Boiler energy efficiency explained

Boiler efficiency is about how much usable heat energy 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 past two decades. Condensing boilers became mandatory in 2005, while 2018's Boiler Plus rules increased efficiency standards for boilers and installations, and new new boilers must be A-rated (minimum 92% ErP efficiency). 

But your boiler might not be running at its advertised efficiency. It can be limited by your heating system set up and the way your boiler is controlled.

Read on to find out why and what you can do to improve your system, for a lighter carbon footprint and cheaper bills. 

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

Why boilers aren't always as efficient as advertised

It's important to think of your central heating as a system, of which your boiler is just one part. 

Other variables such as 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 the system runs. It's important to look at this as a whole and make sure it's all set up as well as it can be.

Modern condensing combi boilers can be warmed sufficiently with heating flow temperatures of 60°C–65°C or lower. But many aren't running as efficiently as they could, because their factory default setting is 80°C, which is hotter than they need to run. 

A gas boiler running at its maximum flow temperature doesn't even condense, so it recovers much less heat than it's capable of. 

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 bleed your radiators and more about balancing 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 that 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. 

Boiler flow temperature

If your boiler doesn't have compensation controls, you can change the heating flow temperature of your combi boiler yourself. 

This is the temperature your boiler heats water to before it passes around your central heating system. Many boilers will start off set to 80oC, but if your home heats up easily, and even overheats, you can try turning your heating flow temperature down to 60°C–65°C.

If you're uncertain then check your manual or Nesta's interactive tool, which walks you through the process. Otherwise, you can ask a qualified heating engineer to do this for you during installation or during your annual service. 

Without insulation and bigger radiators, some homes might heat not properly with lower flow temperatures, and a boiler could cycle more while leaving you chilly. If you've turned it too low, you can dial it back up. You might need to try a few options to see what best suits your home. 

Find out how to lower your combi boiler's flow temperature to reduce your heating bills. 

If you have a boiler with a water tank or cylinder, don't adjust the flow temperature beneath 65°C without consulting an engineer first due to the risk of Legionella bacteria developing in standing water. 

Heating experts The Heating Hub say that owners of Ideal, Vaillant and Worcester Bosch combi boilers could make further savings by turning off the pre-heat function (which is sometimes called Warm Start or Comfort mode). This is normally turned on when your boiler is first installed and keeps some hot water ready in the boiler, minimising the time needed for hot water to come out of the taps.

Getting an annual boiler service is imperative for keeping your boiler running efficiently. You can also use the opportunity to ask your engineer about cost saving measures.

Find out how to get the best boiler service. 

Is it worth buying a new boiler to save money?

Boiler companies sometimes talk up the benefits of replacing your old boiler with a new one to save money. 

But what's often 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.

An investigation into boiler replacement by The Heating Hub found that in Germany boilers are expected to last longer than 20 years, while, in the UK, predicted lifetimes are up to 10 years shorter. 

Getting a new boiler of course entails a cost. 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. There's also an environmental cost to manufacturing and disposing of boilers unnecessarily.

If your boiler is a non-condensing one bought before 2005, this swap could be worth it. But to reduce unnecessary waste and spending, we wouldn't recommend switching an old condensing boiler with a new one unless it's broken, functioning poorly or been condemned. Instead, invest this money in home insulation, spend it on compensation controls or keep it in savings for when you do need a new heating system (be that a boiler, a heat pump or whichever heating system is best for you). 

Choose a reliable brand

If you do need a new boiler, than use our guide to the best boiler brands, where 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, have owners who are satisfied with their purchase and come recommended by an expert panel of Which? Trusted Traders.

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

Read our guide: how to buy the best boiler.

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). 

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 too large for your home means you'll be using more energy to heat your home than you need to, meaning 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, 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're interested in the minutiae of how one boiler's 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 to to get the most efficient boiler possible, you should hone in on this. 

Read our guide on boiler controls or thermostats to find out how to save further money on your heating bills.

Sustainable alternatives

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

The future of gas boilers in homes is still up in the air following a proposed government ban and then a U-turn. But it's likely that in the future many of us will be heating our homes with heat pumps or other low carbon heat systems. 

To find out more about the alternatives, 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 – for example immersion heaters and storage heaters.