Condensing boilers are the most energy efficient, typically being 25% more efficient than non-condensing models. If you still have a old-style non-condensing boiler, then replacing it with a new condensing boiler could save you £45 to £300 a year (Energy Savings Trust, 2021).
All modern boilers that you can buy are condensing boilers and have been for a while. Since 2007, except in exceptional circumstances, building regulations have stated that all new boilers installed in a domestic home should be high-efficiency condensing boilers. So the chances are you already have one if you've ever had a new boiler installed since 2007.
But to make sure you make the most efficient choice, you need to pick the right type of boiler for your home. Getting the wrong type can lead to unnecessarily high bills.
Combi boiler –heats water as you need it, ideal for smaller homes.
Heat-only (Regular) boiler – a heat-only system means you will have a cold water feed and hot water tank where hot water can be stored for later use. They're ideal for larger properties.
System boiler – like a heat-only boiler but more components are built in, meaning they take up less space and you don't need a cold water feed tank in the loft.
Learn more about the pros and cons of each boiler type, and which would suit you best, in our full guide to the different types of boiler
What is the most efficient size of boiler?
Once you've decided on the type of boiler you need, the next key question is the size.
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 too large for your home means you'll be using far more energy to heat your home than you need to, resulting in higher bills and carbon emissions than necessary.
Learn everything you need to know about picking the right size boiler for your home in our best boilers guide
Boiler efficiency ratings explained
Once you know which type and size you need, you can use the boilers' energy efficiency ratings to refine your choice of boiler and pick the most efficient one.
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'). This is great news, as it means almost any modern boiler you can get will be efficient.
You can find each boiler's ErP efficiency ratings in the 'Tech Specs' in our boiler reviews.
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.
You can find each boiler's precise efficiency ratings for winter efficiency and hot water efficiency in the 'Tech Specs' in our boiler reviews under the labels 'Efficiency' and 'Hot Water Efficiency' respectively.
Gas and oil boilers burn fossil fuels, which releases carbon dioxide into the atmosphere. Fossil fuels are also going to run out eventually. This means that despite boilers being very efficient, they are not sustainable.
Carbon emissions from heating our homes accounted for about 17% of the UK's total carbon emissions, according to the Department for Business, Energy & Industrial Strategy.
To tackle climate change, the UK government has committed to reaching net zero carbon emissions by 2050. So if we are going to reach the UK's net zero target, we will all eventually need to heat our home with sustainable energy (such as electricity generated by renewable sources like solar and wind) or sustainable fuels (like hydrogen).
However, at the moment, switching to a non-fossil-fuel boiler isn't necessarily cheap or easy. Most of us will be getting gas and oil boilers for the foreseeable future, so this really makes your choice of boiler a key priority as an efficient one – correctly sized for your home – will still help to reduce your carbon footprint.
Some low-carbon heating systems can be installed now, while others – like hydrogen-ready boilers – won't be available to buy until at least mid-2020s.
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.