Best Combi Boilers for 2018

There are many companies manufacturing boilers that your installer might suggest suits your needs best.  Here is Tradesmenprices’ run-down of Britain’s best combi boiler makers and the prices you can expect to pay for boilers before the cost of installation is added.

Also, where we it’s available, we’ve added extra information about the range of flow rates (how much water can travel through your boiler at any one time and how fast it comes out of your tap or shower) and boiler output.

Boiler output is a way of describing the size of a boiler. The bigger the output, the better. If you get a boiler that’s too small, you won’t get much heat in your home and it won’t produce enough hot water to run a bath. If you get one that’s too big, that’ll cost you money in higher utility bills.

Tradesmenprices rule of thumb – for most flats and smaller homes, 24 to 27kW output will be fine. For bigger houses with 3 or 4 bedrooms, 28-34kW should suit your needs. Anything larger, go for 35kw and above.

Always make sure that, when you get a quote, you ask your tradesman the reason behind their recommendation.

Worcester Bosch

Scoring 5 out of 5 on Trustpilot over 5,554 reviews (at time of writing), Worcester Bosch is Britain’s leading boiler manufacturer by the number of boilers sold.

Formed in 1962 and becoming part of the larger Bosch Group in 1992, all of Worcester Bosch’s current range of boilers have an energy efficiency rating of 90% or above.

There are currently 13 Worcester Bosch combi boilers available:

  • Height in range of boilers – 680mm to 850mm
  • Width in range of boilers – 390mm to 600mm
  • Depth in range of boilers – 280mm to 600mm
  • Minimum flow rate range – 9.8 litres per minute
  • Maximum flow rate range – 20 litres per minute
  • Minimum boiler output – 24kW
  • Maximum boiler output – 42kW


Scoring 98% on 509 Reevoo online customer reviews (at time of writing), Baxi have been important UK manufacturers in the boiler market since 1866.

There are currently 18 Baxi combi boilers available:

  • Height in range of boilers – 700mm to 780mm
  • Width in range of boilers – 390mm to 450mm
  • Depth in range of boilers – 290mm to 345mm
  • Minimum flow rate range – 9.8 litres per minute
  • Maximum flow rate range – 16.4 litres per minute
  • Minimum boiler output – 24kW
  • Maximum boiler output – 40kW


Established in 1874, Valliant’s entire range of domestic boilers were the first to win the Quiet Mark accreditation – the first in the industry – and joined Britain’s list of “Superbrands” the year after.

There are currently 8 Valliant combi boilers available:

  • Height in range of boilers – 720mm to 800mm
  • Width in range of boilers – 440mm to 480mm
  • Depth in range of boilers – 335mm to 597mm
  • Minimum flow rate range – 9.4 litres per minute
  • Maximum flow rate range – 15.2 litres per minute
  • Minimum boiler output – 24kW
  • Maximum boiler output – 38kW


An international company with a dedicated UK presence since 1989 in Telford, Viessmann are focused on delivering effective and efficient boilers to British homeowners – in their current range, 98% or higher of the energy used by one of its boilers is turned into heat.

There are currently 16 Viessmann combi boilers available:

  • Height in range of boilers – 700mm to 1,875mm
  • Width in range of boilers – 400mm to 600mm
  • Depth in range of boilers – 340mm to 595mm
  • Minimum boiler output – 19kW
  • Maximum boiler output – 35kW


Derbyshire-based Glow-worm started manufacturing boilers in 1934. It was bought by Valliant in 2001 but has retained the Glow-worm brand in the UK because of the high regard the British public has for the company.

There are currently 13 Glow-worm combi boilers available:

  • Height in range of boilers – 700mm to 890mm
  • Width in range of boilers – 390mm to 500mm
  • Depth in range of boilers – 338mm to 561mm
  • Minimum flow rate range – 9.8 litres per minute
  • Maximum flow rate range – 19.1 litres per minute
  • Minimum boiler output – 24kW
  • Maximum boiler output – 35kW


Proud holders of a Good Housekeeping Reader Recommended award, the Quiet Mark accreditation, and the Queen’s Award for Enterprise for its boilers, Ideal have been producing boilers since it was founded in 1906.

There are currently 18 Ideal combi boilers available:

  • Height in range of boilers – 700mm to 830mm
  • Width in range of boilers – 395mm to 445mm
  • Depth in range of boilers – 278mm to 330mm
  • Minimum flow rate range – 9.9 litres per minute
  • Maximum flow rate range – 16.4 litres per minute
  • Minimum boiler output – 24kW
  • Maximum boiler output – 40kW


Over seven million households have a Potterton boiler. Now part of Baxi Heating, the company has been making boilers for homeowners, house builders, businesses, councils, and housing associations since the mid-19th century.

