The Ultimate Guide to Combi Boilers 2019


A new, efficient boiler can save you up to £315 per year on your energy bills

Combi boilers are perfect for small households with 1-2 bathrooms

Fill in the form above to receive free boiler quotes from our professional installers

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.

A combi boiler (cool slang for ‘combination boiler’) is a nifty, compact alternative to a traditional boiler, perfect for households that are running low on space. They were originally designed for one-bedroom or two-bedroom homes, but more recent models are capable of supplying much bigger properties.

a combi boiler on a blue wall

On this page, we’ll tell you everything you need to know about combi boilers, including how they work, and how much they’ll cost you. We’ll also give a rundown of the best combi boilers of 2019.

To start collecting free quotes for a new combi boiler, simply fill in this form, and our professional installers will be in touch.

What’s in this guide to combi boilers?

Head straight to a specific section by clicking the links below

What is a combi boiler?

A combi boiler combines two functions in one place: hot water and central heating. So, instead of requiring a hot water tank (like a system boiler), everything takes place within one compact, talented unit. A combi boiler heats up water only when you need it, which is much better for the environment, and great for your energy bills.

How does it 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.

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 the tap or shower end, and you simply won’t believe that it was cold just moments before.

What about central heating?

The central heating process is very similar; cold water quickly turns into hot water, and 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 reheating, and then get going again. It sounds exhausting.

However, although a combi boiler can do both, it can never do both at the same time. It can send hot water to your tap/shower or send hot water to your radiators, but won’t be able to handle both simultaneously.

This doesn’t mean that you won’t have hot radiators while your shower’s running – it just means you won’t be able to turn the heating up until you’re done showering.

how a combi (combination) boiler works

As you can see in the diagram above, your household’s cold mains feed connects directly into the combi boiler. This means that your combi boiler’s water pressure will only ever be as high as the pressure of your cold mains feed – although for most households, this is usually sufficient (around 2.5 bar).

Please note: a combi boiler’s water pressure is affected by the number of outlets being used at any one time. It can manage one hot shower, but have two running together, and you’ll see a decrease in pressure. Turn on a hot water tap, and there’ll be a further reduction. This is why combi boilers are ideally suited to smaller homes.

How is it installed?

The installation of a combi boiler is a fairly straightforward affair. You won’t need to have a cold water tank or a hot water tank fitted, and a combi boiler comes with significantly less pipework than any other type of boiler.

However, unless you’re a qualified plumber, we certainly don’t recommend fitting a combi boiler yourself – especially if it’s going to be powered by gas. Get yourself a Gas Safe registered engineer to do the job for you.

Not only is this a much more sensible way of going about things, but you’ll also invalidate your boiler’s warranty if you perform the installation yourself.

What is a condensing combi boiler?

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. nitrogen 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, too.

Condensing combi boilers are generally around 95% efficient, which means 95% of their energy is used to create heat for you. To put it another way, for every £1 you spend on heating, only 5p of that is wasted.

Compare that to certain older boilers that struggle along at just 55% efficiency (embarrassing). This is why making the switch from an old boiler to a high-efficiency combi boiler can make such a difference to your energy bills.

Combi boilers vs other types

We put the combi boiler head-to-head against the two other popular types of boiler, assessing them based on features such as eco-friendliness, noise, and price. For clarification: the better the star rating, the less noisy the boiler is.

Please note that all the below values are averages only.

Type of boiler
Energy efficiency
ErP rating
Installation cost

We’ve also ranked the four most popular ways of powering a boiler, again based on the same factors. You’ll see that gas and electric come up tops.

Type of boiler
Energy efficiency
ErP rating
Installation cost

As you can see, the world of combi boilers can be a little overwhelming. If you need a little more guidance before taking the leap and forking out all that money, get in contact with our local professionals to receive recommendations and quotes.

How do we assess the best combi boilers?

