Most Efficient Boilers in 2024 – Unlock Big Savings

Last updated: May 3, 2024

The most efficient boilers can save you money on energy bills and reduce your carbon footprint without compromising your heating and hot water.

One in 4 (14.6 million) adults in the UK struggle to cope financially in the UK in 2024 due to rising living costs. Switching to an efficient boiler can help reduce your domestic bills and make your money go further.

Essentially, your gas boiler efficiency relates to how much energy or fuel it burns that is converted into heat energy. Therefore, the more efficient it is, the more fuel is used to heat water instead of being wasted.

For example, a boiler that is 92% efficient will use 92% of the energy to heat the water, while 8% will be wasted.

This wasted energy or heat is mostly emitted via exhaust fumes. In financial terms, this means that for every £1 spent on your heating, 8p is wasted.

The most efficient boilers convert more fuel to usable heat and can save you hundreds of pounds annually while reducing carbon emissions.

This guide explores everything you need to know about the most efficient boilers in the UK in 2024, including top models, their benefits, and how much you can save on energy bills.

Efficient Boilers Key Takeaways:

  • All new boilers sold in the UK are A-rated and utilise condensing technology.
  • This means that they are at least 92% efficient.
  • If you have an old boiler that is non-condensing you can potentially save around £215 to £475 annually by upgrading to a new energy efficient model.
  • To get a quote for a new energy-efficient gas boiler, you can start your application with Which? approved boiler installers Heatable here.

Which Are The Most Efficient Boilers In The UK In 2024?

Here are our top picks of the most efficient boilers currently available in the UK:

Worcester Bosch Greenstar 4000 

One of the most efficient boilers in 2024
Image by Worcester Bosch
  • Efficiency: 94% ErP rating
  • Output Range: 25kW to 30kW
  • Dimensions: 710 (h) x 400 (d) x 310 (w)
  • Warranty: 10 years

The Worcester Bosch Greenstar 4000 offers high-efficiency levels and value for money.

It’s A-rated and has an incredible efficiency of 94%, making it an excellent mid-range model with the most essential features and recent innovations at a competitive price point.

The Greenstar 4000 comes in a more compact design than the Greenstar i for a streamlined installation into your standard kitchen cupboard. 

It features excellent performance and you can choose between 25kW and 30kW power outputs or sizes.

The 25kW size is suitable for small properties and flats and has a flow rate of 10.5 l/min. The 30kW size is suitable for medium-sized properties with higher demand and has a flow rate of 12.3 l/min.

The Greenstar 4000 has a user-friendly LCD display and easy-to-use push control buttons. Thanks to wireless connectivity, you can connect the boiler to Bosch EasyControl and control the boiler remotely from your smartphone or other device.

It comes with a 10-year warranty for parts and labour and an intelligent filling system that ensures your boiler’s pressure is at its optimum level.

Why do we think Viessmann offers the best combi boilers? 

  • Which? Best Buy Award winner – titled as the UK’s “most reliable boiler”.
  • 10-year boiler warranty.
  • State-of-the-art German engineering – no plastic or rubber components (like some competitors).
  • Patented Inox Radial Stainless Steel Heat exchanger (not aluminium found in other boiler brands).
  • The only boiler with Lamda combustion control – maximising efficiency.  
  • Hydrogen ready – for the next generation of fuel.
  • The availability of parts is superb.
  • Excellent customer reviews and aftercare.

Check out our guide on boiler brands to avoid

Viessman Vitodens 100-W

One of the most efficient boilers in 2024
  • Efficiency: 94%
  • Output Range: 19kW–32kW
  • Dimensions: 700(h) x 350(d) x 400(w)
  • Warranty: Up to 12 years

The best combi boiler for homeowners looking to secure a reliable and award-winning combi boiler is the Viessmann Vitodens 100-W boiler.

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The Vitodens 100-W is a top choice if you’re looking for exceptional efficiency, a long lifespan, and quiet operations.

It features an impressive efficiency rating of 94% and the standout Lambda Pro technology that helps maintain maximum efficiency and burn rate.

Operating at only 35dB, the Vitodens 100-W is whisper-quiet and can ensure a peaceful home environment. It has an updated LCD touchscreen display and wifi integration so you can control and diagnose your boiler and even get remote assistance.

Phil Cox, head of Sales at Heatable and former Director of Sales at British Gas notes that what sets the Vitiodens 100-W apart is its “impressive connectivity, allowing for remote access to the boiler, performance monitoring, and the ability to diagnose potential faults.”

