What is a combi boiler?

Combi boilers are an all in one solution. They heat up water so you have hot running water for a bath or shower as well as heating water for your central heating system. They work on demand and don’t store water so you will never run out of hot water again.

Combi boilers take all the water they use directly from the mains, storing little to none within the boiler itself.

It uses a gas burner to rapidly heat the water circling through the boiler, cycling the water routinely through your central heating and back to the boiler as it is repeatedly heated up.

This means that combi boilers create hot water on demand, by heating the mains water, rather than keeping a storage tank somewhere in your house.

So no more running out of hot water if you’ve got a big family (or a partner who takes too long in the shower!) 

Its no wonder then that combi boilers now account for over half of all yearly installations across the UK. In fact, it is projected that combi boilers will account for 78% of the market share by the end of the year.

Photo credit: Andrea Piacquadiowhat is a combi boiler

What are the pros and cons of combi boilers?

Combi boilers massively save on spaceConsumers will have to change their behaviour to match their boiler to get efficiencies advertised
Combi boilers heat water on demand so you never run out of hot waterAs hot water is not stored it can take a some time for hot water to start running
Combi boilers are energy efficient making them cheap to run
Combi boilers use mains pressure so you don’t need to install a pump to get a decent flow rate from your taps.

What are the benefits of a combi boiler?

1) Combi boilers massively save on space

With only the one unit needed and no hot water storage tank required, changing to a combi boiler from a more old-fashioned model can even open up space in your house for more storage as well as saving you money both in the short and long term.

Newer ultra-compact models reduce the amount of space required even more, allowing these boilers to be installed in almost any residence.

2) Combi boilers are very energy efficient making them cheap to run

As a result of UK building regulations, all gas combi boilers these days are required to have integrated condensing technology in order to increase their energy efficiency.

Previously, combi boilers lost as much as 50% of their heat energy through the escaping of combustion gasses. 

Now, however, those gases are redirected through parts of the boiler which extracts their heat through condensation, transferring it to cold water.

This recycling of the combustible gases has allowed combi boilers to become the dominant market force they are today.

3) Combi boilers heat water on demand so you will never run out of hot water

As combi boilers heat your hot water on demand and doesn’t store water inside a tank then you will never run out of hot water.

4) Combi boilers use mains pressure so you don’t need to install a pump to get a decent flow rate from your taps

Another great fact about combi boilers is that they use the mains pressure, so you don’t need any pumps in order to ensure a decent flow-rate from your taps.

Unlike storage tanks, which often just use gravity and valves to control the flow of water, mains water is kept at a certain pressure.

Combined with the expanding effect that heating liquids has, this means that you’ll never have to suffer the indignity of a trickle of water 

Photo credit: Jesse RobertsLarge house, combi boiler

What are the cons of a combi boiler?

1) As hot water is not stored it can take some time for hot water to be generated

One potential disadvantage of upgrading to a combi boiler is the speed at which hot water can be generated.

Whilst the central heating system will work flawlessly, with combi boilers, the time taken to bring the water up to the scalding hot level we all prefer can take a fair few seconds.

A combi boiler only generates large amounts of heated water when instructed to and unless you adjust to your new boiler, it can be frustrating to have to run tepid water out of your tap for up to a minute before the hot water you’re expecting starts to pour forth.

However, in order to ensure that hot water is available on demand, most combi boilers switch themselves on once every hour or so in order to keep a small reservoir of hot water available at short notice.

This ensures that you never have to wait too long for your hot water.

2) Consumers will have to change their behaviour to match their boiler to get efficiencies advertised

The savings that are often advertised on other sites are only if the owner changes their behaviour to match their new boiler.

Recent studies suggest that homes with combi boilers actually use 13% more energy than those without. 

One of the key reasons why is because the energy required to heat up water is much greater than the energy required to keep water hot.

Therefore, if a household is continuously using their hot water in small bursts, the energy consumption could be higher than simply keeping some water hot constantly. 

A good analogy would be the difference between repeatedly switching on and off a lightbulb as opposed to leaving it on for an extended period of time.

However, boiler-makers have queried these studies, arguing that correctly using a combi boiler is always the more energy efficient option, so we recommend keeping an open mind as to whether a combination boiler is best for you or not.

Should I upgrade to a combi boiler?

Whenever old homes have their boilers replaced, homeowners are often advised to upgrade from their old-school immersion heaters (usually located in an airing cupboard separate from the central heating boiler and supplied by a header tank in the roof or attic).

As you can imagine, reducing three items down to one can massively reduce the amount of space taken up by the vital utilities in your home, opening up new spaces for your everyday use. 

Combi boilers are often advertised as perfect for small homes, as they require less space than either system or regular boilers.

They can also be far more energy efficient, if used correctly and, most importantly, they will never run out of hot water as they always heat up water directly from the mains.

This means that a combi boiler can be a great way of reducing your carbon footprint.

What Combi boiler is best for me?

When evaluating which combi boiler is best for you, the main factor to consider is the size of your building and number of occupants.

The more radiators you need to heat at the same time, and the greater the demand for hot water, the larger a boiler you’ll need.

Here is a simple table that you can use to work out what size boiler that you need.

No. Bedrooms & BathroomsPoorly Insulated PropertyModerately Insulated PropertyWell Insulated Property
1 bed & 1 bath20 kW20 kW18 kW
2 bed & 1 bath20 kW20 kW18 kW
2 bed & 2 bath24 kW20 kW20 kW
3 bed & 1 bath28 kW26 kW24 kW
3 bed & 2 bath28 kW28 kW24 kW
4 bed & 1 bath30 kW30 kW28 kW
4 bed & 2 bath32 kW32 kW30 kW
5 bed & 2 bath40 kW34 kW30 kW

Most 3-4 bedroom houses would only need a 20-35kW boiler.

If those bedrooms have ensuites (and therefore you might need to run multiple showers at the same time) it’s recommended to get a boiler at the higher end of this range.

If you think you have a large enough house to justify a combi boiler above 35kW, then it might be worth strongly considering going for a system boiler instead.

Combi boilers can struggle to generate the requisite level of hot water when operating on very large scales, whilst system boilers are only limited by the amount of space in the storage tank.

Unlike in the past, it is no longer recommended practice to buy an oversized boiler to reduce the amount of heat energy lost.

Boiler manufacturers have focussed a lot of research and development into ensuring that the newer models of boilers have little to no waste. In fact, larger boilers can be more inefficient when heating small amounts of water, so figuring out the right size is very important.

Whilst browsing, it’s also good to keep in mind what smart technologies are available with your combi boiler.

Most modern boilers come with integrated technology that allows for greater fine control of your heating and hot water system.

If you’re willing to put in a little extra effort, then you can bring your gas bills right down by intelligent use of smart technology.

You came here wondering “what is a combi boiler”, and we hope you’re now at least a bit wiser on the subject!

We hope you’ve found this blog post informative and interesting, and encourage you to have a look at our other posts on the pros and cons of different boilers. And, if you’re ever asked what a combi boiler is, we hope you can give an excellent answer.


Charlie is a content writer at HomeSage. Having left Oxford University he joined HomeSage in 2020 to help create the UK's best resource in the home improvement sector. In his spare time he enjoys walking his dog Zeus and playing cricket.