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

A combi boiler could save you up to £320 a year because around 90% of the energy it uses is converted into heat.

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?

Combi boilers give you hot water on demand, immediately and for as long as you want. This is why they’re the UK’s most popular type of boiler – they’re efficient, convenient and compact. With a combi boiler, you don’t have to wait for the water to heat up every time you want a bath – or, even worse, find it running freezing cold halfway through your shower.

A combi boiler doesn’t require a water tank or cylinder in the loft or airing cupboard, which saves you space. Instead, all the heating happens within the boiler itself. This also means it’s more economical, because it only heats the water you use, rather than warming up a whole tank.

A combi boiler takes care of both the hot water and the central heating, hence the name – it’s short for combination.

What's In This Guide to Combi Boilers?

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How Does a Combi Boiler Work?

A boiler is essentially a furnace, burning fuel to create warmth, and a combi boiler works on quite a simple principle. Different boilers have different setups, but they all use a device called a heat exchanger, which transfers heat efficiently from the burning gas or oil to the water.

how a combi (combination) boiler works

When you turn the hot tap on, that’s the boiler’s cue to wake up and start working. The water comes through the boiler on its way from the mains to your tap, and as it does so, it flows over a little sensor which tells the boiler to start burning gas or oil. The heat exchanger inside the boiler gets hot; the water flows through the heat exchanger and gets hot too, and arrives in your bath nice and piping.

A similar thing happens when you switch the central heating on. The boiler fires up its gas or oil jets, and a pump inside the boiler starts pushing water through the heat exchanger. The warm water flows out through a pipe into the radiators, making your house nice and toasty, and then back into the boiler where it gets warmed up again (replacing the heat it has lost to your rooms), and so on and so on.

The water for your central heating system, and the mains water that you drink and wash in, come through 2 completely separate pipes.

What Does Condensing Mean?

By law, all new gas and oil-fired boilers in the UK now have to be condensing boilers (with a few exceptions). A condensing boiler is one that saves energy by reusing the heat it would otherwise lose.

When a boiler burns gas or oil, the process produces waste gases – carbon dioxide and steam – which leave the boiler through a pipe called a flue. A condensing boiler has a heat exchanger in the flue, which takes the escaping heat and uses it to help warm the water.

Condensing combi boilers tend to be about 90% efficient. This means 90% of the energy they use is converted into heat, or to put it another way, for every pound you spend, 90p goes directly on heating your home. To put this in context, some older boilers are only around 55% efficient, which means you have to burn a lot more fuel, and consequently pay more, to get the same amount of heat.

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Combi Boiler Pros and Cons

Advantages of a Combi Boiler

Convenience: Ah, the joys of unlimited hot water and not having to wait around for an hour every time you want a bath or shower. Ah, the joys of not having to wash your hands in cold water in the middle of winter, or scald yourself trying to wash up with water from the kettle because there’s no hot water in the tank.

Cost: It is both cost-effective and eco-friendly to heat only the water you need when you need it. With a hot water tank, any warm water you don’t use just sits around losing heat and wasting your money.

Space: You can turn your airing cupboard into an extra storage cupboard without that great big hot water tank.

More space: Because the boiler is only 1 unit, you can generally fit it anywhere in the home, with the caveat that it will need a pipe to get rid of the water vapour. Some boilers are small enough to fit in a kitchen cupboard.

Disadvantages of a Combi Boiler

1 shower at a time, please: Combi boilers don’t always work well when you use more than one hot tap at a time. Because you don’t have a massive hot water tank, and are only heating the water that is coming in off the mains, the boiler may well struggle to supply 2 hot, powerful showers at the same time. There are solutions to this problem – an electric shower is the most obvious one, but you can also get various devices such as mains boosters and accumulator tanks. If you have a large family who all want to shower before work or school in the morning, you may be better off with a system or conventional boiler.

Solar power: If your home has solar panels, it’s tricky to integrate these with a combi boiler, because solar panels need a hot water tank or cylinder. And part of the point of a combi boiler is that you don’t need a tank.

No backup: If your combi boiler breaks down, you have no alternative source of hot water other than the kettle or cooker. A hot water tank with an immersion heater, on the other hand, means you can still heat water even if the boiler isn’t working.

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?

The larger your house, the more powerful boiler you’ll need, since it will need to heat more space and possibly provide more hot water.

what size combi boiler do I need for my house

These figures are guides only, and there’s room for variation within that, depending on how big your rooms are, for example.

The size of the boiler you have now should be a rough guide, bearing in mind that changes may have been made to your house since it was installed, such as an extension or new insulation.

Flow Rate

You may also see manufacturers talking about flow rate. The flow rate is the amount of water which comes through your taps from the mains, measured in litres per minute. Your boiler’s flow rate should more or less match your home’s flow rate – which you can find out by putting a container under the cold tap for a minute and measuring how much water it collects.

Your flow rate multiplied by 3.5 gives you a rough idea of how powerful a boiler you’ll need in kilowatts, assuming your cold water comes in at about 10C. Your installer will be able to check your temperature and flow rate and help you make an accurate assessment.

There is no point buying a more powerful boiler than you need; you will just waste energy. Similarly, a boiler with too low a flow rate will struggle to heat your water properly, but there is no advantage to a boiler with a higher flow rate than your mains water.