There are currently 18 Potterton combi boilers available:

  • Height in range of boilers – all at 780mm
  • Width in range of boilers – all at 450mm
  • Depth in range of boilers – all at 345mm
  • Minimum flow rate range – 9.8 litres per minute
  • Maximum flow rate range – 16.1 litres per minute
  • Minimum boiler output – 24kW
  • Maximum boiler output – 40kW

Since 2005, all boilers in the UK have had to be what is known as ‘condenser’ boilers. These are water heaters fuelled either by gas or oil. They usually achieve over 90% energy efficiency making them a popular choice across the country.

There are two types of condenser boiler – conventional and combination (combi).

From these two, combi boilers are the most popular, accounting for more than half of all boilers installed across the UK. They’re widely considered the best type of boiler for smaller homes in particular.

Types of Combi Boiler

There is only one type of combi boiler although they can vary in size and capacity depending on the type of home they’re made to fit.

Combi boilers are gas-fuelled and get their name from their ability to combine both central heating and hot water facilities into a single unit. They work by heating water directly from the mains so you don’t need to make space for a hot water storage heater or cold-water tank.

In houses with one bathroom, combi boilers are a great choice as they supply water at mains pressure, meaning you can enjoy a steaming hot power shower without worrying about the tank running out or a lack of pressure as the water levels decrease.

Additionally, they’re far smaller than conventional boilers and they can be installed in kitchens or utility rooms, often fitting inside a cabinet to minimise any unattractive looking appliances on show.

Combi boiler cost

The cost of a combi boiler cost will vary depending on the size of your home, as well as the brand, and who you buy it from. Associated costs include boiler cover plans and boiler maintenance fees.

Is your:

  • Existing combi boiler being changed for a new boiler and being installed in the same location?
  • This could set you back around £1,995.
  • Existing combi boiler being changed for a new boiler and it’s being installed somewhere else in your home?
  • This may cost you around £2,600.
  • Existing regular boiler (and its hot water cylinder unit) being replaced with a brand new combi?
  • As this requires more work, this could cost you around £3,000.

The above examples reflect what it would cost you for a mid-range combi boiler installed at a standard labourer’s rate. It’s important to note that depending on where your new combi boiler is installed and which type of boiler you had before. These prices will vary so it’s advisable to get as many quotes as possible before having one installed.

You can use the Tradesmen Prices form just below to secure 3-4 combi boiler quotes with no obligation from local boiler expert. It’s a great way to make sure you pay only what you need to for your new system.

Combi Boiler FAQ 

Why would someone want a combi boiler?

Combination (combi) boilers are a popular alternative to traditional conventional boilers as they can provide an unlimited supply of hot water on demand.

They are space saving, quiet, and typically cheaper to install than a traditional conventional boiler, though this does depend on which boiler you currently have. 

What are the disadvantages of combi boilers?

All boilers have their drawbacks which is why there are so many different types available. It’s important to choose the boiler that does exactly what you need it to do. Combi boilers may be efficient and cost-effective, but they do suffer from several drawbacks. They are:

They require a good level of mains pressure to work effectively.

Because combi boilers heat water directly from the mains, they don’t work well in properties where the mains flow is weak. If this is a problem where you live, you may want to consider a different type of boiler.
They aren’t compatible with all types of shower.

Power showers will not work with a combi boiler as the water pressure is at mains level. However, if your mains pressure is strong, you may be able to achieve power shower pressure through your usual showerhead.
They can’t run more than one shower or bath at a time.

This is a combi boiler’s main drawback and the reason that properties with several bathrooms or taps benefit more having a conventional boiler system. Because combi boilers supply water directly from the mains, running two different hot taps draws the water two separate ways, resulting in decreased water pressure.

They don’t feature a backup water heater.

If your combi boiler malfunctions, there is no backup heater meaning you will be without both hot water and central heating until it is fixed. 

Are there alternatives to combi boilers?

There are several alternatives to combi boilers. The most common alternative to a combi boiler is having a conventional boiler installed.

Conventional boilers use gas to heat a tank of water through cast iron heat exchangers. They are common in older or larger properties where several people may need to use the hot water at the same time (for example across multiple bathrooms, the kitchen, and so on). They do require more space than combi boilers do, which is why they are less common in smaller properties like flats or bungalows.

However, newer conventional boiler models are smaller than their previous counterparts, and have increased efficiency, too, making them still a fairly popular choice in the UK.

If your property is off the main gas grid, you may benefit from oil-fuelled boilers, or Liquid Petroleum Gas fuelled heating. 

What are the main benefits of combi boilers?

