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What is a Combi Boiler?

Combi boiler

A combi boiler is a central heating boiler and water heater in one.

Everything a combi needs is within the boiler itself, meaning there's no need for additional tanks or cylinders – great news if you're low on space!

This guide will help you decide if a combi boiler is the right choice for your home.

Does your home have more than one bathroom? Check out the system boiler, which can be better for homes with higher hot water demands.

What is a combi boiler?

There are 3 types of boiler: combi, regular and system.

The combi boiler is the only one that doesn’t store hot water in a cylinder. Instead, the combi fires up to heat water on demand. This is great if you’re looking to save space (cylinders can be quite bulky), but not such good news if your property has a high demand for hot water (e.g. two or more bathrooms).

Thanks to their size, combi boilers have quickly become the most popular boiler type in the UK, but that’s not the only benefit.

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.

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.

Advantages of a combi boiler

Aside from the space saving nature of this boiler, there are plenty of other reasons to consider a combi.

  • Higher efficiency = Lower energy bills

    As of 2018, all new combi boilers installed in the UK are legally required to be at least 92% efficient. This means that for every £1 spent heating your home, only 8p is lost. This could significantly reduce your home’s carbon footprint, as well as lower your heating bills.

  • Condensing

    By law, all new combi boiler installs must be 'condensing', which means that any heat which would usually escape through the flue pipe, is captured and reused to preheat cold water as it enters the boiler. This process, used by all condensing boilers, is known as a 'Flue Gas Heat Recovery System' and it means the boiler doesn't have to work as hard to heat the water, therefore using less energy.
    Replacing a boiler with less than 70% efficiency with one of the most efficient condensing units could save you up to £300* on your heating bills in one year.

  • Easy to install

    As they’re an all-in-one unit (and can be fitted almost anywhere!) combi boilers are relatively easy to install, meaning a quicker installation, and a potentially lower cost.

When comparing quotes, be sure to ask the engineer if they will perform a powerflush of the heating system. A powerflush is a requirement for any new boiler installation as it helps clear any dirt, debris and magnetite from the radiators and pipework. It also ensures that your warranty remains intact.
  • Hot water on demand

    Combi boilers produce hot water on demand, so there’s no need to use extra energy to keep a storage cylinder hot.

  • Mains water pressure

    A combi boiler takes its water supply directly from the mains unlike traditional systems, which rely on a tank in the loft. This means that you should get a much stronger flow of hot water.

hot water tap


There are a number of advantages to these boilers, but there are a few disadvantages you should also consider:

  • Not suitable for large homes

    Combi boilers aren't the right fit for every home. You’ll want to consider a system boiler if your home is 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 if it needs to supply two or more 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, that 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’re not sure a combi boiler is right for your home, compare combi with system boilers.

Best combi boilers

Once you have determined the combi boiler is the right fit, it’s time to choose your boiler. 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, the following gas boilers come out on top year after year:

Manufacturer Boiler model Central heating outputs (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 they’re also A-rated for efficiency. Year after year, they're recommended by our network of 5,000+ installers and dominate the top spots in numerous consumer review polls.

Find out more about the best gas combi boilers on the market.

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What size combi boiler do I need?

We know combi boilers are small, but when we talk about ‘boiler size’, we mean the output rating, measured in kilowatts (kW); not the physical dimensions.

Most manufacturers offer a handy, interactive guide on their website which will help you determine which size boiler you need, so it’s good to make use of this free service.

Additionally, you can find out more about: What size boiler you need.

Fuels used by combi boilers

Whilst gas combi boilers are the most commonly used, other types of fuelled boiler are available for those homes without a gas connection. These include:

Most off-grid homeowners turn to oil, mainly because it’s the most affordable option. Electricity is by far the most expensive, while LPG sits in the middle.

Your other option is to move away from the boiler altogether and consider a renewable heating system.

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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.

Combi boiler Max. warranty Average price (without installation)
Worcester Bosch Greenstar i 7 years £800 – £950
Vaillant ecoTEC Plus 10 years £1,000 – £1,400
Ideal Logic+ 7 years £700 – £900
Baxi EcoBlue Advance 10 years £1,000 – £1,300
Glow-worm Energy 7 years £800 – £900
Viessmann Vitodens 100-W 10 years £900 – £1,000

Even if you’re working with a restricted budget, there are many manufacturers offering reliable and efficient boilers with a low price tag. Find out more about the best cheap boilers.

Cost of combi boiler installation

On average, a combi boiler installation costs between £500 – £700, for a straightforward boiler replacement (replacing an old combi boiler in the same location).

If the installation is more complex – let’s say you’re moving the boiler or switching boiler type – costs could increase to £1,500 – £2,800.

As boiler installation costs can vary so much, you should get quotes from at least three different engineers. That way, you can be confident that you’re getting the most competitive price – get your quotes today.


*If you’re replacing a G-rated boiler with an A-rated heating system with a room thermostat, controls and thermostatic radiator valves in a detached home. According to the Energy Saving Trust


About the author


Adam is our resident home heating expert. His experience and advice has helped millions of customers improve the efficiency of their homes and save money.

approved by engineer This article has been checked and approved by a Gas Safe registered engineer.

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