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What is a Combi Boiler?

Combi boiler

Combi is short for combination boiler; basically it is a combination of a water heater and central heating boiler in one compact unit.

We've created the ultimate guide to combi boilers with everything you need to know to get a new combi boiler for your home. From types, manufacturers, sizes, prices and free installation quotes, we can help you find the best combi boiler for the best price without the hassle.

What is a combi boiler?

There are 3 types of boiler: regular, system and combi. While traditional regular boilers are fed a supply of cold water from a large feed and expansion tank in the attic, a combi boiler takes its water supply directly from the mains delivering a stronger pressure (as does a system boiler). And, while both regular and system boilers need a hot water cylinder to store hot water, a combi will produce hot water on demand, i.e. when you turn a tap on.

Combi boilers are preferred in the majority of new boiler installations in the UK because of their high energy efficiency, convenience and compact size.

Diagram of how a combi boiler works in the home

Combi boilers work by heating water directly from the mains, so you don't need a hot water storage cylinder or cold water tank.

Diagram of how a combi boiler works in the home

Combi boilers work by heating water directly from the mains, so you don't need a hot water storage cylinder or cold water tank.

Condensing combi boilers

The vast majority of new combi, system and regular boilers manufactured and installed now are condensing boilers. What a condensing boiler has that non-condensing boiler does not is a Flue Gas Heat Recovery System. As any boiler works it produces waste flue gases and heat and in an old, non-condensing boiler these gases would be wasted.

A Flue Gas Heat Recovery System captures the heat from these waste flue gases and recycles it to preheat the new, cold water as it enters the boiler from the mains. Essentially, this means the boiler doesn't need to work as hard to heat the the water and therefore uses less energy. Condensing boilers have a condensing pipe enables the condensed vapour to drain away while the boiler is working.

Best combi boilers

There is a huge selection of new combi boilers on the market with many manufacturers offering multiple ranges. However, in terms of overall reliability and performance, these gas boilers come out on top year after year:

Manufacturer Boiler Model Available Sizes (kW) Efficiency
Worcester Bosch Greenstar i 25, 30 94%
Vaillant ecoTEC Plus 25, 32, 35, 38 92%
Ideal Vogue Max 26, 32, 40 94%
Baxi 800 Platinum 24, 28, 33, 40 92%
Viessmann Vitodens 200-W 1.9 – 35 94%

All of these boilers are described as being able to fit in a standard kitchen cupboard and are A rated for efficiency. They're widely recommended by our network of 5,000 installers up and down the UK and dominate the top spots in numerous consumer review polls.

To find out more about the best gas combi boilers on the market, take a look at our comparison articles:

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Types of combi boiler

Although we've listed the best gas combi boilers here as they're the most commonly installed, combi boilers are also available in oil, LPG or electric in a range of sizes.

Gas combi boilers

Natural gas combi boilers are the most commonly installed in the UK and if your home is connected to the gas network it is usually the best option in terms of keeping running costs down.

Oil combi boilers

For homes not on the gas network an oil combi boiler is a popular choice. These systems need a tanker of oil to be kept outside your home which needs to be topped up by a supplier as and when it runs out. Read our oil review of the Best Oil Boilers to find out which manufacturers come out on top.

LPG combi boilers

LPG stands for Liquefied Petroleum Gas and it is a combination of gaseous hydrocarbons, produced from natural gas and oil extraction (66%) and from oil refining (34%). These setups are the same as oil boilers, i.e. the boiler needs to be supplied with fuel from a tank in your garden and refilled when running low.

Electric combi boilers

An electric combi boiler is slightly different to the others in that it doesn't burn fuel to heat water; instead the water passes over an electric element as it would in a hot water cylinder with an immersion heater. They are a popular alternative for homes which aren't connected to the gas grid and oil is either impractical or too expensive. They are often limited as to the amount of hot water they can produce and electricity is relatively expensive so they aren't generally recommended for large homes. Which are the Best Electric Boilers?

Advantages of a combi boiler

High energy efficiency

As of 2018 all new combi boilers installed in the UK are legally required to be at least 92% efficient which is the highest level of efficiency on the market, meaning only 8% of the fuel the boiler uses is wasted. This could significantly reduce your home's carbon footprint and potentially lower your heating bills.

Lower energy bills

With efficiency of 92%, for every £1 you spend on energy, only 8p is wasted. When you consider that some of the older non-condensing boilers only reached 55% efficiency, it represents a significant saving. For example, when replacing a boiler with less than 70% efficiency with one of the most efficient condensing units you could save up to £315 on your heating bills in one year.*

Compact size

Imagine finally being able to get rid of that bulky water tank and / or storage cylinder that sits in your attic or airing cupboard. Unlike system or traditional boilers, combis are a single combined unit. To be more specific, they don't require a separate water tank or hot water cylinder and can usually fit snugly into a kitchen cupboard. This saves a huge amount of space in your home. This is particularly great news for those living in small homes or flats.

Easy to install

Because they're an all-in-one unit, combi boilers are relatively easy to install as there's no need for extra elements and pieces. Easy installation usually means a quicker installation which equals a cheaper installation. And, as a combi doesn't need to be connected to a hot water cylinder or feed tank in the attic, it can be fitted pretty much anywhere in the home. Great news all round!

