Skip to main content

How to increase boiler efficiency

A person modifying boiler settings to increase boiler efficiency

Keeping your boiler running efficiently is vital if you want to avoid paying more than you should on your energy bills.

This guide will help you understand exactly what an energy-efficient boiler means, which measurements to look out for, how to find the most cost-effective boiler for your home and tips for keeping your boiler efficiency as high as possible.

Table of contents

  1. How efficient is my boiler?
  2. Check your boiler’s energy efficiency rating in 3 steps:
  3. How is boiler efficiency measured?
  4. ErP energy rating
  5. Which are the most efficient boilers?
  6. 6 ways to increase boiler efficiency
  7. FAQs
  8. Boiler cover and services from HomeServe

How efficient is my boiler?

A boiler’s ‘energy efficiency’ is the percentage of the total energy used by the boiler to provide useful heating. For example, a modern boiler (i.e. made in the last 10 years) might have 94% efficiency. This means 94% of the energy it uses goes to heating your home and only 6% is ‘lost’, or used to run itself.

Non-condensing heat-only boilers older than 20 years might typically have 60% efficiency, with 40% of their energy being lost. This is because some heat is wasted in the form of hot gases released from the flue.

However, modern condensing boilers capture some of the heat from the released gases and use it to heat water returning from your central heating system. This means they need less heat from the burner. Any new boiler you buy to replace an old one will be a high efficiency appliances boiler and therefore it will be more energy-efficient.

As your central heating costs represent more than half of your annual energy bill, you can assume that your overall energy budget will increase by purchasing a new boiler.

Check your boiler’s energy efficiency rating in 3 steps:

1. Look for the energy efficiency sticker

Most modern boilers have a sticker either on the boiler itself, or somewhere in the pack given to you when it was fitted that shows the rating of your boiler.

2. If you don’t have a sticker, find the model number

Usually, there’s a little sticker or plate with the brand name (like Vaillant, Worcester Bosch, Ideal) and the exact model name/number, e.g. ‘Ideal Independent C30’. If it’s not obvious, it could be underneath the boiler where the pipes come out, or under the flap at the front where the dials are.

3. Now type in your model number online at the PCDB database

Virtually every boiler ever made is on this database and it’s very easy to find your boiler’s rating. Look for the seasonal efficiency figure, as this gives the fairest reflection of how the boiler performs over the course of a year.

How is boiler efficiency measured?

There are a few different systems in use, but the main two are ErP (Energy Related Products Directive) and SEDBUK 2009. These days we’re all accustomed to seeing A-G grade energy ratings, which are based on the Europe-wide ErP directive (you can find out more below).

Boiler efficiency ratings

The A-G grades that you may be aware of were introduced to make it easier to understand the percentages below:

  • A: 90% efficiency and above (all modern condenser boilers)
  • B: 86–90%
  • C: 82–86%
  • D: 78–82%
  • E: 74–78%
  • F: 70–74%
  • G: below 70%

ErP energy rating

The A-G grades shown above are based on the ErP, a piece of European legislation signed in 2009. It considers the boiler’s lifetime energy consumption, including its manufacturer, transport and installation, and how energy efficient it is when in use. All new boilers get an A rating on the ErP system.

SEDBUK efficiencies

SEDBUK 2009 is the other main system we use to judge new boiler efficiency. It stands for ‘Seasonal Efficiency of a Domestic Boiler in the UK’ and is focused on the year-round or seasonal efficiency measure we discussed earlier. It gives you an exact percentage score for your boiler, but doesn’t include the lifecycle factors as the ErP rating does.

The previous SEDBUK 2005 system is featured on older boilers and is still used by many manufacturers. Like the ErP system, SEDBUK 2005 gives A-G ratings; A-rated boilers score over 90% for efficiency, while G-rated boilers score under 70%.

Older boiler energy efficiency ratings

If you’re an owner of an older boiler, check the PCDB database for your energy efficiency rating. However, a rough guide is:

  • Over 25 years old: 60% efficient
  • 20 years old: 70% efficient
  • 15 years old: 80% efficient
  • 10+ years old: 85% efficient

If you’ve got a non-condensing boiler that is more than 15 years old and less than 80% efficient, it’s very likely that replacing it with a condensing boiler that is 90%+ efficient will pay you dividends over the next few years – especially during the cold winters!

Which are the most efficient boilers?

There are a number of things to consider when looking at choosing the right boiler for your home:

The size of your home

Larger homes with more radiators and bathrooms may require a larger heat output than smaller homes.

Type of boiler

There are three main types of boiler: combi, heat-only and system.

A heat only boiler could be used on either an open vented or sealed heating system, whereas the system boiler will be used on a sealed system.

System boilers are often found in modern homes with an ‘unvented’ hot water cylinder. Combi boilers produce instant hot water and they’re a single unit that generates all the hot water and heating for the home.

6 ways to increase boiler efficiency

To use a combi boiler more efficiently – and these tips work with heat-only and system boilers too – there are a few things you can do:

1. Upgrade your boiler

According to The Energy Saving Trust, a new energy-efficient boiler with a full set of heating controls could save you between £100 and £200 a year in heating costs.

