A Guide to Combi Boilers: How Do They Work and How Much Do They Cost?

A combi boiler converts around 90% of the energy it uses into heat, and could save you up to £320 a year.

You can get a new combi boiler and have it installed from as little as £1,000.

Combi boilers are a great space saver, with no need for a water tank or cylinder.

What is a combi boiler?

The boiler is the beating heart of any home. It’s the fiery furnace that keeps your rooms toasty and your showers steamy, working tirelessly in the background. Unfortunately, we only notice how much we love our boilers once they conk out on us. What a thankless life they lead.

On average, heating your home makes up around 60% of energy bills, so it’s important you find a boiler that takes things seriously. A combi boiler (cool slang for ‘combination boiler’) heats up water only when you need it, which is much better for the environment and great news for your bills. It’s a ‘combination’ because it deals with the central heating and the hot water at the same time (yes, it’s very talented).

Here’s everything you need to know about combi boilers.

What's in this guide to combi boilers?

Head straight to a specific section by clicking the links below

How does a combi boiler work?

A combi boiler never has a day off; it’s on constant standby. Even while you’re sleeping, the loyal combi boiler will be on high alert, able to deal with your watery requests at a moment’s notice.

how a combi (combination) boiler works

Instead of storing loads of heated water in a big tank, a combi boiler does things a lot more last minute. When you turn on the hot tap or start running a shower, cold water will whoosh from the mains, go through the combi, and get all fired up. This magic happens because of the heat exchanger, which is powered by gas, oil or electricity (see more below). Hot water will come tumbling out at your end, and you simply won’t believe that it was cold moments before.

The central heating process is very similar; cold water quickly turns into hot water, which is then sent on a journey through the radiators of your house. Once the water has lost its warmth, it will go back to the boiler for more, and then get going again. It sounds exhausting.

What does condensing mean?

By law, every new boiler in the UK has to be condensing, so it might help to know what that means.

All boilers have a flue, which is basically a little pipe on an external wall that all waste gases (e.g. carbon dioxide) go through before floating off into the world. The sad thing is that these gases still have heat that can be used, so a condensing boiler has a ‘flue gas heat recovery system’ for this very reason. The gases are snatched up before they have a chance to leave the flue and are used again, heating up more water. This is doing the environment a solid favour and reducing your energy bill.

Condensing combi boilers are generally around 90% efficient, which means 90% of their energy is used to create heat for you. To put it another way, for every £1 you spend on heating, only 10p of that is wasted. Compare that to certain older boilers, who struggle along at just 55% efficiency. How embarrassing.

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Combi boilers: pros and cons

If you’re still not sure if a combi boiler is what you need, allow us to help you weigh up the good and the bad.

Advantages of a combi boiler

You've got the power. The lack of storage tank means that the water is drawn directly at mains pressure, so you can enjoy a proper power shower without needing to buy an extra pump. You’ll have a blast!

Extra wriggle room. What’s more, the compactness of the combi boiler means there’s space in your house for something more enjoyable. With hot water cylinders and cold water tanks out of the way, think of the exciting objects that could take their place. Your loft will be a lot roomier, and your airing cupboard could just become a nice place to stand in.

Speedy installation. When you buy a new boiler, the initial cost of installing it can be pretty hefty, but a combi boiler barely takes any time to install. It has way fewer parts and significantly less pipework than other types of boiler. It’s simple: a shorter installation time means a smaller fee.

No heat loss. With other types of boiler, energy is wasted on keeping the tank of water permanently hot. Heat escapes (it always does), and the boiler has to use more energy to replace it. Not with a combi. By heating water only when it is required, barely any energy is wasted, and you save money on your bills.

Your pipes are less likely to freeze. Why? Because a combi boiler has very few pipes. You can’t have frozen pipes if you don’t have pipes to freeze.

Disadvantages of a combi boiler

It can't deal with too many people. As a combi boiler draws water directly from the mains, it can’t support multiple outlets at the same time. If you have two hot showers running, there’ll be a pressure drop. Add a third shower and the pressure will drop some more. Chuck a few hot taps into the mix and you’ll start to feel sorry for it. A house with lots of people and numerous showers is not really the place for a combi.

But wait! There are solutions. You can install an accumulator tank (costing between £500-£1000), which will store some extra high-pressure water and give your combi a hand on busy shower days. Another option is a mains booster (costing around £300-£500), built to receive incoming mains water and give it some added oomph.

There's a 'combination' trade-off. The combi boiler combines central heating with hot water in one magical mixture, but they don’t get along 100% of the time. Whenever you turn on a shower or a hot tap, the central heating will pause and all priority will be given to your hot water needs. Once the hot water stops running, the central heating will kick off again. This doesn’t sound ideal, but the central heating pause will rarely last long enough to cause a problem - unless you take unreasonably long showers (stop doing that).

