• Get the best quotes, fast!
  • Over 5,000 engineers across the UK
  • Gas Safe registered engineers

Need a New Boiler?

Get FREE no-obligation quotes now!

  • No delays! Get quotes today
  • Compare quotes & get the best prices
  • Gas Safe & OFTEC registered engineers
 4.8 stars

1,567 reviews

 4.8 stars

1,567 reviews

Combi Boiler Fires Up Then Cuts Out: Find Out Why

Nick Geary
By: Nick Geary
Updated: 6th July 2022

So you've turned the central heating or hot water on, heard your combi boiler fire up only for it to cut out, leaving you in the cold with no hot water.

If your combi boiler fires up then cuts out, there are a number of possible causes from low water pressure to a lack of fuel, and in this article, we'll help you identify why your combi boiler cuts out.

Need a new combi boiler?

Get FREE quotes for for a replacement boiler today.


What can cause a combi boiler to cut out?

There are a number of reasons why a boiler would cut out and fail to provide your property with central heating and hot water. These range from serious faults that need the expertise of a heating engineer to more minor issues that are safe to tackle yourself.

Possible reasons why your combi boiler is firing up only to cut out again include:

Causes of a Combi Boiler Cutting Out Signs
Low water pressure The pressure gauge will read below 1 bar.
Faulty pump Boiler or pump overheating, pump making loud noises, central heating taking a long time to heat up.
Airlock Water not coming out of certain taps or radiators not heating up properly
Blockage in the heating system Boiler overheating, strange noises coming from the central heating system, radiators not turning out properly.
Frozen condensate pipe You have a condensing combi boiler with an uninsulated external condensate pipe and there's been a period of freezing cold weather.
Insufficient fuel supply Could be an issue with the meter (if you have a prepayment meter check that it's been topped up), supply pipes or lack of local fuel stored (LPG and oil).

Low water pressure

Without sufficient pressure, a combi boiler won't work properly.

Fortunately, checking the boiler's water pressure is simple as there's a pressure gauge on the unit. The pressure gauge should be showing a pressure of between 1 and 2 bar, anything lower than 1 is a sign of low boiler pressure.

Low water pressure is often the result of a leak somewhere in the system or soon after the radiators have been bled.

Leaks aren't always obvious as they could be within the system, take a look around your home for wet or damp patches. If you notice even a tiny leak, you should turn the mains water supply off and contact a heating engineer immediately.
When bleeding the radiators, air is released from the central heating system, which can result in a drop in water pressure.

Increasing boiler pressure is something that can be done without the help of a professional heating engineer – although we highly recommend hiring one if you feel even the slightest bit unsure.

Find out how to repressurise your boiler in 8 simple steps.

Faulty pump

When you turn the central heating on, the hot water needs to be circulated around the system, this is possible thanks to a central heating pump. Water passes through the pump before circulating around the central heating system but over time, it's possible for debris to build up within the pump, causing a blockage. This is most common in older systems and a heating engineer will be able to perform a hot flush of the pump to clear out the blockage.

It's possible to prevent a blockage in the pump by having the boiler serviced annually by a Gas Safe registered engineer.


An airlock is the result of air being trapped in the hot water or central heating system which acts as a blockage to prevent the hot water from circulating.

There are a couple of ways to clear air from the hot water and central heating system which can be done without having to call out a professional and we've put a guide together to take you through the steps: Airlock in Pipes? Here's How to Clear It.

Blockage in the heating system

As hot water circulates around the central heating system it can contain bits of debris such as limescale, sludge and rust which can eventually build up and form a blockage.

In addition to causing your combi boiler to cut out soon after firing up, a blockage in the central heating system can reduce the efficiency and life expectancy of the unit too.

To prevent a build-up, it's highly recommended to have a magnetic filter installed. A magnetic filter collects the rust and debris in the water of the central heating system before it has a chance to build up and cause a blockage.

If there's already a blockage in the heating system, having a magnetic filter installed won't make any difference. Instead, you will need a heating engineer to perform a powerflush of the heating system which will clear out the system and allow the hot water to circulate efficiently again.

Once the powerflush has been carried out, a magnetic filter could be installed to prevent another blockage again in the future – your heating engineer will probably recommend having one fitted too.

Need a new combi boiler?

Get FREE quotes for for a replacement boiler today.


Frozen condensate pipe

The condensate pipe leads waste acidic water outside and disposes of it down a drain. Being outdoors means that the condensate pipe is prone to freezing during the winter, which can cause a blockage.

A blocked condensate pipe will prevent the waste liquid from being disposed of and the boiler will cut out as a result.

To remove the blockage, the frozen condensate pipe needs to be thawed out. If you feel confident enough to do this yourself, find out How to Thaw a Frozen Condensate Pipe with our step-by-step guide. Otherwise, don't hesitate to contact a professional heating engineer.

Combi not receiving enough fuel

Whether your boiler runs on gas, oil or electricity, there's a chance that it might not be receiving enough fuel. Without a sufficient amount of fuel, the combi boiler won't be able to operate properly.

If you have a prepayment meter, ensure that it's topped up, otherwise gas won't be making its way to your property. If you store the fuel locally (LPG or oil), then it's worth checking if it needs topping up.

Under Gas Safe regulations, gas combi boilers need to be connected to the gas meter by 22mm gas supply pipes at the very least. Many older heating systems have 15mm pipes which are unlikely to supply enough gas to the boiler. A Gas Safe registered engineer will be able to carry out a repiping of the central heating pipes. Find out what's involved when it comes to Replacing Central Heating Pipes for a Combi Boiler.

For oil combi boilers, an OFTEC technician will be able to assess the supply of oil to the boiler, while an electrician can take a look at the electricity supply for electric boilers.

Still having problems with your combi boiler?

The best way to get your combi boiler working properly again is to hire a heating engineer to diagnose the boiler, find the fault and get it repaired.

At Boiler Guide, we have a network of fully-qualified heating engineers across the UK and you can get free quotes from those based in your area by completing our simple online form. Up to 3 engineers will be in touch to provide their quotes, allowing you to compare them and be confident that you're getting the most competitive price for work.

Need a new combi boiler?

Get FREE quotes for for a replacement boiler today.


Nick Geary

About the author

Nick Geary

Nick has a wide range of experience writing about conventional and renewable heating solutions, and is always increasing his knowledge by researching any new heating technologies on the horizon.

Need a new boiler?

Get FREE no-obligation quotes now!

Excellent 4.8 out of 5

Find local heating engineers

Find approved boiler engineers
in your area.

Begin your search by entering your postcode below.