Boiler efficiency: how efficient is your boiler?

14 September 2023 | Aimee Tweedale

As much as 15% of the UK’s carbon emissions come from the way we heat our homes.1 That’s why energy efficiency is such an important topic right now. 

Making sure you’re using gas and electricity efficiently means you’re helping to fight the climate crisis. And it could save you money on your bills, too. 

Gas boilers play a huge part in our everyday lives. 85% of us in the UK2 use them to heat our homes and our water. So keep reading to find out everything you need to know about boiler efficiency, and how to boost yours.

What is boiler efficiency?

An efficient boiler uses the least amount of energy – and money – to keep your home warm and cosy.

An inefficient boiler does the opposite. It uses more energy, which means it costs more to run. And it’s not so great for the planet either. 

So it’s a good idea to look at energy efficiency when you’re choosing a new boiler.

How efficient is my boiler?

Boiler efficiency is measured in percentages. If your boiler is described as ‘90% efficient’, it means that around 90% of the boiler’s energy goes into heating your home. The other 10% is lost as heat, or used to power the boiler itself. 

Age can be a good way to guess your boiler’s efficiency. While modern condensing boilers have to be A-rated with an efficiency of 90% or more, older non-condensing boilers can be a lot lower. A boiler over 15 years old could be 85% efficient, but one over 20 years old could be only 70%. 

Another way to check boiler efficiency is through ratings. We’ll get on to those next.

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Boiler efficiency ratings explained

In the UK, we rate boiler efficiency in different ways – depending on the age of your boiler. Here’s an introduction to 2 of the most common systems, and how to understand them.


We generally use ErP (Energy-related Products Directive) for newer boilers. Their energy efficiency is rated using letters of the alphabet, from ‘A’ to ‘G’. Boilers with an ‘A’ energy rating are the most efficient, and boilers with a ‘G’ rating are the least.


If you have an older boiler, its energy efficiency might be rated using the Seasonal Efficiency of Domestic Boilers UK (SEDBUK) system. The energy efficiency of these boilers will be shown as a percentage.

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How much could I save with an efficient boiler?

Upgrading your standard heating system to an energy-efficient boiler could be a smart investment. 

Heating makes up around 53% of energy bills a year, so a boiler upgrade could have a big impact. As well as giving you peace of mind, it might save you money in the long run. 

The Energy Saving Trust worked out how much money you could save by updating your old boiler. You can see their findings in the table below. Or find out what else affects your gas and electricity bills with our energy guides.

What you could save each year with a new A-rated boiler

Old boiler rating Annual savings semi-detached houseAnnual savings detached houseAnnual savings mid-terraced houseAnnual savings mid-floor flat

6 ways to improve your boiler’s efficiency

Here are our top tips to keep your boiler and central heating working as they should.

1. Get a routine boiler service

Just like a car, your boiler needs regular servicing to keep it running smoothly and safely. It can help make it more efficient, find out if something’s wrong, and stop small problems from turning into bigger ones.

All of our boiler cover plans come with a yearly service – as well as a 24/7 helpline, unlimited callouts, and access to our network of Gas Safe engineers. So it could be your easiest option if you’d prefer to leave it to the experts.3

2. Bleed your radiators 

As well as improving your radiators’ heat efficiency, this simple task could lead to cheaper bills. 

To work out if your radiators need bleeding, turn on the heating. If they take a while to warm up or have cold spots at the top, it could be worth doing.

Read our guide on how to bleed your radiator.

3. Top up the pressure

Low pressure is bad news for a boiler’s efficiency and your energy bills. High pressure can also prevent it from working properly. So it’s best to be somewhere in the middle – between 1 and 2 bar of pressure. 

Read our guide on how to repressurise your boiler, to make sure it's always running as smoothly as possible.

4. Insulate your pipes

You could cover your central heating pipes with foam tubes. They’re easy to fit yourself, and you can buy them from most DIY shops. They reduce heat loss from the pipes, so your boiler won’t have to work as hard.

5. Run your heating every month – even in summer

Another way to increase your boiler’s efficiency is to turn on your heating for 15 minutes every month, right through the year. It might feel a bit silly if it’s hot outside, but it’ll keep your boiler active and could prevent problems in winter.

6. If all else fails, consider upgrading to a high-efficiency boiler

Regular boiler services and maintenance will help you keep things running efficiently and avoid a boiler breakdown. But the time will always come when you need to replace your boiler. And sometimes, it’s a good idea to do that sooner rather than later, as old boilers can be more expensive to run.

If you’re ready to take the leap, have a look at our boiler replacement options. You can also check the prices and potential savings of combi boilers, and learn about the future of gas boilers in the UK.