Here’s a breakdown of all the key features we’ve used to assess combi boilers. You’ll notice that some of these aren’t included in the tables above, but you’ll certainly find them in our more technical tables below (where we discuss specific models of combi boiler).


Easily one of the most important factors to consider when you’re making a significant investment. Even if a certain type of boiler is perfect for your home, you’ll always be restricted by your budget.

Energy efficiency

A boiler’s ‘energy efficiency’ refers to the percentage of energy created that is used by your house vs. the energy that goes to waste. Naturally, the more inefficient the boiler, the more money you’ll be spending to heat your home. Therefore, it’s really important that you get a high-efficiency boiler.

You’ll see energy efficiency measured in two ways.

1) Firstly, the ErP rating (previously known as SEDBUK) assesses the boiler’s efficiency in two categories: hot water and central heating. This is given as two separate grades (from A-G), and most boilers on the market today achieve an AA rating.

2) Secondly, there’s also a percentage, which refers to the proportion of fuel that is converted into usable energy for your home. For example, if you had a boiler with 95% efficiency, that would mean for every £1 you spend on gas/electricity, you’d lose 5p on wasted energy.

It is now law in the UK that all boilers must have at least 92% efficiency.


Every boiler makes a noise, ranging from a whispery whir to a loud hum. The quieter the better, obviously. A boiler’s noise is measured in decibels (dB), but this might not mean anything to you. To use an everyday example, a human conversation typically ranges between 40-60dB, so ideally you should be looking for a boiler that’s 50dB or below.

a close-up of a boiler pressure gauge


Normally measured in kilowatts (kW), a boiler’s ‘output’ refers to how much hot water it can generate at any given time. If you live in a one-bedroom or two-bedroom house, a 24-27kW boiler will probably be sufficient, capable of running about 10 radiators and one bathroom. Meanwhile, in a larger house (i.e. three or four bathrooms), a 28-35kW boiler would be more suitable.


The key culprits in any boiler’s ‘waste gases’ are nitric oxide and nitrogen dioxide (collectively known as NOx), which aren’t great for the environment. NOx goes around causing reactions that create ozone and acid rain, which in turn damage forests and lakes (and lead to health problems). A boiler’s NOx output is measured in mg/kW, and is typically around the 20-40 mark.

Did You Know?

Replacing your boiler with a newer combi boiler could help you to save up to £300 a year on your heating bills. By using less energy, you can cut your carbon footprint down a substantial amount.


You always have to consider how much space your new boiler is going to take up. Fortunately, the key advantage of combi boilers is that they’re all fairly small, and some of them are even the size of a kitchen cupboard.

Flow rate

Specific to combi boilers, ‘flow rate’ refers to the quantity of hot water that can pass through a tap in one minute (measured in litres per minute, or l/m). Although each combi boiler comes with its own ‘flow rate’, your hot water pressure will ultimately be determined by the pressure of your cold water mains feed.

Pros and cons of combi boilers

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.

Is a combi boiler right for you?

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. For example, a 24-27kW combi boiler can heat up to ten radiators and provide hot water for one bathroom.

However, combi boilers are not limited to small properties. Certain models can reach up to 50kW of power, and are capable of serving multiple bathrooms.

what size combi boiler do I need for my house

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

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

Gas 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 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.

Electric 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.

How much does a new combi boiler cost?

A new combi boiler typically costs around £700, although the cost of installation will push the total up to around £2,500. Check out the table below for a general idea of combi boiler costs.

Run (annual)
Buy + install
Combi (gas)£700£1,850£551£3,101£2,550

For more information, take a look at our detailed guide to boiler costs in 2019. On this page, we also cover system boilers, conventional boilers, and biomass alternatives.

Cheap combi boilers

If you’re looking for a combi boiler that’s reliable and affordable, consider the Baxi 200 Combi.

Endorsed by the Energy Saving Trust, Baxi’s 200 Combi is efficient, easy to install, and can be operated using a smartphone or tablet. It ticks all the boxes without costing too much.

The data below is for the 24kW model.

