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What is a Combi Boiler?

Combi boilers are the home heating dream that the human race has been striving towards since the discovery of fire. Here's what they are, how they work and what they cost.

Combi boilers or combination boilers aren’t truly appreciated until something goes wrong, and when they do go wrong, boy, you'll know.

If you are on the hunt for a new boiler replacement, you have probably encountered the various different types of boilers available. 

Combi boilers are the most frequently installed in UK households, so if you live in an average sized property, it’s likely the best choice for you. 

First, why are Combi Boilers so popular? 

Combi boilers provide both hot water and central heating from a single unit making them the most efficient type of boiler. In fact, modern combi boilers are up to 98% efficient. 

Imagine if Usain Bolt was your postman, but he also did your bins. A combi boiler is like that... basically.

Due to their compact design, they are also ideal for homes with limited space, being able to be installed into small spaces like kitchen cupboards.

What is a Combi Boiler? 

What is a Combi Boiler?

Combi is short for combination boiler and is labelled as such since it provides both hot water and heating from a single unit.

This is an advantage over older home heating systems that were composed of an immersion heater, fed with water from a header tank in the attic. 

The immersion heater needed to be turned on well in advance of requiring hot water, this meant that water wasn’t on-demand and since they are extremely inefficient, they cost a considerable amount of money to operate.

They are also much larger, made up of multiple components and so required much more room to house. 

Curious to know which boiler brand performed worst? Find out in our report of the worst boilers

How does a Combi boiler work? 

How Do Combi Boilers Work?

Cold water is fed into the combi boiler directly from the mains water supply, the gas fuel is then burned to produce heat, which is transferred from the combustion chamber to the water via the heat exchanger.

...It's all very clever and complicated.

When the temperature in your home drops, your thermostat will detect this, send a signal to your combi boiler, and it will fire up to generate the required heat.

Once the desired temperature has been achieved, the boiler will reduce its heat output and maintain the status quo, helping to reduce energy waste, while keeping your home warm. 


If someone uses a hot tap, shower, or bath within the home, the diverter valve will direct hot water to the outlets while they are in use. Once the tap is turned off, the diverter will be restored to the central heating position. 

Hot water can be directed to the central heating or the water outlets, but never both simultaneously.

Is a Combi Boiler Suitable for your Home?

Is a Combi Boiler Suitable For Your Home?

Combi boilers are supplied with water directly from the mains water supply and rely on the pressure from the mains to provide an adequate flow rate.

Therefore, if the mains pressure is relatively low, a combi boiler may not be appropriate for your home. 

Got a large home with multiple bathrooms? You'll want a system boiler

Combi boilers have two heat outputs, one for domestic hot water and one for the central heating.

Providing on-demand hot water takes much more effort than hot water for the central heating, therefore, it’s important to select a boiler that matches your homes domestic hot water demands. 

What size Combi Boiler do you need?

What Size Combi Boiler Do You Need?

When considering boiler size, it typically refers to the boiler’s power output and not the dimensions of the boiler.

A combi boilers power is measured in kilowatts (kW), so put simply, the greater the kW, the greater the capacity of the boiler to meet your properties hot water and heating requirements.

For instance, if you live in a small home with one bathroom, then a small (low power) combi boiler is probably going to be recommended.

However, if you live in a larger home with multiple bathrooms, then a much larger sized boiler will be required to meet the heating demands.

Here's a quick table for reference;

  Minimum kW Size  
What size boiler for... 1 Bath/Shower  2 Baths/Showers  
2 Bedroom House 26kW 29kW
3 Bedroom House 29kW 35kW
4 Bedroom House 29kW 35kW
5 Bedroom House 35kW 35kW

For a shed load more advice on boiler sizes, see our guide.


Pros of Combi Boilers

Pros of Combi Boilers

#1 - They are extremely efficient 

Combi Boilers Save MoneyIf you are looking for a boiler that's highly energy efficient, a combi (combination) boiler might be the solution. They can be used to heat the home as well as its water, saving you a chunk on heating bills.

#2 - Combi boilers have a compact size

Combi Boilers Sizes

Their compact size makes them ideal for even the smallest of properties, and since they use the mains as the water source, you will not have to make space for a cumbersome water tank or cylinder.

#3 - Hot water on-demand 

Combi Boiler hot Water

Another plus is that because they can instantly heat the water you need; you will not have to worry about having enough water in the hot water storage tank or it running out. 

#4 - Installation location more flexible 

Combi Boiler Location

Since a combi is relatively small and it is a single unit, there is typically more flexibility as to where you can have it installed within your home. This isn’t always the case with system and regular boilers that require a storage tank and/or overhead tank, which can severely limit the options. 

Cons of Combi Boilers

Cons of Combi Boilers

There are a few drawbacks to consider.

#1 - Limited flow rate 

Combi Boiler Flow Rate

Water flow rates are reduced when two or more outlets are being used at the same time. For this reason, combi boilers are typically not suitable for larger properties or those with multiple bathrooms. 

