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What is a Combi Boiler?

Combi boiler

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

So everything a combi needs to heat your home and hot water is all within the boiler itself. That means there’s no need for any tanks or cylinders. Great news if you’re tight on space.

This is the ultimate guide to combi boilers and includes everything you need to know to get a new combi boiler for your home.

Does your home have more than one bathroom? Then you’d be better off with a system boiler.

What is a combi boiler?

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

The combi boiler is the 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 you have high demand for hot water. And by ‘high demand’, we mean two or more bathrooms and a large family.

Thanks to being so compact, combi boilers have quickly become the most popular boiler type in the UK.

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.

Condensing combi boilers

By law, all new combi boiler installs must be condensing. And that’s no bad thing.

Boilers that aren’t condensing (non-condensing), aren’t very efficient. Much of the heat they make goes straight out of the flue pipe and not towards heating your home. Condensing boilers hold onto more of this heat thanks to a Flue Gas Heat Recovery System.

A Flue Gas Heat Recovery System captures the heat from the waste flue gases and recycles it to preheat the new, cold water as it enters the boiler from the mains. This means the boiler doesn't need to work as hard to heat the the water and uses less energy.

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 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. 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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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

Condensing combi boilers have efficiencies around 94%. This means that for every £1 spent heating your home, only 6p is lost to the heat that leaves through the flue pipe.

When you think that some older non-condensing boilers only reach 55% efficiency, that’s a pretty big 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 £300* 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 almost anywhere in the home. Great news all round!

When comparing quotes, be sure to ask the engineer if they will perform a powerflush of the heating system. A powerflush should be part of any new boiler installation as it clears any rust, debris and limescale from the pipes.

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’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 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 boiler may be the better solution.

What size combi boiler do I need?

We know that combi boilers are compact. But when we talk about ‘boiler size’, we mean the output rating, measured in kilowatts (kW).

A boiler’s output rating is a measurement of its power. So, a boiler with a big output rating, will be able to meet higher demand for heating. With that in mind, it’s important not to go for the most powerful boiler you can afford. Do that and your heating bills will rocket. Go too low and it will struggle to meet demand.

So it’s important to get boiler sizing spot on.

The best way to do this is by getting the expert opinion of a Gas Safe registered engineer – which you can do right now. To give you an idea of what you’re looking for, take a look at the table below:

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

Be careful: On the whole, combi boilers aren’t recommended for large homes with 2+ bathrooms. In these cases, a heating engineer will often advise that you go for a system boiler with a hot water cylinder.

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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

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 solid warranty.

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 the best cheap boilers.

Cost of combi boiler installation

On average, you’re looking at combi boiler installation costs of 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 to a new spot 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. So .

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*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.

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