Baxi 200 Combi 24kW
ErP ratingAA
FuelGas condensing
Noise (decibels)48
Power output (kW)24
Nitrogen oxides (mg/kWh)38
Size (mm)700 x 395 x 279
Weight (kg)26
Typical price£613.63*
Warranty2 years standard**
Combi flow rate at the hot water tap (@ 35°C temp rise)9.8l/m

*Price information sourced from Excluding the cost of installation.

**Available when registered within 30 days of installation and serviced annually.

The best combi boilers in 2019

We’ve scoured the market and handpicked the seven best combi boilers of 2019, based on key features such as efficiency, power output, warranty, and price. All information has been sourced from the boiler models’ official datasheets, apart from the prices, which are from

We’ve ranked the boilers in terms of their efficiency, but other features might be more important to you.

This is by no means an exhaustive list. You can learn more in our complete guide to the best boilers of 2019, which includes system and conventional boilers, too.

Power output
Standard warranty*
Best for
Worcester BoschGreenstar 8000 Style30-50kW94%5 yearsPower output
VaillantecoTEC pro24-30kW94%2 yearsA bit of everything
VaillantecoTEC Plus Combi Store 93838kW94%5 yearsQuietness
IdealLogic+ Combi24-35kW94%7 yearsStandard warranty
ViessmannVitodens 200-W26-35kW98%3 yearsEnergy efficiency
BaxiDuo-tec24-40kW92%7 yearsEco-friendliness
PottertonAssure25-36kW93%2 yearsSmall size

*All models listed here (except the Potterton Assure) have 10-year warranties available, dependent on particular T&Cs.

That’s a swift breakdown of the top seven, but here’s a little bit more detail about each of the boilers, including the best options for smaller and larger homes.

Worcester Bosch Greenstar 8000 Style

The best combi boiler for power output

The Greenstar 8000 Style is one of the most powerful combi boilers on the market, with output capability ranging from 30kW to a mighty 50kW. This means it can power up to three bathrooms, with an impressive flow rate of 17.9l/m (for the 50kW model).

This boiler also comes with an Intelligent Filling System, meaning it’s wired to correct any pressure drops automatically, saving you the faff. What’s more, it has a colourful LCD display, and wireless connectivity to a smart thermostat (called Bosch EasyControl).

You can get the Greenstar 8000 Style in five different outputs: 30kW, 35kW, 40kW, 45kW, or 50kW. The data below is for the 50kW model.

Greenstar 8000 Style 50kW
ErP ratingAA
FuelGas condensing
Noise (decibels)53
Power output (kW)50
Nitrogen oxides (mg/kWh)24
Size (mm)780 x 440 x 365
Weight (kg)56
Typical price£2,180.98
Warranty5 years standard, with up to 10 years available (T&Cs apply)
Combi flow rate at the hot water tap (@ 40°C temp rise)17.9 l/min

Vaillant ecoTEC pro

The best combi boiler for a bit of everything

Hand-built in Vaillant’s award-winning factory in Derbyshire, the ecoTEC pro is a solid performer in all categories. Its easy-to-use setup is a big plus point for users, and the boiler offers an impressive 94% efficiency. There’s also an LPG alternative available for off-grid households where gas isn’t an option.

The ecoTEC pro comes in three different outputs: 24kW, 28kW, or 30kW. The data below is for the 24kW model.

Greenstar 8000 Style 50kW
ErP ratingAA
FuelGas condensing
Noise (decibels)49
Power output (kW)24
Nitrogen oxides (mg/kWh)27
Size (mm)720 x 440 x 338
Weight (kg)38
Typical price£964.28
Warranty2 years standard, with 7 to 10 years available when you book a Vaillant Advance installer
Combi flow rate at the hot water tap (@ 35°C temp rise)9.6 l/m

Vaillant ecoTEC Plus Combi Store 938

The best combi boiler for quietness

With just 43 decibels of noise, the Vaillant ecoTEC Plus Combi Store 938 is one of the most peaceful combi boilers on the market, having secured a Quiet Mark accreditation. This is an award given to ultra quiet, reduced-noise designs by the Noise Abatement Society. The folks there just hate noise.