Hot water on demand

They produce hot water on demand so there's no need to use extra energy keeping a storage cylinder hot. And there's no waiting around for the tank to refill after the water is gone.

Mains water pressure

A combi boiler takes its water supply directly from the mains unlike traditional systems which rely on gravity from a tank in the loft; this should mean you get a much stronger flow of hot water.

hot water tap

Disadvantages of a combi boiler

Not suitable for large homes

Combi boilers aren't the right fit for every home. You may want to consider a system or conventional boiler if your home is particularly large and / or has several bathrooms. This is because a combi boiler takes its water supply directly from the mains and heats on demand. It doesn't store hot water so f it needs to supply several outlets at once it will have to split the flow making it weaker.

Not ideal for homes with weak mains pressure

Make sure you have an adequate supply of water coming from the mains. If the mains pressure is weak or inconsistent, your showers and taps will be too. It may be possible to install a pump to strengthen the flow but this will add to the cost.

Not always compatible with old pipework

You also need to consider that if you're replacing an old heating system which was designed to handle the weaker flow from a feed tank in the attic, your existing pipework and radiators may not cope with mains pressure. If you need to have these elements replaced this will add the cost of your new combi boiler.

If you have a higher demand for hot water, weak mains pressure then a system or regular boiler may be the better solution.

What size combi boiler do I need?

It's really important that you get the right size or output of boiler for your home. If the boiler is too small it will not be able to meet your hot water demand, and if it's too big you will end up wasting energy and spending more money on bills.

In addition to your hot water requirements, the boiler also needs to be able to keep up with the strength of the mains pressure as it flows through the boiler. A professional will be able to measure this and advise you as to the best size. Complete our enquiry form today and up to 3 qualified installers will be in touch to arrange a visit to your home. To give you a headstart, this is the general guideline set out by industry professionals:

Number of Bedrooms Number of Bathrooms Number of Radiators Recommended Output
1-2 1 Up to 10 24 - 27kW
2-3 2-3 Up to 15 28 - 34kW
4+ 3+ Up to 20 35 - 42kW

IMPORTANT: Combi boilers are not generally recommended for larger homes with more than 2 bathrooms as they only provide a single flow of hot water that will weaken if it needs to supply multiple taps or showers at the same time. In these cases an engineer will often recommend that you opt for a regular or system boiler as these include a separate hot water cylinder which stores a higher volume of hot water.

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Combi boiler prices

Combi boiler prices tend to range between £500 - £1,500 for the boiler itself plus extra for the installation.

Manufacturer / Boiler Model Standard Warranty Extended Warranty? Average Price (excluding installation)
Worcester Bosch Greenstar i 5 years Up to 7 years available £800 – £950
Vaillant ecoTEC Plus 5 years Up to 10 years when installed by a Vaillant Advance Installer £1,000 – £1,400
Ideal Logic+ 7 years with 10 years on heat exchanger N/A £700 – £900
Baxi EcoBlue Advance 10 years N/A £1,000 – £1,300
Glow-worm Energy 7 years Up to 15 years available when installed by a Glow-worm Club Energy Installer £800 – £900
Viessmann Vitodens 100-W 5 years Up to 10 years available if fitted by a Viessmann Trained Installer £900 – £1,000

Combi boiler installation

Cheap combi boilers

Buying a boiler is an important investment as you’ll be relying on it for your heating and hot water for many years to come, so you should try to spend as much as you can afford to and choose a high quality model with a substantial warranty. Luckily, even if you’re working with a restricted budget, there are several manufacturers which offer reliable and efficient but also relatively cheap boilers. If you are looking for a cheap boiler which doesn’t compromise on quality, take a look at combi boilers by Glow-worm, Vokera and Ideal.

Cost of combi boiler installation

On average, you are looking at between £500 – £700 for a straightforward boiler replacement, i.e. replacing an old combi boiler with a combi boiler in the same location. If the installation is more complex and you are changing either the type or location of your boiler the installation cost could increase to £1,500 – £1,800.

The cost of a combi boiler installation will vary from provider to provider and is impacted by lots of factors like any new piping or radiators needed. This is why it is important to source at least 2 or 3 different quotes. Only then can you compare and make sure you're getting the best deal out there.

NOTE: All condensing boilers (whether combi, system or regular) require a condensing pipe to drain the condensed vapour and dispose of it down a drain. If the installer can't add a pipe where the existing boiler is then it may need to be relocated which will add to the cost of installation.

Get quotes for a new combi boiler

At Boiler Guide we have a network of over 5,000 Gas Safe registered and highly recommended installers across the UK. Send us an enquiry telling us what you're looking for and we'll provide you with up to combi boiler 3 quotes from reputable installers in your area.

There is absolutely no-obligation to take any of the quotes forward and we don't charge you a penny for this service.

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*Source: Figures based on installing a new A-rated condensing boiler with a programmer, room thermostat and thermostatic radiator controls (TRVs) in a gas-heated home from an older boiler with a programmer and room thermostat. Based on fuel prices as of August 2019 for a detached house.


About the author


Adam is our lead content writer who spends most of his time researching and writing articles for the Boiler Guide website. You can rely on him to keep you up to date about the future of home heating.

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