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1 June 2021

Which boilers are most energy efficient?

Find out which boilers are the most energy efficient and how it's measured.
Radiator being adjusted
Matthew Knight

The cost of heating your home, particularly during the colder winter months, makes up about 60% of energy bills.

If you need a new boiler, make sure you buy one from one of the best boiler brands.

Energy efficient boilers

The first question you should ask yourself if you want the most energy efficient boiler is what type of boiler should you buy. 

All modern boilers that you can buy are condensing boilers, which are much more efficient than traditional non-condensing boilers. According to the energy saving trust, you could save as much as £340 a year on your gas bill if you're trading in an old-style non-condensing boiler for a new condensing one.

But to make sure you make the most efficient choice, you need to pick the right type of boiler for your home.

What is the most efficient type of boiler?

  • Combi boiler: heats water as you need it and takes up less space as it doesn't need a storage tank, making them ideal for smaller homes.
  • Heat-only boiler: sometimes known as a conventional boiler, a heat-only system means you will have a hot water tank or cylinder where hot water can be stored for later use. They're ideal for larger properties.
  • A system boiler: just like a heat-only boiler but more components are built in, meaning they take up less space and you don't need a tank in the loft. 

Getting the wrong type of boiler for your home is a shortcut to unneccesarily high bills. Learn more about the different types of boiler and which would suit you best in our full guide to the different types of boiler.

What is the most efficient size of boiler?

Once you've decided on the type of boiler you need, the next key question is the size. The size of the boiler in the context of efficiency is its heat output, measured in kW. 

The higher the heat output of the boiler, the more energy it will be using when it's switched on. 

In most cases, larger homes with more radiators and more bathrooms will need a larger heat output than smaller ones. 

Learn everything you need to know about picking the right size boiler for your home in our free guide on how to buy the best boiler

Boiler efficiency ratings explained: ERP and Sedbuk

You only need to begin to concern yourself with a boiler's energy efficiency rating once you have found the correct type and size of boiler for your home.

But once you know which type and size you need you can use the boilers' energy efficiency ratings to refine your choice of boiler and pick the most efficient one. 

ErP energy rating

All modern boilers are rated for efficiency under the new European energy label introduced in September 2015. Nearly all modern gas condensing boilers available in the UK get an A rating on this label, so it can be more useful to use the Sedbuk 2009 efficiency rating if you are interested in the minutiae of how one boiler compares to another. 

Sedbuk, which stands for Seasonal Efficiency of Domestic Boilers in the UK, is a system used to classify and compare boilers on how efficient they are at converting fuel into heat.  

Sedbuk efficiencies

Traditionally, Sedbuk ratings were shown on labels as a letter – these ranged from A-G. But these letter labels have been withdrawn to avoid confusion with the European energy labels, which use similar ratings but are based on different principles. 

There are currently two different types of Sebuk ratings, called SAP scores, that you should look out for when you are buying a boiler. 

  • SAP winter efficiency: this is measured by an independent laboratory and is the measured energy efficiency of the boiler including space heating and hot water. It's calculated to an industry standard called SAP 2009. There are only marginal differences between modern condensing boilers on this measure. 
  • SAP hot water efficiency: the hot water efficiency figure is how efficient the boiler is at producing hot water for your taps. There tends to be a much bigger difference between boilers on this measure and so if you are keen to get the most efficient boiler possible, it should be this figure that you hone in on. 

You can find each boiler's precise efficiency ratings for winter efficiency and hot water efficiency in our boiler reviews.

If all of this sounds too complicated then you can also use our boiler efficiency star rating, which you can find in the 'Test results' tab of our boiler reviews.

Which are the most efficient boilers?

In the table below, Which? members can see a selection of the most efficient boilers according to the boilers' ERP efficiency ratings for hot water and space heating. 

If you're not a Which? member, sign up now to reveal the boilers in the table.

The boiler brands included in the table are all Which? Best Buys, so you can count on the fact that they will be reliable and well loved by owners and engineers.

Most efficient combi boiler for a large home

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Best Buy

With two A's on its ERP efficiency label, this is one of the most efficient combi boilers you can buy. It's also available in a heat ouptut as high as 50kW, so it could be perfect for even a large home. It's also from an incredibly reliable brand, which is well loved by its own customers and heating engineers.

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Most efficient heat-only boiler

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An efficient and affordable heat-only option from a very reliable brand. It's heat output doesn't go as high as some others, so it might not be the best choice for a very large home, but for most medium to large homes it's an efficient and reliable choice.

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Most efficient System boiler

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Best Buy

This System boiler is one of the most efficient you can buy and from a very reliable brand. If you have a very large home, you may need something bigger, but this System boiler is efficient and comes in various heat outputs suitable for small to family sized homes.

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Tables last updated 01 June 2021

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