How efficient is my gas boiler?

Find out what the energy efficiency of your boiler means, how it’s measured, and some of the different factors that can affect how efficient your boiler really is.

Can a new boiler lower your gas bills?

Yes, absolutely. Every new boiler you can buy in the UK today is at least 90% efficient, or A-rated. But older boilers were made to different standards. They can be as low as G-rated – which means less than 70% efficient.

One of the most common upgrades for British Gas engineers is swapping an older D-rated boiler for a modern A-rated one. This could save a typical household up to 23% on their annual gas bills. What this actually means in pounds and pence depends on what type of home you have – and how well insulated it is.

What does a boiler’s energy efficiency mean?

You rely on your boiler for hot water all year round. And in the colder months, you rely on it for heating too.

Doing all that takes a LOT of energy – over half the average home’s total energy use, in fact. So the efficiency of your boiler has a big impact on your finances as well as your carbon footprint.

A boiler’s efficiency is the amount of energy that actually goes into heating your home, rather than being lost or wasted.

If your boiler is 90% efficient, only 10% of the energy is lost.

But if your boiler’s efficiency is 70%, you’ll lose 30%.

Of course, you still pay for 100% of the energy you use – so getting the maximum heat from that energy makes a lot of sense. What can you do to help improve your boiler’s efficiency? Just follow these steps to find out more.

How is boiler energy efficiency measured?

ErP and SEDBUK are the main efficiency ratings for boilers. They’re calculated in slightly different ways, but essentially tell you the same thing.

What is ErP?

ErP stands for ‘Energy-related Products’. It’s the European rating system for boilers and other heating appliances.

Every new boiler has an ErP energy label, which ranks its efficiency from A-G. By law, all modern gas boilers in the UK are A-rated, which means they’re highly efficient.

What is SEDBUK?

There are two different versions of the SEDBUK rating system, which stands for ‘Seasonal Efficiency of a Domestic Boiler in the UK’.

SEDBUK 2009: The 2009 system gives each boiler an exact percentage score. This is what to look for if you really want to get into the detail, but the difference of a few percentage points won’t have a massive impact on your bill. The big savings come from swapping an old boiler for a new one.

SEDBUK 2005: This is the old system still used by lots of manufacturers. Like ErP, it rates boilers from A to G. A-rated boilers score over 90% for efficiency, while G-rated boilers come in under 70%.

Why are new boilers more energy efficient?

In the UK, a 2007 law made it compulsory for all new boilers to be ‘condensing’ boilers. This is a modern design that’s much more efficient than the old style of boiler.

Condensing boilers use the warmth from the hot gases expelled in the flue, rather than just letting all that heat go to waste. A condensing boiler will always be at least 90% efficient, or even more.

So, if you’re wondering how efficient your home’s boiler is, the answer is probably ‘very’ – but only if it was bought and installed recently.

Why should you upgrade an old boiler?

Older boilers are much less efficient than modern condensing boilers.

The energy efficiency of an older boiler can be as low as 60% – which is a G rating. With a G-rated boiler, 40% of the energy you pay for is going to waste rather than heating your home.

That means upgrading an old boiler to a newer, more efficient model is a great way of saving money.

And because more energy efficiency means lower CO2 emissions, it’s good for the planet too.

How much can I save with a new boiler?

This table shows how much you could save in an average year if you swapped an older boiler for a new A-rated model. As you can see, the type of home you live in also makes a difference.

Detached house Semi-detached house Terrace house Flat (mid floor)
D (78-82%) £265 £215 £180 £80
E (74-78%) £320 £240 £200 £90
F (70-74%) £410 £270 £225 £100
G (less than 70%) £580 £385 £320 £145

Source: Energy Saving Trust

Figures are calculated using fuel prices as of January 2024. These are estimated figures based on installing a new A-rated condensing boiler with a programmer, room thermostat and thermostatic radiator controls (TRVs) in a gas heated home from an older boiler with a programmer and room thermostat. Savings will vary depending on the size and thermal performance of your home.

What type of boiler is most energy efficient?

If you’re choosing between a conventional, combi or system boiler, none of them is necessarily more energy-efficient than the others. But picking the right one for your property and family size will mean you’re using it in a more energy efficient way.

For example, if you’re in a small property with a small family, a combi boiler might be your best option. A combi boiler makes hot water on demand, so you won’t be storing hot water that never gets used.

But if you have a large property and a big family, a combi boiler won’t be the most energy-efficient choice. A system or conventional boiler is more likely to meet all your needs while helping to keep the bills as low as possible.

You can learn more about the different types of boilers here.

Is boiler size important for energy efficiency?

Yes, choosing the right size boiler for your property and your family has a big impact on energy efficiency.

When we talk about boiler size, we’re really talking about its heat output. This is measured in kilowatts (kW).

If your boiler isn’t big enough, it will struggle to provide the heat you need. But if it’s too big for what you actually need, you’ll end up wasting a lot of energy in your heating and hot water.

Take a look at this article for more detailed guidance on choosing the right boiler size. It also has a handy calculator to help you work out what’s best for your property.

Looking for a new boiler?

If you need advice on anything to do with boilers, our team of heating advisers are always here to help.

You can book a free appointment and one of our expert engineers will come and look at your boiler and central heating system. They’ll talk you through the best options for your home and give you a fixed price quote.

Or if you prefer to do it all online, that’s easy too. Get started by answering a few simple questions and we’ll recommend the best boiler for you.