No emergency backup. If a traditional boiler gives up, you can normally use an immersion heater to provide some short-term relief. With a combi boiler, that isn’t an option, so you’re stuck using the kettle. The silver lining is that immersion heaters are expensive to use, so at least you don’t even have the option to spend bucket loads of money on hot water.

It doesn't like solar panels. There isn’t really much compatibility between combi boilers and solar thermal technology. Solar panels usually need a hot water tank and a combi boiler can’t think of anything worse.

If a combi boiler sounds like the right system for you and your property, why not compare quotes by hitting this button and hearing from our suppliers.

How Much Would a New Combi Boiler Cost You?

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What size combi boiler do you need?

You don’t want to do your house a disservice and give it a boiler that’s too small. Likewise, if you overpower your house then it might start getting ideas above its station. Finding the boiler that’s the perfect size for your house is very important.

To put it simply, larger houses need bigger and more powerful boilers. Check out the guide below to get a better idea (to be used as a rough indicator only).

what size combi boiler do I need for my house

Flow rate

You’ll hear the phrase ‘flow rate’ bandied around a lot when talking to suppliers and installers about boilers. And boy do they like to talk about boilers!

‘Flow rate’ refers to how much water can pass through a boiler at any one time, which determines how fast it can come out of a tap or shower head. It’s measured in litres per minute and is based on how fast the boiler can heat up the water. A boiler with a lot of power (measured in kW) will be able to produce a pretty fast flow rate.

However, as a combi boiler draws water directly from the mains, it’s the mains pressure that you need to think about. There’s no point buying a combi boiler that can pump out 14 litres of hot water every minute if the mains is only supplying water at 10 litres per minute. You need to work with what you’ve got.

Here’s the lovely Francis, a Gas Safe registered boiler engineer, discussing what size combi boiler is right for your home and hot water needs:


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Small combi boilers for small properties

It’s in flats and small houses that combi boilers come into their element. You really see them take on a whole new level of confidence.

Nobody wants the boiler to be the centrepiece of their home, and the compactness of a combi boiler is perfectly suited to small properties. In fact, many combi boilers are so dainty that they can fit inside a kitchen cupboard, hidden from view and allowing you to make the most of your precious space.

If you live in a small property with only one bathroom, a small entry-level combi boiler should provide you with all the heating and hot water you need. Small combi boilers usually have a power output of roughly 24kW to 27kW, but they can be as small as 12kW. A 24kW combi boiler should be big enough to heat up to ten radiators and provide hot water to one bathroom.

Brands such as Worcester Bosch produce a range of compact combi boilers for small properties. These measure just 69cm x 39cm x 28cm in size, making them the smallest combi boilers on the market. You won’t believe how dinky they are.

A certified boiler engineer will advise you on the right size combi boiler for your flat or house, so that its power output and flow rate match up to your home's specifications.

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Should I get an oil, electric or gas combi boiler?

Although fiercely independent, combi boilers can’t power themselves; you need to use oil, electricity or gas. So what do you go for?

Gas-powered combi boilers

Mains gas is the cheapest option.

A gas-powered combi boiler needs to be serviced every year by a qualified, registered gas engineer who is on the Gas Safe Register (formerly 'CORGI registered'). A malfunctioning gas boiler is serious business, as it can release carbon monoxide and poison the air in your home, so a carbon monoxide detector is highly advisable.

To have a gas-powered boiler, your home needs to be connected to the gas grid. The majority of UK households are on the gas grid, although about 4 million of them (or around 15% of the country’s population) are not – and that’s when you need an alternative.

Oil-powered combi boilers

This is your next cheapest option if you’re not on the gas grid, generally costing around 75% less than electric. It’s also very eco-friendly, allowing you to use bio-diesel or recycled fuels.

Unfortunately, it does demand a bit of physical space, as you need an actual tank of oil sat outside your home. Also, since you won’t have an unlimited supply of oil, you’ll need to make sure you don’t run out.

Electricity-powered combi boilers

If your house is off the gas grid and there isn’t any space for a tank of oil outside, electricity is a pretty handy choice. It’s the most expensive of the three options, but the installation is very straightforward – and what’s more, electric-powered boilers can be up to 99% efficient. Also, they don’t have a flue (because there are no waste gases), which means the boiler doesn’t have to be on an outside wall. You’ll also dodge the carbon monoxide risk that comes with gas-powered boilers.

On the negative side, the manufacturing of electricity can often have a huge carbon footprint. Your energy bills will be pretty big, and electricity prices are always on the up. Plus, a power cut will leave you in a world of coldness.

