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Boiler Energy Efficiency

By Matthew Knight

Once you have discovered how boilers are rated for energy efficiency, find out which boilers are the most efficient by using our boiler reviews.

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

You can keep heating costs down and still keep your home cosy by making smart heating choices. Installing an energy efficient boiler can save you hundreds of pounds a year. Read on to find out all you need to know about boiler efficiency, including how much you're likely to save by installing a new, efficient boiler.

Lower heating costs: heating products

The age and type of boiler you have affects how efficient it is. A modern condensing boiler is the most efficient boiler on the market – find out more in our guide to condensing boilers.

If everyone in the UK with gas or oil central heating installed a high-efficiency condensing boiler with full sets of heating controls, we would save enough energy to heat nearly 1.9 million homes for a whole year and save around 6.7 million tonnes of CO2, according to the Energy Saving Trust.

The efficiency of a boiler is normally expressed as a percentage. Since October 2010 only boilers that are 88% or more efficient can be installed in homes, and most of the boilers you can buy now are between 88% and 91% efficient. 

More recently boiler efficiency is stated as a rating on a boilers ErP energy label from A-G. Nearly all modern gas condensing boilers get an A energy rating.

It's worth noting that you wouldn't expect to see a difference in your gas bills if you choose a boiler that was 91% efficient over an 88% efficient boiler, or between one A rated boiler and another. The biggest saving comes from making a leap from an old inefficient boiler to a new one.

Boiler efficiency rating - how it is calculated

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: how boilers compare

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Sedbuk ratings were shown on labels as a letter  - these ranged from A-G. But these letter labels have been withdrawn to avoid confusion with the European energy labels, which use similar ratings but are based on different principles.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

  • Sedbuk 2005 - this older system rates boiler efficiency from A to G, with A-rated boilers being more than 90% efficient
  • Sedbuk 2009 - this newer system shows boiler efficiency as a percentage, rather than A-G.

All new boilers should be rated using Sedbuk 2009, but you may still see older models which were tested under the old Sebuk 2005 system.

A Sedbuk 2005 90% efficient A-rated gas, oil or LGP boiler works out as 88% efficient under the new Sedbuk 2009 system. The percentage difference between Sedbuk 2005 and 2009 is mainly down to differences in efficiencies between using a boiler for heating an entire home in the winter months and only using it for heating water in the summer. 

Manufacturers often prominently display the Sedbuk 2005 rating in their manuals because it gives them a higher efficiency figure.

ErP energy rating

All modern boilers will be 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 eficciency rating if you are interested in the minuti of how one boiler compares to another.

Will an efficient new boiler save you money?

If you replace an old G-rated boiler with a modern A-rated boiler with a full set of heating controls, the Energy Saving Trust estimates you'll save around £340 a year on your gas bill. This does not take into account the cost of installing a new boiler.

The table below shows how much different types of boilers will cost you in energy bills, depending on the size of your house. (A boiler that's 15 years old or more is classed as 'old' in these calculations. A lightweight boiler can be attached to a wall; a heavyweight boiler is floor standing.)

Annual fuel costs with different boiler types (source: Sedbuk)
Boiler type Flat Semi-detached Detached Efficiency
Old gas heavy weight 779 GBP 1204 GBP 1705 GBP 55%
Old gas light weight 659 GBP 1019 GBP 1442 GBP 65%
New non-condensing 549 GBP 849 GBP 1202 GBP 78%
New condensing 481 GBP 744 GBP 1053 GBP 89%

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