Boiler energy efficiency explained

Boiler efficiency is about how much usable heat you get from your boiler for every unit of energy you put into it. The more efficient your heating system, the less energy you'll be using to heat your home. 

Minimum boiler efficiency standards have come a long way over the last two decades. Condensing boilers became mandatory in 2005, while 2018's Boiler Plus rules increased efficiency standards for boilers and installations. 

Currently, new boilers must be A-rated, with minimum 92% ErP efficiency. This is good news for consumers trying to keep control of their heating bills. 

But there's a catch. In people's homes,  boilers often don't manage the efficiency advertised. 

Condensing boilers are designed as efficient systems that recover heat that would otherwise be lost. But how effectively they can do that job is limited by the energy efficiency of your home, the way your boiler is controlled and the design of your heating system.

It's important to think of your central heating as a system, of which your boiler is just one part, and make sure it's all set up as well as it can be.

While an A-rated boiler can be over 90% efficient in some homes, modern electricity-run heat pumps are typically capable of 200-400% efficiency. Read more in our guide to heat pumps explained.

Why boilers aren't always as efficient as advertised

Modern condensing combi boilers are designed to run with low flow temperatures which means that the water circulating through your radiators isn't incredibly hot. 

But it's very common that modern boilers don't run as efficiently as they could, only using high flow temperatures such as the max setting, which is usually 80°C. A gas boiler running at its maximum flow temperature doesn't even condense, so it recovers much less heat than it's capable of. 

Boilers belong in heating systems and other variables, like the size of your radiators, the quality of your boiler controls, your home insulation and the upkeep of your system (such as radiator balancing) all affect how it runs. 

This means that consumers may have a false sense of security when buying a boiler with a high efficiency rating if it's not installed as a part of an efficient system. Simply put, an A-rated boiler isn't a surefire way to 90%+ efficiency in reality. 

Find out how to balance and bleed your radiators to keep your central heating running properly.

How can I make my boiler run more efficiently?

The best way to guarantee that a new boiler is working in a way that delivers value for money is to have it installed with weather compensation or load compensation controls.

This will vary its flow temperature according to your home's heating need. A heating engineer should be able to sort this out for you and different boilers are compatible with different controls. 

Other changes which can make your home heating more efficient, whether your boiler is new or old, are to insulate your loft and walls and fit larger radiators, which enable your central heating to run at lower temperatures. 

You can change the heating flow temperature of your combi boiler yourself. If you're uncertain then asking a qualified heating engineer to do this for you during installation or servicing is a good idea. 

Without insulation and bigger radiators, some homes might not be able to be heated properly with lower flow temperatures and a boiler could cycle more while leaving you chilly. So try turning your heating flow temperature down to 60°C - 65°C, but dial it back up if your home can't easily get to the temperature set on your thermostat. 

If a trusted technician sets your boiler flow temperature to its maximum, they should be able to justify this to you. 

If you have a boiler with a water tank, it will be difficult to run it at the efficiency potential of a combi boiler. Don't adjust the flow temperature beneath 60°C without consulting an engineer because these systems work differently and storing water in a cylinder introduces some hygiene risks. 

More on: How to lower your combi boiler's flow temperature to reduce your heating bills

Is it worth buying a new boiler to save money?

Be cautious about buying into marketing that promises big cost savings just because you've bought a new boiler.

Boiler companies may talk up the benefits of replacing your old boiler with a new one to save money. For example, British Gas says that you could save £840 a year by getting a new boiler. 

But what's buried in the small print is that this presumes that you're replacing a G-rated boiler with an A-rated one that's working highly efficiently. This is convincing enough to shift units, but simply buying a new model isn't guaranteed to deliver this cost saving.

Also, getting a new boiler comes with a lot of costs. Just swapping one combi with another will likely set you back between £1,500 - £3,000, so you need significant efficiency gains to recoup that money in the near future.  

To reduce unnecessary waste and spending, we wouldn't recommend switching an old condensing boiler with a new one unless it's broken or been condemned. If your boiler's an older condensing model, it's best to keep your boiler for as long as it will work and instead invest in home insulation, or save money in preparation for your next heating installation and purchase compensation controls.

You will find greater savings if your boiler is a non-condensing one bought before 2005 and this swap can be worth it, but the amount you'll save is still proportionate to the efficiency of your system so bear this in mind.