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Potential energy savings with a combi boiler

These figures from the Energy Saving Trust provide an estimate of how much swapping an old, relatively inefficient gas boiler for a new A-rated combi could save you in England, Scotland and Wales:

Old boiler rating
Detached house
Semi-detached house
Mid-terrace house
Detached bungalow
Mid-floor flat
70 to 74%
74 to 78%
78 to 82%

Don’t forget, though, that you don’t have to choose a combi to save money: a new-style conventional or system boiler will also cut your heating costs due to its vastly increased efficiency over old models.

How do you know how energy efficient your boiler is?

As you’ve probably noticed, a boiler’s efficiency is expressed as a percentage. Since April 2018, it’s been a law in the UK that all boilers have to be at least 92% efficient. UK boilers are all measured on the SEDBUK (Seasonal Efficiency of Domestic Boilers) system.

Make sure your new boiler has a decent score; you don’t want your home to be the least eco-friendly place on the street.

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How much do combi boilers cost?

As a general rule, the price of combi boilers starts at between £400 and £500 and goes up to around £3,000. At the lower end of the price scale, brands like Biasi and Volkera offer a number of cost-friendly options, e.g. the Ideal Exclusive 24kW combi, which comes with a five-year warranty, and retails at around £520.

The more powerful your boiler is, the more expensive it is likely to be, so the least expensive options are usually suitable for a one-bedroom or two-bedroom property. For a three-bedroom property (which will probably need a 28kW boiler) prices start from around £700 or £800, while for four or more bedrooms you’ll probably be looking at at least £1,000.

These prices are for the boiler only - you’ll also have to pay to have it installed, and this is not a job you can do yourself, unless you are a Gas Safe registered engineer.

If you’d like to find out how much it’ll cost to have a new combi boiler installed in your property, click this button, fill in the form, and you'll receive bespoke quotes from our suppliers.

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See our combi boiler prices page for more detailed information on the costs of combi boilers.

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How much does a combi boiler installation cost?

combi boiler installation

The more work your installer has to do, the more expensive installing your new boiler is going to be. If you are replacing a condensing combi boiler with a newer one, this is likely to involve the least amount of work: between £500 and £700.

Replacing a non-condensing boiler with a condensing one in the same place will involve a bit more work, including possibly a new flue and changes to the pipework, so this will probably cost between £600 and £1,100.

In both scenarios, if your system is very old, it may need a clean, which could set you back an extra £500.

If you’re installing a new condensing boiler in a different place, expect to pay between £1,100 to £1,500, plus £200 for cleaning the system.

Swapping an old-style boiler for a combi is the most expensive option, as taking away the tank and cylinder and reworking the whole system is a lot of work. You’re looking at anywhere between £1,400 and £2,200, including cleaning the system.

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Best Combi Boiler Brands in 2019

Worcester Bosch

Worcester Bosch is one of the UK market leaders, with 55 years’ experience in manufacturing boilers, and has a strong reputation. It has also received a 5-star rating on the Trustpilot reviews website and even holds a royal warrant. As you might have guessed from the name, it has its UK headquarters in Worcester.


Viessmann has been making boilers since 1917 and, despite employing 12,000 people around the world, remains a family-owned business with its UK office in Shropshire. Viessmann has won numerous awards, including most recently, a German award known as Specialist Trade Partner No 1, based on the service it has given to other heating companies.


Vaillant is among the UK market leaders, with a 4-star rating on reviews website Trustpilot. The company has been making heating appliances for more than 140 years and is one of Europe’s largest heating manufacturers, yet remains a family-owned business. It is increasingly focusing on renewable energy.


Ideal is a traditional British brand which has been making boilers since 1905. The Hull-based company was the second UK boiler manufacturer to receive a Quiet Mark from the Noise Abatement Society (for its Logic+ range), and it also won a Good Housekeeping Reader Recommended award in 2017.


Grant is a Wiltshire-based manufacturer which has been making boilers for more than 35 years. The company is particularly focused on tackling high energy prices and carbon dioxide emissions, although it is still best known for its oil-fired boilers.


Baxi is another traditional UK brand, founded in Preston in 1866. Its boilers tend to be strong on cost and efficiency - all new Baxi boilers are endorsed by the Energy Saving Trust. 98% of customers on the reviews website Reevoo said they would recommend Baxi, and the company won the energy efficiency award and the customer service award in the Gas Industry Awards 2017. Baxi's 400 Combi was a finalist in the heating product of the year category in the Heating and Ventilation News Awards 2017.

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Next steps

To find out which type of boiler might best suit you and your home, hit the orange button, complete the form and we’ll help you get a competitive quote from a trusted installer in your area.

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