Boilers: Boiler energy efficiency

Looking at gas bills

Cutting back on your gas use will help save money as the price of fuel rises

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 by making smart heating choices – from choosing the most energy efficient boiler and the most cost-efficient heating appliances, right down to draught proofing your letterbox – to keep your home cosy.

If you know that you want a new boiler why not go straight to our boiler reviews.

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

According to the Energy Saving Trust, 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

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 89.7% efficient. 

It's worth noting that you wouldn't expect to see a difference in your gas bills if you choose one that was 98.7% efficient over an 88% efficient one. 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

Scrren shot of the Sedbuk boiler efficiency database

The Sedbuk boiler efficiency table lists the efficiency of nearly 3,000 boilers

Traditionally, Sedbuk ratings were shown on labels as a letter A-G, but these letter labels have been withdrawn to avoid confusion with the European energy labels, which use similar ratings but are based on different principles.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

  • Sedbuk 2005 - this older system of Sebuk ratings rates boiler efficiency from A to G rating, with A-rated boilers being more than 90% efficient.
  • Sedbuk 2009 - this newer system of ratings 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.

You can check how the different types of ratings compare using the chart you'll find under our list of frequently asked questions about boilers.

Under the new Sedbuk 2009 system, a Sedbuk 2005 90% efficient A-rated gas, oil or LGP boiler works out as 88% efficient. The 2% difference between Sedbuk 2005 and 2009 is mainly down to differences in efficiencies between using a boiler for space heating in the winter months and only using it for water heating in the summer. Manufacturers often prominently display the Sedbuk 2005 in their product literature because it gives them a higher efficiency figure.

Will an efficient new boiler save you money?

Based on replacing an old G-rated boiler with a modern A-rated boiler with a full set of heating controls the EST estimates savings of around £340 per year on your gas bill. This does not take into account how much installing a new boiler will cost.

The table below illustrates just how much a modern condensing boiler could cut your gas bills by, 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 typeEfficiencyFlatBungalowTerracedSemi-detachedDetached
Old gas heavy weight55%£779£1020£1062£1204£1705
Old gas light weight65%£659£863£898£1019£1442
New non-condensing78%£549£719£749£849£1202
New condensing89%£481£630£656£744£1053

These figures are based on a gas unit price of 4.36p per kWh (April 2013)

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