Category - Boilers
Guide author photo

Will Scholfield, Engineer

31 Jan : Updated 27 Nov ● 11 min read

Boiler efficiency: a complete guide

Ensuring your boiler is functioning efficiently is imperative to keeping your bills down, saving money on breakdown repairs and reducing your carbon footprint. What's more, a highly efficient boiler brings hot water on tap, a warm home and a more comfortable way of living.

Whether you’re a homeowner or landlord, or you rent a property - saving energy is something we all want to get on board with - having an efficient boiler is just one of the many ways to ensure this. This guide aims to provide a deep dive into boiler efficiency, including what it is, how you can maintain a healthy boiler and your current efficiency rating.

What is boiler efficiency?

Boiler efficiency is the measure of how well a boiler converts its energy content of fuel into heat. In other words, it compares the amount of fuel transformed into usable energy and heat to the amount of fuel wasted during the combustion process. 

Boiler efficiency ratings span from A-rated to G-rated. 

How is a boiler's efficiency rating calculated?

Measured in percentage, a boiler's energy efficiency is calculated as the amount of energy that goes into heating your home and isn't wasted. To calculate boiler efficiency, divide total energy output by total energy input and multiply by hundred.

As an equation, this would look like this: η=(Energy output)/(Energy input) X 100

Understanding your boiler's efficiency rating

Boilers employ a variety of rating systems to reflect the energy efficiency of various brands and models. The two most common ones used by boiler manufacturers in the UK are ErP and SEDBUK. Let's take a look at these in a bit more detail.

What is ErP?

ErP stands for Energy-related Products and is a directive that applies to each member state in the European Union as well as the UK.

Using a scale of A to G (with A-rated appliances being the most efficient and G-rated appliances being the least efficient), it denotes the energy efficiency of a boiler to indicate to consumers how much energy that specific model will waste whilst heating their home.

In practice, A-rated boilers will have lower carbon footprints and prove more cost-effective to run than lower-rated alternatives.


The ErP scale previously provided electrical appliances such as boilers with an energy-efficiency rating of between A++++ and G. But, more recently, this scale has been readjusted to avoid confusing consumers. 

Why has the scale changed?

The main reason behind the change was the fact that technological innovations have led to increasingly efficient boilers being made available.

Whilst 10-15 years ago, a particular model of a boiler may have achieved an ErP rating of A, the release of more advanced technologies means that this would no longer be accurate. So, to ensure that the efficiency rating of our appliances is up to date, the EU and UK decided to reset the scale and relabel appliances accordingly.

What is SEDBUK?

Unlike ERP, SEDBUK (Seasonal Efficiency of Domestic Boilers) is a rating that is represented as a percentage as opposed to a letter. The higher the percentage, the better the score. SEDBUK is used to measure the average annual efficiency achieved in a typical domestic boiler or appliance.

How energy efficient is my boiler?

Since April 2018, all new boilers, by law, must have an efficiency rating of A and perform at 90% or above. Roughly, the energy efficiency of a boiler by age is as follows:

  • Over 25 years old: 60-70% efficient
  • 20 years old: 75% efficient
  • Over 15 years old: 80-85% efficient

Upgrade to a brand new A-rated boiler

Upgrade to a brand new A-rated boiler

Save up to £840 a year on energy bills with a more energy efficient boiler.

Find a boiler

How does your boiler's energy efficiency help you to save money?

Over half of your annual energy costs go into heating and hot water, so an efficient boiler is essential for saving money. All modern boilers that are appropriately maintained burn their fuel effectively, but they eventually lose some heat in the hot gases that escape up the flue.

Did you know?

A boiler flue is a pipe that brings in air from the atmosphere to help burn the fuel and also takes boiler exhaust gases outside the house and releases them into the atmosphere. This is because, when burning fuels like gas or oil, boilers produce dangerous byproducts like carbon monoxide. As such, a flue must be fitted correctly to guarantee the safe operation of any boiler that runs using combustible fuels.

What are the most efficient types of boilers?

Here are the most common boiler types and their energy-saving rating:

  • Standard boilers - This style of boiler is the more traditional type. They are fitted to a system that includes a hot water storage cylinder and feed and expansion tanks, which are often situated in the loft space. Older boilers of this type are generally 70-80% efficient. However, their more modern counterparts are more efficient, circa 90%, so they are also A-rated efficiency. More space is needed for this boiler and system, down to the fact you would need room for the above-mentioned hot water cylinder and tanks.  
  • System boilers - The system that this boiler is attached to is filled directly from the water mains. Apart from that, this boiler is very similar to a standard boiler, with the only real difference being that the water pump for this boiler system is situated inside the boiler, along with other controls to control system water pressure. Again, these appliances are available with an A-rated efficiency rating. 
  • Combi boiler - Combination boilers are very efficient, some with over 90% (even up to 98%) efficiency. Installing a new combi boiler, depending on your usage, could save you money each year of its use.

Condensing boilers

All of the above boiler types are now only available as condensing boilers. This means that they extract heat from the flue gases (products of combustion from the burnt fuel), and it cools to a point where it condenses back into water. This also promotes the higher operating efficiency levels that are now required for all new appliances.

Combi, system and standard boilers: Which type is the most efficient?

The efficiency of a boiler is determined by several factors, including its type, size, brand, and how it is used. Combi, system, and standard boilers are the three most common types of boilers used for heating in residential and commercial properties.

Let's take a closer look at each type and its efficiency characteristics.

Combi boilers

Combination boilers, often shortened to combi boilers, are units that provide hot water for taps and central heating without needing separate water tanks or a hot water storage cylinder. They heat water on demand, meaning they do not store hot water and are space efficient due to the fact they do not need a water tank or a hot water storage cylinder. 