Look at our insulation advice guides to see the ways you can reduce heat loss in your home - and how to get grants to help pay for it.

Boilers from brands that scored well in our reliability and engineers' survey.

Over on our guide to the best gas boiler brands, Which? members can see which boiler brands are well-rated by customers and engineers. If you're not a Which? member, sign up now to see our boiler results.

Our Best Buy boiler brands make boilers that are proven to be reliable in people's homes, owners are satisfied with their purchase and Which? Trusted Traders recommend them.

Remember: even an A-rated boiler isn't efficient if it's installed as a part of an inefficient system. 

What is the most efficient size of boiler?

The size of the boiler – in the context of efficiency – is its heat output, measured in kilowatts (kW). The higher the heat output of the boiler, the more energy it will be using when it's switched on. 

In most cases, larger homes with more radiators and more bathrooms will need a larger heat output than smaller ones. 

However, getting a boiler size that's much too large for your home means you'll be using more energy to heat your home than you need to, resulting in higher bills and carbon emissions than necessary. 

Boiler efficiency ratings explained

ErP energy rating

All modern boilers are rated for efficiency under the European energy label ErP which stands for 'Energy-related Products' (introduced in 2015). It rates boiler efficiency from A+++ to F, with the former being the most efficient boiler system you can get.

Combi boilers have two ratings: space-heating (the letter under to the radiator symbol) and water heating (the letter under the tap symbol). Space-heating is how efficient the boiler is at heating your radiators, while the water heating component is how efficient the combi boiler is at heating your hot water for showers and taps. 

Heat-only and regular boilers only have the space-heating part of the ErP label.

The ErP label also shows boilers sound power level measures in decibels (dB). The higher the number, the louder the boiler is while operating. Finally, it also shows the boilers heating output in kW.

Nearly all modern gas condensing boilers available in the UK get an A rating on this label. Additionally, in 2018, the UK government introduced 'Boiler Plus' legislation which requires all boilers in England to have a minimum ErP efficiency of 92% (or 'A-rated'). 

SEDBUK efficiency rating

If you are interested in the minutiae of how one boilers efficiency compares to another, it can be more useful to use the SEDBUK 2009 efficiency rating.

SEDBUK stands for Seasonal Efficiency of Domestic Boilers in the UK, and is a system used to classify and compare boilers on how efficient they are at converting fuel into heat.  

Traditionally, SEDBUK ratings were shown on labels as a letter, but these letter labels have been withdrawn to avoid confusion with the ErP labels, which use similar ratings but are based on different principles. 

There are currently two different types of SEDBUK ratings – called SAP scores – that you should look out for when you are buying a boiler. 

• SAP winter efficiency – this is measured by an independent laboratory and is the efficiency of the boiler including space heating and hot water. There are only marginal differences between modern condensing boilers on this measure. 
• SAP hot water efficiency – this figure measures how efficient the boiler is at producing hot water for your taps. There tends to be a much bigger difference between boilers on this measure, so if you are keen to get the most efficient boiler possible, this figure that you should hone in on. 

Got the most efficient boiler? Read our guide on boiler controls or thermostats to potentially save further money on your heating bills

Sustainable energy and fuels

Gas and oil boilers burn fossil fuels, which releases carbon dioxide into the atmosphere. 

However, at the moment, switching to a non-fossil-fuel boiler isn't necessarily cheap or easy. Many of us will be living with boilers for some time yet, so this really makes using your boiler effectively a key priority as an efficient one – with compensation controls and correctly sized for your home – will still help to reduce your carbon footprint.

Low carbon heating can be cheaper to operate than fossil fuel based heating. Under the Energy Price Guarantee (as of January 2023), a heat pump needs to operate at 280% efficiency to cost roughly the same as a gas boiler. With a well designed system, a heat pump can run more efficiently than this on most days through the heating season.

To find out more about the low carbon solutions available to you now, read our expert guides on:

• Solar water heating Solar thermal panels that use heat from the sun to heat water to use in your home.
• Ground source heat pumps A network of water pipes buried underground, extracting natural heat from the ground that is then transferred to your home heating system. 
• Air source heat pumps Works in a similar manner to a ground source heat pump, but instead extracts heat from the air, which is then boosted to a higher temperature using a compressor.
• Electric central heating Reduces carbon emissions from your home (though only zero carbon if the electricity is produced by renewables) – for example immersion heaters and storage heaters