Here, Francis, a Gas Safe registered boiler engineer, discusses what size combi boiler is right for your home and hot water needs:


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Small Combi Boilers for Flats and Houses

If you live in a small property with only 1 bathroom, a small entry-level combi boiler should provide you with all the heating and hot water you need.

Small combi boilers are ideal for compact flats or houses, as they are often dinky enough to be tucked inside a kitchen cupboard, hiding the boiler from view and saving you lots of space. Brands such as Worcester Bosch produce a range of compact combi boilers for small properties, which measure just 69cm x 39cm x 28cm in size, making them the smallest combi boilers on the market.

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 comfortably heat up to 10 radiators and provide hot water to 1 bathroom.

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. After all, there's no point installing a bigger combi boiler than you need.

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Should I get an Oil, Electric or Gas Combi Boiler?

Most boilers in the UK run on mains gas, and it’s generally considered the cheapest option, but there are alternatives you can choose, either out of preference or because your home isn’t connected to the mains gas supply. Electric or oil-fired boilers are the 2 most common options but solid fuel boilers are also an alternative.

Oil Combi Boilers

Oil-fired boilers work in a similar way to gas boilers, but by burning oil instead of gas. However, whereas gas boilers draw gas from the mains supply, there is no mains oil supply: you'll need to have your oil supply delivered and stored in a tank.

Oil is a more efficient fuel than gas, but that doesn’t mean oil-fired boilers are cheaper than gas boilers: the prices of both, along with the price of electricity of course, can fluctuate, and historically oil has been more expensive. Also, since you won’t have an unlimited supply of oil, you’ll need to make sure you don’t run out.

Electric Combi Boilers

Electric boilers don’t burn fuel or produce waste gases, so they can be incredibly efficient – up to 99%. Because they don’t have a flue, they’re usually pretty compact, and you have more freedom over where you install them as they don’t need to be on an outside wall. For the same reason, installation is usually cheaper. Electric boilers are very quiet, and unlike with gas, there is no risk of carbon monoxide poisoning from a malfunctioning electric boiler.

However, despite the increased efficiency, electricity tends to be more expensive than gas. The process of producing the electricity itself is quite carbon-intensive, so the claim that electric boilers are more eco-friendly is dubious. Electric boilers don’t usually have the capacity to heat as much water as gas boilers. And of course if you have a power cut, you have no hot water or heating at all.

Gas Combi Boilers

If you have a gas combi boiler, you must have it serviced every year by a qualified, registered gas engineer. An installer or engineer is legally required to be on the Gas Safe Register (what used to be CORGI registered). A gas boiler which isn’t working properly can release carbon monoxide, which is poisonous, so we advise having a carbon monoxide detector near your boiler too.

For oil-fired or solid fuel boilers, the installer or engineer should be registered with OFTEC.

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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?

A boiler’s ErP (Energy related Products) rating tells you how efficient it is. Ratings go from G to A+++, with G being the lowest and A+++ being the highest. Almost all new boilers have an A rating, as the law requires all new boilers to be at least 88% efficient.

You might also see a SEDBUK rating, which stands for seasonal efficiency of domestic boilers in the UK. This will usually be expressed as a percentage, such as 88% efficient. (though, just to be confusing, before 2009 SEDBUK ratings were expressed as letters from A to G, but were different from the ErP rating).

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Combi Boiler Prices

As a general rule, the price of combi boilers start at between £400 and £500 and go 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. The Ideal Exclusive 24kW combi, which comes with a five-year warranty, retails at around £520.

The more powerful your boiler is, the more expensive it is likely to be. So the least expensive options are likely to be those suitable for a 1 or 2-bedroom flat, or a 2-bedroom house. For a 3-bedroom house, which will probably need a 28kW boiler, prices start from around £700 or £800, whereas for 4 bedrooms plus, you’ll probably be looking at £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, hit this button to get a quote from our suppliers.

Get a Bespoke Combi Boiler Quote for Your Home

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

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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 2018

Worcester Bosch

Worcester Bosch is one of the UK market leaders, with 55 years’ experience in manufacturing boilers, and has a strong reputation. It was the top brand in Which?’s annual survey in 2017, receiving 5 stars from both engineers and customers, and 5 stars for reliability. Worcester Bosch was the most highly recommended brand by engineers and many of its models were rated as Which? Best Buys. 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 was the second most popular brand in Which’s annual survey, receiving 5 stars from customers and 5 stars for reliability. The company 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. Many of its models were rated as Which? Best Buys.


Vaillant is among the UK market leaders and was rated highly by both customers and engineers in last year’s Which? survey. It was the second most popular brand with engineers, scoring 5 stars for ease of fixing problems and build quality. It also has 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 scored well with both customers and engineers in the Which? survey, and won a Good Housekeeping Reader Recommended award in 2017. Ideal was also the second UK boiler manufacturer to receive a Quiet Mark from the Noise Abatement Society, for its Logic+ range.


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. The brand was not listed in Which?’s satisfaction survey but a number of its boilers were Best Buys.


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. The company was ranked in the top 5 brands in the 2017 Which? survey, receiving a 4-star customer rating, and 98% of customers on the reviews website Reevoo said they would recommend Baxi. The company also won the energy efficiency award and the customer service award in the Gas Industry Awards 2017, and its Baxi 400 Combi was a finalist in the heating product of the year category in the Heating and Ventilation News Awards 2017.

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

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