Or, taking out a boiler cover plan could be your best option. You can keep your current boiler, but have peace of mind in case anything does go wrong. We can even repair an existing problem for free if you’re an OVO customer, or at a lower price through Home Recover. All you have to do is pick the plan that works for you.4

Children playing on the floor by a radiator

When should I get a new boiler?

Getting a new boiler is a big decision. It can be one of the most expensive choices you make for your home. 

But as you can see in the table above, if you’ve got an inefficient boiler, you could be losing out on as much as £6305 a year. And that’s not including any potential repair costs.

So it might be time to think about getting a new one if:

  • Your boiler is more than 10 years old
  • Your boiler breaks down or needs repairs often
  • Your heating bills are going up for no obvious reason
  • Your boiler is leaking, smells, or makes strange noises
  • Your heating and hot water take a while to come on

Find out more about how much it costs to get a new boiler.

Where should I get a new boiler?

You can get an A-rated boiler from CORGI HomeHeat (part of the OVO family), which could save you up to £630 a year on your energy bills. Everything will be set up by a friendly Gas Safe engineer, and they’ll even give you extras – like a free smart thermostat from tado°. So if you’re ready to make the switch, get a quote online in just a few minutes.6 

For more information, our boiler guides can help you through the process:

Which are the most efficient boilers?

When it comes to choosing the most efficient boiler, there are 2 main types to look out for.

Condensing boilers

At the moment, these are the most efficient ones out there. They’re called condensing boilers because they can recover more energy from condensing vapour.

Most new boilers are this type. That’s because it became UK law in 2005 that any new gas boiler fitted should be a condensing boiler.7 

If you have a G-rated non condensing boiler, switching to a condensing boiler could save you as much as £630 per year.

Combination boilers 

Combi boilers, as they’re usually known, provide hot water on demand. They’re compact, quick to install, and super efficient. 

A typical condensing combi boiler can come with an efficiency rating as high as 98%.8

But remember, when choosing a boiler, you should think about the size of your home, how much hot water you use, and the type of fuel you have access to. A combi boiler might not be the best option if you need to heat a home with multiple bathrooms.

Find out more about the different types of boiler.

Boiler efficiency and the environment

Heating our homes takes up a lot of energy. So boiler efficiency has a big part to play in reducing our impact on the planet. 

Green boilers are more efficient, even though they still come with a footprint. They’re made using new technology that’s changing the heating industry for the better. Here’s how:

  • Emissions – these boilers are engineered to a high standard. They’re designed to meet updated emission guidelines and energy efficiency measures.
  • Efficiency – green boilers reduce heat loss and maximise heat transfer.
  • Cost – they run on less fuel compared to older, traditional boilers.
Couple dancing in the kitchen

Low-carbon heating alternatives

In the next few years, the UK government is expected to start phasing out gas boilers. That’s because we urgently need to cut down our carbon emissions. 

So what does it mean for all of us with gas boilers? Well, at some point in the future, it’s likely we’ll be using renewable electricity to heat our homes instead.

Some current low-carbon heating options on the market include:

  • Air source heat pumps – these take heat from the air, and use it to warm up your water and your home. Find out more in our guide to air source heat pumps.
  • Storage heaters – smart, electric heaters that can charge up using cheaper energy overnight, then warm your home in the day.
  • Solar thermal panels – just like the panels that use the sun to generate electricity, these can turn sunlight into heat. Find out more in our guide to solar panels.
  • Hydrogen boilers – these boilers burn carbon-free hydrogen instead of natural gas. Find out more in our guide to hydrogen boilers.

Get boiler, plumbing, heating, or electrics repair with Home Recover

At OVO, we want to be there for you in an emergency – whether you’ve got cover or not.

If something goes wrong with your boiler, plumbing, or electrics, we’ve got you. With Home Recover, we’ll fix it for free for OVO customers (or £79 if you get your energy elsewhere) when you take out our Complete cover plan.9

Our experienced, independent network of engineers is already trusted by over 200,000 UK homes.10 And with our 24/7 emergency helpline, we’ll help you get up and running again as soon as possible.

Sources and references:



3 Exclusions, eligibility criteria, cancellation fees and terms and conditions apply.

4 Exclusions, eligibility criteria, cancellation fees and terms and conditions apply.

5 According to the Energy Saving Trust (July 2023), a detached household moving from a G-rated boiler to a new A-rated condensing boiler with a programmer, room thermostat and thermostatic radiator controls (TRVs) could save up to £630 a year. Savings will vary depending on the size and thermal performance of your home. Here are their calculations.

6 Exclusions, eligibility criteria, cancellation fees and terms and conditions apply.



9 Exclusions, eligibility criteria, cancellation fees and terms and conditions apply.

10 OVO Energy and CORGI HomePlan are part of the OVO family, and as a group serviced 215,927 boilers in 2022.