This boiler is only available in one capacity – 38kW – which means it’s highly suited to larger households (i.e. two or three bathrooms). Check out the key data below.

Vaillant ecoTEC Plus Combi Store 938 38kW
ErP ratingAA
FuelGas condensing
Noise (decibels)43
Power output (kW)38
Nitrogen oxides (mg/kWh)35
Size (mm)720 x 440 x 600
Weight (kg)60
Typical price£1,573.99
Warranty5 years standard, up to 10 years available when fitted with a Boiler Protection Kit by a Vaillant Advance installer
Combi flow rate at the hot water tap (@ 35°C temp rise)15.9l/m

Ideal Logic+ Combi

The best combi boiler for warranty

Most combi boilers come with 2-5 years standard warranty, but the Ideal Logic+ Combi offers an impressive 7 years. There’s also a 10-year warranty available, although this is subject to specific terms and conditions (such as annual servicing from an Ideal engineer).

This boiler is endorsed by the Energy Saving Trust, and it’s compact enough to fit inside a cupboard. Plus, its transparent condensate siphon means that you can easily spot any debris, and deal with it before it causes problems.

The Logic+ is available in three different capacities: 24kW, 30kW, or 35kW. The two larger sizes have LPG alternatives, but the 24kW model is gas-only. The data below is for the 24kW model.

Ideal Logic+ Combi 24kW
ErP ratingAA
FuelGas condensing
Noise (decibels)48
Power output (kW)24
Nitrogen oxides (mg/kWh)28
Size (mm)700 x 395 x 278
Weight (kg)29.2
Typical price£791.98
Warranty7 years standard, up to 10 years available subject to T&Cs
Combi flow rate at the hot water tap (@ 35°C temp rise)9.9l/m

Viessmann Vitodens 200-W

The best combi boiler for energy efficiency

Looking to get the most out of your energy bill spend? The Viessmann Vitodens 200-W has 98% efficiency, meaning only 2p of each £1 you spend on heating your home goes to waste. Unsurprisingly, this boiler was endorsed by Which? as a Best Buy in September 2018.

With fancy components like a MatriX cylinder burner and an Inox-Radial heat exchanger, the Vitodens 200-W is a high-functioning genius.

It’s not really suited to small, one-bedroom properties, as it’s only available in three fairly hefty capacities: 26kW, 30kW, or 35kW. The data below is for the 35kW model.

Viessmann Vitodens 200-W 35kW
ErP ratingAA
FuelGas condensing
Noise (decibels)51
Power output (kW)35
Nitrogen oxides (mg/kWh)39
Size (mm)850 x 450 x 375
Weight (kg)40
Typical price£1,605.23
Warranty7 years standard, up to 10 years available subject to T&Cs
Combi flow rate at the hot water tap (@ 35°C temp rise)9.9l/m

Baxi Duo-tec Combi

The best combi boiler for eco-friendliness

If you hate nitric oxide and nitrogen dioxide as much as Mother Nature does, the Baxi Duo-tec Combi will align perfectly with your feelings. This boiler produces just 20g of NOx per kWh, which is impressively low. Whenever there’s acid rain or a local polluted lake disaster, you’ll know you had practically nothing to do with it.

On top of its mere trickle of NOx, this boiler also comes with a 7-year standard warranty, matching Ideal’s Logic+. However, there’s a tradeoff; the Duo-tec is slightly too big to fit inside a kitchen cupboard.

You can get the Duo-tec in four different capacities: 24kW, 28kW, 33kW, or 40kW. If you want an LPG alternative, we’re afraid that only applies to the 28kW model. The data below is for the 24kW model.

