Boiler efficiency: how efficient is your boiler?

20 July 2021 | Aimee Tweedale

Did you know that as much as 15% of the UK’s carbon emissions comes from the way we heat our homes1?

That’s why energy efficiency is such a hot topic right now. Making sure that you’re using gas and electricity efficiently means you’re helping to fight the climate crisis. And, as a nice bonus, it can save you money on your bills, too!

As the key to home heating for 85% of us in the UK2, gas boilers have a big part to play. Keep reading to find out everything you need to know about boiler efficiency, including how to make your boiler more efficient. 

What is boiler efficiency?

An efficient boiler is one that 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, as well as being worse for the planet. 

That’s why it’s a good idea to look at energy efficiency when you’re choosing a new boiler. In the long run, you could save yourself a lot of money by choosing a more efficient model. 

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 to heating your home. The other 10% is lost as heat, or used to power the boiler itself. 

Age can be a rough indicator of 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. One over 20 years old could be only 70% – definitely room for improvement. 

Another sign of efficiency are ratings, so we’ll get on to those next.

Cold shower? No thanks!

Get boiler and central heating cover

Get a quote

Boiler efficiency ratings explained

In the UK we use a few types of boiler rating systems. Luckily, they’re not as complicated as they sound.

What is ErP?

ErP (Energy related Products Directive) is used for modern boilers. It’s shown as a certified letter rating: A+++ boilers are top of the class, and G boilers are the lowest. 

What is SEDBUK?

SEDBUK stands for the rather catchy Seasonal Efficiency of Domestic Boilers UK – the rating system used before ErP was introduced in 2015. SEDBUK ratings are displayed as a percentage, to avoid confusion with ErP’s letter rating system. 

SEDBUK 2009 is the rating used since the system was adjusted in 2009, and SEDBUK 2005 is the older system used before this.  

Switch to OVO Energy

Effects of an inefficient boiler: how much money could I save with an efficient boiler?

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

You’ll enjoy the peace of mind that comes with knowing you’ll have hot water and a warm house. And, over time, it should save you money. With heating accounting for around 53% of energy bills per year, this upgrade could have a big effect. Check out what else affects your gas and electricity bills

The good people of The Energy Saving Trust worked out how much money you could save by updating your old boiler. See the table below for their findings, depending on the age and efficiency rating of your boiler3!How much you could save by replacing an old boiler

6 ways to improve your boiler’s efficiency

Wondering how to make your boiler more efficient? Here are the top tips to keep your boiler and central heating system ticking along nicely.

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, detect whether something is wrong – and prevent small issues turning into big problems.

So always book a boiler service with a Gas Safe registered engineer – every year.

2. Bleed your radiators 

Along with the perk of improving your radiators’ heat efficiency, this minor maintenance task should lead to cheaper bills. 

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

Read our guide on how to bleed your radiator in 7 easy and quick steps.

3. Top up the pressure

Low pressure is bad news for a boiler’s efficiency and energy bills. Equally, high pressure can prevent it from working properly. So make sure it’s in the sweet spot – between 1 and 2 bars of atmospheric pressure. 

Read our guide on how to repressurise your boiler to make sure it's always up and running.

Boiler pressure too high? We’ve got a guide for that, too. 

4. Insulate your pipes

Cover your central heating pipes with foam tubes from a DIY store – they’re easy to fit, and you can do it yourself! It’ll reduce heat loss through the pipes, meaning your boiler doesn’t have to work quite so hard.

5. Run your heating every month – even in summer

Another way to increase your boiler’s efficiency: pop your heating on for 15 minutes every month, right through the year. Yes, even when it’s hot outside! Doing this might feel a bit odd, but it’ll keep things active and avoid any problems during 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 the undesired boiler breakdown. But the time will always come when you need to replace your boiler. Sometimes it’s sensible to do that sooner rather than later, as old boilers can be expensive to run.

If you’re ready to take the leap, find out about the prices and potential savings of combi boilers here. Think it’s time for a new boiler? See our boiler replacement options.

Find out more about the future of gas boilers in the UK

Children keep warm by the radiator

When should I get a new boiler?

Getting a new boiler is never a decision to be taken lightly. It can be one of the most expensive choices you make for your home. 

But as you can see in the table up above, if you’ve got an old, inefficient boiler in your house, you could be losing out on as much as £200-300 per year.

Plus, getting your boiler repaired can set you back hundreds of pounds each time. So if yours is breaking down often, it might be time to think about getting a new one. 

These are some of the key signs that your old boiler could be on its last legs:

  • It’s more than 10 years old
  • It breaks down or needs repairs often
  • Your heating bills are going up for no obvious reason
  • It’s leaking
  • It’s giving off a weird smell
  • It makes strange noises, like popping or hissing
  • Your heating and hot water are slow 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, award-winning boiler from CORGI HomeHeat (part of the OVO family). Here you’ll also find out how to tell if it could be time for a replacement. OVO members can visit their online account for all the latest offers from CORGI HomeHeat. 

Thinking about buying a new boiler? Read our practical guide on types of boilers and how to choose the right one for your home.

Which are the most efficient boilers?

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

Condensing boilers

Currently the most efficient out there, condensing boilers can recover more energy from condensing vapour – that’s where they get their name.

Most new boilers are condensing boilers. That’s because it became UK law in 2005 that any new gas boiler fitted should be a condensing boiler4

If you’ve still got an old non-condensing boiler in your home, with a G efficiency rating, switching to a condensing boiler could save you as much as £310 per year5!

Combination boilers 

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

A typical condensing combi boiler can come with an efficiency rating as high as 98%6.

But remember, when choosing a boiler (and working out how efficiently it’ll run) you should also take into account the size of your home, how much hot water you use, and the type of fuel you have access to. A combi boiler may not be the best option for you if you need to heat a large home with multiple bathrooms. 

Find out more about the different types of boiler and how to be more efficient with heating and hot water.

Boiler efficiency and the environment: eco boilers

Heating our homes takes up a lot of energy. So boiler efficiency has a big part to play in reducing our impact on the environment. Modern green boilers are more efficient, but they do still have some impact on the environment. 

Green boilers are engineered using newer technology that’s changing the heating industry for the better. Here are 3 important ways: 

  • Emissions: these newer boilers are engineered to abide by strict emission guidelines and energy-efficiency measures. Standards are high.  
  • Efficiency: green boilers are manufactured to reduce heat loss and maximise heat transfer. An efficient system is better for business and the climate. Win win.
  • Cost: green boilers use less fuel to run than older, traditional boilers.
Couple dancing in the kitchen

Low-carbon heating alternatives to gas boilers

In the coming years, the UK government is expected to start phasing out gas boilers. This is because we need to urgently cut down our carbon emissions. 

What does this mean for all of us with gas boilers in our homes? 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
  • Storage heaters: smart, efficient electric heaters that can charge up using cheaper off-peak energy during the night, then keep you toasty all day
  • Solar thermal panels: just like the panels that use the sun to generate electricity, these can turn sunlight into heat!
  • Hydrogen boilers: these boilers simply burn carbon-free hydrogen rather than natural gas – find out more in our guide to hydrogen boilers

Find out more about low-carbon home heating options and our Zero Carbon Heating Trial

Sources and references:







7 OVO's 2019 analysis of academic research into the effectiveness of feedback on energy consumption behaviours indicates that being able to monitor your energy usage by installing a smart meter and having access to energy insights information can lead to a reduction in energy use. The reduction in energy use results from the customer making changes to their habits by curtailing home energy use through actions such as optimising heating settings or switching lights off. The actual savings achieved will vary depending on individual energy use and the nature of changes made to a customer's energy habits.