Advanced technologies like the Inox-Radial heat exchanger and the modulating MatriX-Plus cylinder burner contribute to the Vitodens 100-W’s exceptional performance and efficiency.

Despite its compact size and lightweight design, the Vitiodens 100-W is also very powerful, making it an excellent choice for medium to large houses.

Why else do we think Viessman Combi boilers are a superb option? 

  • Awarded Which? Best Buy since 2018 onwards
  • Low CO2 emissions to help ease your carbon footprint and conscience.
  • Outstanding value thanks to superior internal components that ensure a longer-lasting boiler.

Alpha E-Tec NX Combi Boiler (Most Reliable Affordable Boiler)

One of the most efficient boilers in 2024
  • Efficiency: 93% ErP rating
  • Output Range: 28kW–38kW
  • Dimensions: 712(h) x 440(d) x 255(w)
  • Warranty: 10 years

If you are looking for a combi boiler that offers maximum value for money, then the Alpha E Tec NX combi boiler is a superb option.

It’s accredited as A-rated with a 93% efficiency and comes in sizes or power outputs of 28kW and 38kW. The 28kW size has a flow rate of 12.1 l/min while the 38kW size has a flow rate of 15.9 l/min.

The Alpha E-Tec NX comes with a stainless steel heat exchanger, which is unheard of amongst budget boiler brands. Stainless steel is stronger than aluminium and can withstand extreme heat.

You don’t have to worry about longevity or corrosion with the Alpha E Tec, making it one of the most reliable combi boilers in the lower price range.

The expansion vessel of the Alpha E-Tec NX, which helps control pressure, is zinc-plated, allowing it to withstand corrosion and last much longer than standard models. 

The Alpha E-Tec has a Good Homes Magazine Award and countless 5-star reviews on Trustpilot.

It’s compatible with all the popular smart thermostats, including Tado, Hive, and Google Nest. You can integrate the boiler into the rest of your smart home hub, and take greater control of your heating.

Why else do we think Alpha Combi boilers are a superb option? 

  • Good Homes Magazine Approved (2024).
  • Competitive price point.
  • Superior parts and design at an affordable price.

Worcester Bosch Greenstar 2000 

One of the most efficient boilers in the UK
Image by Worcester Bosch
  • Efficiency: 93% ErP rating
  • Output Range: 25kW–30kW
  • Dimensions: 700(h) x 300(d) x 400(w)
  • Warranty: 7 years standard

The Worcester Bosch Greenstar 2000 is an excellent choice if you have a small home and are looking for an industry-leading boiler at a competitive price.

It’s the brand’s quietest model and is perfect for small homes with one bathroom thanks to its compact size. The Greenstar 2000 is Quiet Mark certified and can seamlessly fit between your cupboards without sticking out.

With an ErP rating of 93%, the Greenstar 2000 is a super-efficient workhorse that comes in 25kW and 30kW power outputs (sizes).

It’s hydrogen-ready, and you get a 7-year parts and labour guarantee when it is fitted with a system filter. It also comes with a 10-year guarantee on the primary heat exchanger for added peace of mind.

Hydrogen ready: Can operate on a hydrogen and natural gas mix and if the government boosts “green gases” in the UK gas grid, you can trust the boiler will continue working without needing a complete switch to another boiler in the future.

The Greenstar 2000 features a user-friendly digital display and comes in an ultra-modern design with easy-to-remove side panels for convenient maintenance and servicing.

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It’s an ideal choice if you’re on a budget and offers perks you can expect from a premium brand while keeping your hot water and heating running reliably all year round.

Why do we think the Greenstar 2000 is an excellent option?

  • Which? Best Buy Award
  • A swanky exterior design combined with a powerful, efficient, and reliable interior. 
  • Planet-friendly packaging that’s great for your home and the environment

What Is Boiler Efficiency?

Boiler efficiency refers to how well your boiler converts fuel into heat. Manufacturers generally calculate it as a percentage of how much fuel the boiler converts into usable energy and how much is lost in the process. 

If your boiler is 90% efficient, only 10% of the energy is lost.

But if your boiler’s efficiency is 70%, you’ll lose 30%.

Of course, you still pay for 100% of the energy you use – so getting the maximum heat from that energy makes a lot of sense.

Generally, the higher the boiler efficiency, the less fuel it uses, eventually helping you reduce energy bills and lower carbon emissions.

How Is Boiler Efficiency Rated?

The main efficiency ratings for boilers include:

ErP Rating System:

ErP stands for ‘Energy-related Products’. It’s the European rating system for boilers and other heating appliances.

Every new boiler has an ErP energy label, which ranks its efficiency from A-G. By law, all modern gas boilers in the UK are A-rated, which means they’re highly efficient.