Baxi Duo-tec Combi 24kW
ErP ratingAA
FuelGas condensing
Noise (decibels)50
Power output (kW)24
Nitrogen oxides (mg/kWh)20
Size (mm)780 x 450 x 345
Weight (kg)40.2
Typical price£722.98
Warranty7 years standard, up to 10 years available subject to T&Cs
Combi flow rate at the hot water tap (@ 35°C temp rise)9.8l/m

Potterton Assure Combi

The best combi boiler for small size

You’ll barely notice the Potterton Assure Combi. Measuring just 700m in height, 390mm in width, and 285mm in depth, this scrunched up combi boiler will easily tuck inside one of your kitchen cupboards. It also weighs less than 30kg, which is at least 10kg less than the average combi boiler.

Naturally, a boiler this small is quick and easy to install, meaning you won’t have an engineer round for too long. The Assure Combi is available in outputs of 25kW, 30kW, or 36kW, with LPG alternatives for the 25kW and 30kW models. The data below is for the 25kW model.

Potterton Assure Combi 25kW
ErP ratingAA
FuelGas condensing
Noise (decibels)48
Power output (kW)25
Nitrogen oxides (mg/kWh)31
Size (mm)700 x 390 x 285
Weight (kg)29.2
Typical price£796.06
Warranty2 years standard, subject to registration and annual service
Combi flow rate at the hot water tap (@ 35°C temp rise)10.2 l/m

Combi boiler grants

Not sure you can cobble together the funds for a new boiler? The UK Government’s Energy Company Obligation (ECO) might be of help to you. If you can meet certain requirements, you could be eligible to have your old boiler replaced for free (or at least subsidised).

So what are the requirements? Crucially, you need to be living in social housing, and own a non-condensing boiler that’s a minimum of five years old. Take a look at the official government page to find out how to apply, although after September 2019, the scheme will apply only to gas boilers.

Problems with combi boilers (FAQs)

Here we delve into some problems that combi boiler owners sometimes have to deal with. However, we strongly advise that you don’t attempt to fix boiler issues on your own – especially if you have a gas boiler.

A helpful Gas Safe registered engineer is usually just a phone call away.

Why does my combi boiler have low pressure?

There are several reasons why your combi boiler might have low pressure, such as a water leak in the system, a broken pressure valve, or if you’ve recently bled one of your radiators.

It’s possible to repressurise your boiler by turning the small tap on your boiler’s filling loop (and increasing the pressure back to 1.5 bar), but this isn’t recommended as a solo enterprise, particularly if you have a gas boiler. Phone an engineer.

Why is my combi boiler not heating my radiators?

Hot water running, but no central heating? You might have pockets of air trapped in your radiators, which can lead to unequal heat distribution (and cold patches).

You can bleed your radiator to release the air, but bear in mind this could also lead to an overall drop in your system’s pressure. This sounds like a catch-22, but it’s easy to fix low system pressure (see above).

Why is my combi boiler not turning on in winter?

If your combi boiler isn’t playing ball on an ice-cold day, this is probably because its condensate pipe is frozen. For a handy explanation of what to do, check out Viessmann’s guide to thawing a frozen condensate pipe.

Why is my combi boiler switching itself off?

A combi boiler that keeps switching itself off might simply be begging you for a service.

Something could be wrong with the flue pipe, and this can affect the entire system. It’s simple: arrange a boiler service, have the flue pipe checked for any blockages/corrosion, and make sure the burners and jets get cleaned. Your boiler might just need repressurising, but this is something that an engineer should decide.

Finding a boiler installer

If a combi boiler sounds like the right thing for your home, your next step is to find the most suitable model and a reliable installer – all for a competitive price. That’s where we come in. Simply fill in this short form, and our bunch of local, professional installers will be in touch. Happy heating!

Charlie Clissitt Content Manager

When it comes to all things eco-friendly and economically savvy, Charlie is a spectacularly woke millennial. A university-educated solar panel scholar with an eye for detail, when Charlie isn’t writing, he’s tucked up in bed watching his favourite black and white film.