The older a boiler gets, the more expensive it could be to repair as old parts may fall out of production and get more difficult to replace. You can avoid a mid-winter boiler blowout by investing in a new boiler during Autumn before the nights start getting much colder.

2. Schedule a boiler service

Taking care of your boiler with regular check-ups keeps your boiler running in mint condition, protects you from any unexpected outages, and prolongs the lifespan of your boiler. Read our 5 reasons to book an annual service then book your annual boiler service here. This ensures your check-up is carried out by a Gas Safe Engineer.

3. Ensure radiator and pipework efficiency

If your radiators are in good working condition, your boiler can do its job and is less likely to develop a fault. So make sure you look after your whole system by bleeding your radiators regularly, or replacing them if one becomes a problem.

Bleed your radiators

When you bleed your radiators, you release any air that’s become trapped inside – releasing the air in the radiator allows the full surface area of the radiator to provide heat, therefore the heat efficiency of your system. This means a warmer home and cheaper energy bills.

Check our in-depth guide on how to bleed a radiator if you think your radiators have cold spots.

Use a system cleanse

Instil the purpose and function of inhibitors and the protective measure they provide. I would also put that if a system cleanse is required on a regular basis other fundamental problems with the heating system may exist.

Check out our central heating inhibitor explained – if you feel it’s not a job for you, don’t sweat it – get a Gas Safe registered engineer to do it for you.

A central heating system cleanse is like a spring clean for your boiler, flushing out any dirt, sludge, rust or debris that may have built up in your heating system. Left unchecked, these bits and bobs could cause blockages, hardcore corrosion, poor efficiency and even a boiler breakdown. A power flush is just one way to do this.

Check out our article, which has a step-by-step guide – if you feel it’s not a job for you, don’t sweat it – get a Gas Safe registered engineer to do it for you.

4. Get the correct pressure

Unless you have a heat-only boiler that works from an open system, for example, your boiler pressure gauge should usually measure between 1 and 1.5. If the pressure is too low, you could receive high energy bills due to inefficiency.

If it is too high, your boiler will be under unnecessary pressure and could be more likely to develop faults. If you see that the dial on your boiler is indicating an unusual reading, you could reduce the pressure by re-pressurising your boiler.

5. Check you’re not losing heat

Even if your boiler is working at its most efficient level, you may be losing heat through gaps in old windows and poorly insulated walls, so be sure to think about all the elements of your household when seeking to enhance your home’s overall efficiency.

Every detail counts. Simple pipe lagging can also improve boiler efficiency as they insulate and reduce heat loss, keeping your hot water hotter for longer periods.

Check out our energy saving tips for home for more detail on how to plug those gaps for an airtight home.

6. Control the temperature

Turning down your thermostat by as little as one degree could reduce your heating bills by almost 10% (Energy UK). It might not sound like much, but a slight tweak could really make a difference if you use your central heating on a regular basis; we recommend setting your temperature at the lowest comfortable level.

Smart thermostat technology is designed to do exactly this for you; it could make your heating more efficient by working out the best schedule for your home based on when you’re in and which rooms you use most often. After learning your heating routine, the smart thermostat tells your boiler when to heat up, which could prevent you from wasting energy and potentially help lower your energy bills.

It’s worth noting that if you’re struggling at all with boiler efficiency and your boiler is 10 years old or older, it’s probably coming to the end of its life and you’re due for a major, money-saving upgrade. Failing that, tick off the energy-saving tips in our list and those little tweaks will carry your hot water hero through the upcoming seasons with no problems.


What is boiler efficiency?

Boiler efficiency is all about how well your boiler uses energy to provide heating. It’s shown as a fraction of the total energy used e.g. 94% efficiency or ‘useful energy’ for a modern boiler, and 6% lost or ‘wasted energy’.

Which type of boiler is most efficient?

Modern gas fired condensing boilers recirculates the latent heat from the released gases to heat the water coming back from your central heating system. This means condensing boilers don’t need as much heat. The size and age of your home affect which type of condensing boiler you need to buy.

How efficient is a 20-year-old boiler?

A 20-year-old boiler is very likely to be a non-condensing boiler with a poor energy efficiency rating. You can find out your boiler’s energy efficiency rating here.

Is it better to undersize or oversize a boiler?

Boiler size is measured by the output in kilowatts (kWs), so it’s all about accurately matching your boiler size or ‘power’ to the amount of heat and hot water you need. Avoid undersizing (leaving you with insufficient power/heating) or oversizing (causing unnecessarily high energy bills), by getting a professional to recommend your boiler size.

Boiler cover and services from HomeServe

If you need assistance to make sure you’re as energy efficient as possible, our boiler cover comes with a free boiler service in the first year, which will help keep your boiler working efficiently.

Remember: modern ‘A’ rated boilers are the most efficient, so if you’re struggling, it may be time to upgrade or at least get a service. Either way, we can help you get your boiler efficiency up to standard.

Share this post

Share this post

Still looking for more help & advice?

Our help & advice articles cover Plumbing, Home heating, Electrical, Energy-saving and Home maintenance.

View all articles