Combi boilers are compact, fitting into kitchens with ease, and relatively quiet compared to conventional boilers – the most you might hear is a quiet rumble as the heating or hot water is turned on.

They also supply water directly from the mains, which means hot water is always available and there is no decrease in water pressure, even after extended periods of use.

Combi boilers also come in a broad range of styles and sizes, so there is nearly always going to be a model that will suit your home. 

Who can I buy a combi boiler from?

Thousands of companies across the UK offer the supply and installation of combi boilers. The reason for their popularity among both consumers and installers is that they’re widely considered to be a more eco-friendly and cost-effective alternative to conventional boilers.

You may find that if you live in a council property where the boiler needs replacing, you could access a grant or subsidisation for a combi boiler. The government funded ECO scheme is behind this initiative, and you can find out if you are eligible for it on their website.

If you want to buy your combi boiler through an independent company, it’s worth knowing the difference in pricing between small and large organisations, and why they occur.

A larger company will have to pay sales commissions, and they’ll be subject to higher running costs. This means the price you pay is significantly higher than what you would pay a smaller company for the same combi boiler and installation.

If you have a quote for a combi boiler already, you may want to compare it with a few more through Tradesmen Prices. If you fill in the form at the bottom of this page, we’ll put you in touch with 3-4 local tradesmen who will be able to offer you competitive prices for the supply and installation of your new combi boiler, as well as advice on which model you would benefit from choosing. 

Should my combi boiler installer be a member of anything (trade body, government registration scheme, etc?)

Your engineer must be on the Gas Safe Register (this used to be known as Corgi Registered). The Gas Safe Register contains both individuals and companies with the knowledge and experience to handle installations, repairs, and maintenance of any equipment which runs on gas. To check if an installer is on the Gas Safe register, you can click here.

If a potential installer is a heating technician, look out for MCS certification or OFTEC accreditation. 

How long does it take to install a combi boiler?

If you’re looking at a straightforward combi boiler swap (i.e. swapping one combi boiler with another, in the same location) the job should be done in a day. In fact, if the engineers start in the morning, the installation may be finished by the time you come home from work.

Installing a new combi boiler in a different location to your old one will take longer as your pipework will need to be diverted. Depending on where the new combi boiler is being installed, you can expect the work to take 1-2 days.

Should you wish to change an older boiler for a combi, this may take around 3-4 days. Of course, this depends on the complexity of the installation and how much access the engineer has to it. They will need to remove the old hot water cylinder and re-pipe the entire central heating system.

Before you get your combi boiler installed, your central heating system will have to undergo a ‘flush’. This is to get rid of any debris inside the system such as rust from the insides of your radiators. This can be achieved using a hot chemical flush, or a power flush.

Depending on the age and size of your central heating system, it’s likely you’ll be choosing one or the other – older systems (especially those containing old pipework and copper joints) can suffer strain from a power flush, causing leaks to occur.

Depending on how many radiators there are in your property, a power flush can take up to 8 hours whereas a hot flush takes around 1 or 2 hours. 

Before I get a combi boiler, what should I ask a potential installer?

Since working with gas is a specialist area, it’s wise to ask your potential combi boiler installer to prove that they’re Gas Safe registered.

Additionally, you may want to consider asking them: 

How long will it take to install my combi boiler?

Although this page can give you a good estimate, the actual installation time of your boiler is entirely down to your individual situation. Your potential boiler engineer will be able to confirm how long the installation will take, as well as speak to you about any complications that may arise. 

What boiler output size do you recommend?

Choosing the right size boiler is essential. The right size boiler will produce the hot water you need and won’t run up your energy bills higher than expected. Make sure that, with every tradesman you get a quote from, you ask them the reason behind their boiler output size recommendation.
Will I need to redecorate?

Re-piping can sometimes cause disruption in your home, so it’s worth asking your engineer if they’re going to redecorate or simply clean up after they fit your combi boiler.

Which boiler cover plans are available to me?

As with any appliance, combi boilers require servicing to keep them in top condition and working effectively. Ask your combi boiler engineer about coverage plans, and how much they will cost. Often, you’ll be offered full boiler cover (including annual maintenance, call-outs, and part replacement) at a monthly premium, but plans will differ depending on your provider.

Why should I get combi boiler quotes from Tradesmen Prices?

At Tradesmen Prices, we work with local suppliers of combi boilers to bring you 3-4 quotes you can compare against ones you may already have. We check their history and credentials to make sure they:

  • Are genuine traders
  • Are Gas Safe Registered
  • Will offer you competitive prices

Also, because you’ll have several quotes, you’re likely to see prices decrease as they compete for your custom. Fill in the form above to get started.

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