pump
Fuel SupplyLack Of Fuel, Pay Meter Empty
Frozen Condensate PipeCheck Pipe Outside

Water Pressure

For your boiler to circulate hot water effectively and efficiently, it needs to have an optimal level of pressure in the system. Low boiler pressure will cause your boiler to fail to meet demand, while high boiler pressure will cause all sorts of other problems.

Boiler pressure should ideally be around 1.5 bar. However, you should always check your boiler’s manufacturer’s recommendations in the manual. Yes, read the manual, if you don’t have it anymore, to a quick search for Boiler model + manual and you should be able to find a digital copy.

The boiler will cut off if the pressure is too low. One of the most common causes of low boiler pressure is a leak somewhere in the home heating system. Check for water around all of your radiators, pipes. Turn off the water supply and contact a qualified heating engineer if you notice even the tiniest sign of a leak.

Cold patches on the radiator, especially at the top, are a clear symptom of trapped air in the system. This trapped air will function as an obstruction, preventing the hot water from flowing freely around the radiators. This means they’ll struggle to achieve the desired temperature, putting additional load on your boiler. You’ll need to bleed the radiators to get rid of the air and get the hot water flowing again. 

Heat Exchanger

The life expectancy of a heat exchanger is no more than 7 years. If your heat exchanger has worn out or become damaged, then replacing it is the obvious choice. However, keep in mind that heat exchangers are costly and might not be worth investing in if your boiler is old and burnt out. We would recommend you to buy a new boiler instead of giving the original one for repair.


The temperature in a room is measured by the thermostat. When the temperature dips below a specific level, it will activate the heating system. Because a faulty thermostat doesn’t accurately measure the temperature, the boiler will cycle on and off more frequently. You should also check where the thermostat is located. If it’s in a drafty hall, these changes in temperature may be what is causing the problem.

You might want to investigate getting a smart thermostat, which can be programmed and adapt to your home’s heating preferences automatically.

Water Pump

Your water pump must be working correctly for your central heating system to function effectively. If there is any issue with the water pump, then obviously water is not going to be pumped around the system Water is not being pumped around adequately because of a defective pump. Debris builds up in the pump over time can produce a blockage, which is especially frequent in older systems.

To fix the problem, a Gas Safe engineer will need to perform a hot flush of the pump. Ignoring this issue could mean you have to buy a replacement pump when your existing one craps out, setting you back around £300.

Fuel Supply

Any disruption to your boiler’s fuel supply will cause it to turn off. Before contacting your energy provider to discover whether you’re having problems with your fuel supply, make sure that any other gas appliances in your home are still working properly.

If you have a pay meter, make sure it’s full, and if you heat with an alternative fuel like oil, LPG, or biofuels like wood pellets, make sure you have enough.

Frozen Condensate

Frozen Condensate is a common complaint during winter months when the condensate in pipes gets frozen. In simple terms, a condensate pipe is a drainage pipe used to discard acidic water out from your boiler. If the temperature around this condensate pipe drops, then this acidic water can freeze. A frozen pipe will block the water flow and as the boiler keeps sending more water, the ice barrier will send it back to the boiler. To fend off the damage, the boiler cuts out and displays an error.

Another way to figure out whether your pipe is blocked is when the boiler makes a gurgling noise as a result of the blockage. You can reverse this blockage by using warm water to free the ice and resetting the boiler.

If Your Boiler Fires Up Then Turns Off, Don’t Ignore It

A boiler with a short cycling cycle should not be overlooked. It’s happening for a reason, and that reason must be addressed in order to avoid more issues and restore your heating.

If you ignore a short-cycling boiler, you could end up with:

  • More Expensive Heating Bills
  • A Large Boiler Repair Bill

Last Updated on November 10, 2021

Frequently Asked Questions

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

While there is a myriad of reasons why the boiler could switch on and off which shouldn’t be worried about, you must keep a check on your boiler frequently. Maintaining a boiler is an easy task and if done consistently then it will save you a lot of damage and money in the long run.

What is boiler Short-Cycling?

When a boiler’s burner cycles on and off for short periods of time, it’s known as short cycling. Short cycling causes a lot of wear and tear on the boiler, as well as increased energy use. Because of the short cycling, the heating load is much lower than the boiler’s heat output.

When is the right time to call a gas safe registered engineer?

In short, if your gas boiler is creating havoc and continuously switching off and on, then you need to get in touch with a registered gas safe boiler engineer. They will free you of the headache and find the fault and fix it immediately. However some issues can be mended at home, so carefully read through the guide above and you will find the answer to your boiler misery in no time.

David Thompson

Author: David Thompson

Dave has been involved in the UK building sector for over 3 decades and has developed a particular interest in energy efficiency in the home.

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