Virtues of the Combi.png

They are generally regarded as highly efficient in terms of energy consumption because they only heat the water that is required at the time, thereby reducing energy waste. The efficiency of a combi boiler varies slightly depending on the model, brand, usage and age. 

As a rule, domestic home heating appliances lose efficiency as they age, with poorly-maintained boilers losing efficiency far quicker than well-maintained ones. To keep your boiler running at optimum efficiency, ensure you get it serviced annually by a qualified engineer.

System boilers

System boilers are typically found in homes with multiple bathrooms with a higher demand for hot water. They are usually connected to an unvented hot water cylinder, which can provide mains pressure hot water to various outlets simultaneously. 

You can also use system boilers in conjunction with other home heating systems and renewable energy technologies, such as solar panel systems.

Standard boilers

Also known as conventional boilers, this boiler type requires feed and expansion tanks in the loft and a hot water cylinder to store and heat water.

Standard boilers heat the primary water in the system, which is then pumped around your radiators and through a coil which heats the water you bathe in, which is stored in the hot water cylinder. The cylinder then feeds your hot water outlets. 

While standard boilers are dependable and can provide consistent hot water and heating, they can be less efficient than combi and system boilers. This is due to the fact that they store hot water in a cylinder, which can lead to heat loss and energy waste.

The actual efficiency of a boiler depends on various factors, including its design, components, usage and demand, and maintenance.

Condensing boilers vs non-condensing boilers: Which type is more efficient?

Condensing boilers are more efficient than non-condensing boilers as they extract and utilise much more heat from the combustion process than non-condensing boilers.

If you're interested in a new boiler, you can use our easy Find a Boiler tool to select the perfect boiler for your home. Our engineers can also install it for you the very next day at no additional charge.

Need a new boiler?

Need a new boiler?

Buy or subscribe to a new A-rated model online

Find a boiler

Why aren't boilers always as efficient as advertised?

Boilers, like many other appliances, are designed and tested under controlled conditions to determine their efficiency. However, when they're installed in our homes, they can sometimes struggle to reach these optimum efficiency levels as a whole range of variables come into play. 

Below are a few reasons why your boiler might not be performing at the efficiency level advertised.

  1. Installation quality - The installation of a boiler plays a critical role in its performance. If a boiler is not installed correctly, it may not operate efficiently.
  2. Regular maintenance and servicing - Boilers require regular maintenance and upkeep in order to function at peak performance. If a boiler is not maintained correctly, it may gather dirt and debris (also known as sludge) within the system pipework and radiators, have combustion problems, or suffer from other operational issues that could lower its efficiency over time.
  3. Operating conditions - If your boiler is constantly working at a high temperature, its efficiency will decrease over time. Think of it like a car; continually driving at higher speeds in a low gear would wear down the gearbox over time. Consider lowering the temperature of your boiler in milder months to extend its lifespan.

How can you maximise the efficiency of your boiler?

Higher energy bills, sluggish radiators, and a heating system that never seems to function properly can all be caused by an inefficient boiler. There are several ways to maximise the efficiency of your boiler, which can help you cut energy costs and reduce your environmental impact. 

Here are some of our experts’ top tips for maximising the efficiency of your home’s boiler.

Regular servicing

Plan routine boiler maintenance to keep it in good working order. You should have your boiler serviced at least once a year to ensure it functions correctly and safely.

Not sticking to an annual boiler service could affect your boiler’s performance; you'll be more likely to experience a boiler breakdown, and you could even invalidate your warranty.

Discover our Home cover plans, starting from just £7.99 per month.

Keep any pipework clean

The build-up of central heating sludge can cause major defaults in keeping homes warm, toasty and running at the best possible efficiency. 

Applying a chemical inhibitor will lower the corrosion rate and stop the accumulation of scale and sludge, preserving the system's efficiency. 

Bleed your radiators

Taking good care of your radiators and bleeding them at regular intervals can improve the efficiency of your boiler. Air trapped in the radiator system can reduce its effectiveness, resulting in inefficient heating. 

Learn the what, why and how of bleeding your radiators in our expert's guide.

Get the correct size of boiler for your home

To maximise energy efficiency, you want a boiler that's not too big or too small. Calculating the perfect size of boiler for your home involves thinking about the size of your property, the number of radiators and bathrooms you have, and the hot water demands of anyone living there. 

Read our full guide on choosing the right size boiler for your home to learn more.

Use smart controls

Technologies like smart thermostats enable you to control the settings of your home's central heating system easily, intelligently and remotely.

They also allow you to adjust your heating on a room-by-room basis, sending hot water to the radiators in the rooms you use and leaving those in empty spaces turned off.

Whilst additional tech might seem like an unnecessary expense, getting a smart thermostat fitted with your boiler will actually end up saving you money in the long run - potentially up to £311 a year

Smart controls come in all types, shapes and sizes, with different options costing between £50 and £350. Compared with the potential annual savings to be made, yours could have paid for itself in a little over 12 months.  


At BOXT, we work to keep your home's energy bills and carbon footprint as low as possible, which is why we offer our customers the latest smart technologies with every boiler installation.

Speak with our team for more information about how we can help you save money and energy.

How much could a new A-rated boiler save on your energy bills?

To maximise boiler efficiency and save as much as possible on your energy bills, ensure you take a close look at the energy efficiency ratings and compare models. The best boiler for you will depend on your home's size, type, number of radiators and hot water needs. 

Read our expert's in-depth guide to learn more about the potential energy savings you could enjoy by getting a new A-rated boiler.

Improve your home's energy efficiency

Improve your home's energy efficiency

A-rated boilers at fair, fixed prices

Find a boiler