#2 - Reliant on mains water pressure 

Combi Boiler Mains Water Pressure

Ultimately, a combi boiler can only heat the water which gets fed to it from the mains. So, if your mains water pressure is low, your combi boiler won't work very well.

Combi boiler key points: 

  • The compact design makes them ideal for properties with limited space
  • Do not need a hot water storage cylinder
  • Do not need a cold-water tank.
  • Typically, not suitable for larger properties with multiple bathrooms/or a high hot water demand
  • May not be suitable for areas that suffer from low mains water pressure

Combi boiler – The best combi boilers available

Best Combi Boilers

Worcester Bosch Greenstar i Combi Boiler Payment Plans:

Worcester Bosch Boiler Finance

Fixed prices from £1,780 (get your quote online)

APR Deposit Loan Amount Term Monthly Payment Interest Total Payable  
9.9% £0 (0%) £1,780 10 Years £22.98 £977.36 £2,757.36
9.9% £780 (Max) £1,000 10 Years £12.91 £549.09 £2,329.08
9.9% £0 (0%) £1,780

3 Years




9.9% £780 (Max) £1,000 3 Years £32.01 £152.39 £1,932.39


Alpha E-Tec Combi Boiler Payment Plans:

Alpha Boiler Finance

Fixed prices from £1,595 (get your quote online)

APR Deposit Loan Amount Term Monthly Payment Interest Total Payable  
9.9% £0 (0%) £1,595 10 Years £20.59 £875.78 £2,470.78
9.9% £595 (Max) £1,000 10 Years £12.91 £549.09 £2,144.08
9.9% £0 (0%) £1,595

3 Years




9.9% £595 (Max) £1,000 3 Years £32.01 £152.39 £1,747.39

*finance examples to be used for illustrative purposes only. Get a finance quote online for final figures.

Best Combi Boilers (By Model)

#1 - Worcester Bosch Greenstar CDi Compact 

#2 - Viessmann Vitodens 050-W Combi 

#3 - Alpha E-Tec

#4 - Baxi 600 Combi 

#5 - Ideal Logic+ Combi 


The above is our definitive list of the best combi boilers available from each of the manufacturers in our best boiler brands ranking. 

In our view, these are the best boiler models offered by some of the UK's biggest and most reputable boiler manufacturers. They offer the perfect combination of excellent customer reviews, warranty periods, technical brilliance, efficiency, and power. 

How long do combi boilers last? 

How Long Do Combi Boilers Last?

Boilers vary in their quality and therefore their lifespan, so it is essential that you consider this when shopping for a new boiler.

Although it may be tempting to opt for a cheap boiler to save money on the upfront costs, in the long term this may end up costing you more if the boiler is of inferior quality. 


In general, combi boilers will last between 10 and 15 years, but this will largely depend on the quality of the boiler you purchase and if you have it serviced regularly.

You can typically gauge the quality of a boiler from the warranty offered, for example, a boiler brand that offers a 10-year warranty will be of higher quality than a boiler with a 5-year warranty. 

What is a Condensing Boiler?

What is a Condensing Combi Boiler?

When shopping for a new boiler, many people get confused by the terms condensing and non-condensing.

Today, the law states that all new boilers must use condensing technology, so there is not really any need to be concerned about securing a condensing boiler

Rather than a type of boiler, condensing is more of a technical attribute. For example, condensing boilers have a much larger heat exchanger and produce additional heat from the fuel, which would have been wasted in non-condensing boilers, making them much more efficient. 

Old, non-condensing boilers were as much as 50% less efficient since a lot of the heat produced was lost via the combustion exhaust gases. However, new boilers are designed in such a way that this heat is captured and used to heat water too. 


All condensing boilers are at least 90% energy efficient, can harness excess energy to heat your home, are better for the environment and will help to reduce your heating bills. 

For context, when you spend £1 on fuel only 10p is wasted, compared to older non-condensing boilers, which were around 70% efficient, this can make a substantial impact on your energy bills.

Check our guide on choosing the best boiler for a 3 bedroom house

Looking for the best deal on a new combi boiler? 

If so, get your new boiler installed with Heatable, here’s why: 

  • Gas Safe installation within 24 hours
  • Thousands of happy customers – an average score of 4.9 on Trustpilot (that’s better than the market leader).
  • Price match guarantee – if you find a cheaper quote, we will match (must be like for like)
  • Multiple payment options - interest free available
  • 10 year guarantee
  • Fixed price guarantee – no hidden costs
  • Save your quote until later

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Heatable is registered in England and Wales No. 08804726 (VAT No. GB205805920 ). Trading address is 1 Station Road, Great Sankey, Warrington, Cheshire, WA5 1RH. Heatable acts as a credit broker (not as a lender) and offers credit products through a panel of lenders. Heatable is authorised and regulated by the Financial Conduct Authority (reference no. 805259).

[1] Price based on a like for like combi swap in a 2 bed, 1 bathroom terraced home with a standard flue

[*] Installation dates are subject to installer availability

[ยน] only available on combi boiler installations

Financial Support: If you are in receipt of certain benefits, you may be entitled to claim a free boiler grant.