The ErP uses the following scale to determine boiler efficiency:

  • A-rated boiler – 90%+ efficient.
  • B-rated boiler – 86-90% efficient.
  • C-rated boiler – 82-86% efficient.
  • D-rated boiler – 78-82% efficient.
  • E-rated boiler – 74-78% efficient.
  • F-rated boiler – 70-74% efficient.
  • G-rated boiler – Below 70% efficient.


SEDBUK stands for ‘Seasonal Efficiency of a Domestic Boiler in the UK.’

Like ErP, it rates boilers from A to G. A-rated boilers score over 90% for efficiency, while G-rated boilers come in under 70%.

Those who prefer the SEDBUK system believe it offers a more accurate reflection of the boiler’s yearly efficiency.

Which Is The Most Efficient Boiler Type?

Condensing boilers are the most efficient boiler type. They’re more efficient than older boilers because they recover more heat from the exhaust flue gas and uses it to heat the central heating water.

If your boiler was installed before 2005, it’s very likely to be a non-condensing G-rated boiler under the Energy Related Products (Erp) Directive.

This is because of Boiler Plus Legislation was passed by UK parliament in 2005, which essentially banned the installation of non-condensing boilers.

If your boiler is G-rated, it’s likely to be somewhere between 50 and 70% efficient, meaning that it will cost more to run than a modern condensing boiler.

Does Age Affect Boiler Efficiency?

The age of the boiler is important when it comes to boiler efficiency.

In fact, with wear and tear gas boilers lose their ability to maintain their original and optimum efficiency. The parts get rusted, and the system may get blocked by sludge and debris.

So, if your boiler is over 10 years old, no matter what ERP rating it has, it’s not going to be as efficient as it used to be. 

The table below shows you the estimated decrease in efficiency with boiler age.

Age of BoilerEfficiency PercentageEfficiency Rating
0+ Years90% +A
10+ Years85% +B/C
15+ Years80% +C/D
20+ Years70% +E/F
25+ Years60% +G

When Should You Replace Your Boiler?

If your boiler is very old, i.e. installed prior to 2005, it’s likely to be a G-rated boiler and very inefficient. In such a case you should start making savings to upgrade to a modern A-rated combi boiler.

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Additionally, if your boiler is very old, i.e. over 15 years old (regardless of its ErP rating), it will no longer be operating at its optimum efficiency, in which case it will be worth upgrading your boiler for a new model.

Other signs that can show you it’s time to replace your boiler include:

Higher Energy Bills

Old and inefficient boilers will waste a considerable amount of energy when heating your home, which is bad for your pockets and the planet.

If you’ve noticed your utility bills creeping up in spite of no major changes to your household’s energy usage, it’s likely that it’s time for a new boiler.

Gurgling or Banging Noises

Most of the time, the sounds and noises you’ll hear coming from your home’s central heating system are normal. They can come from pipes expanding and contracting, water moving through the system, or the boiler firing up.

They can also be caused by a blockage or build-up of sludge, but a power flush can quickly fix this.

However, if you hear any new, excessively loud, or persistent sounds like kettling, banging, gurgling, or clunking, it can be a sign of a more serious problem, especially if it’s accompanied by vibrations.

You Can’t Find A Needed Part

Older boilers are sometimes impossible to repair since the parts needed to do the job aren’t in production anymore.

If you can’t get hold of the components, replacing your boiler with a newer model is the only way to go.

The parts may also be too expensive for the repair to be worthwhile. Therefore, it may be better to invest in a new boiler that is more energy-efficient and reliable than the current system and less likely to need repairing in the near future.

Frequent Breakdowns

A new boiler will make the most financial sense if you find yourself needing to call an engineer more than once a year (on top of your regular servicing) to fix new or recurring problems.

Before too long the amount you spend to fix frequent breakdowns will amount to the cost of buying a new boiler.

Wondering how much a new boiler costs? Check out our guide on options available if you need a new boiler but can’t afford it, as well as boilers on finance and government boiler grants here.

Check out our video on how to vet boiler brands/models in the UK:

How Much Can The Most Efficient Boilers Save?

According to the Energy Savings Trust, replacing an old gas boiler with a new A-rated condensing boiler can save you from £215 to £475 annually, depending on your home’s size and thermal performance.

Choosing the right boiler size in terms of output in kWs is crucial for efficient heating and cost savings.

A boiler that is too small will struggle to heat your home, while a boiler that is too large will waste energy and increase your energy bills. It can also cycle on and off more frequently, leading to increased wear and tear on the system and reduced efficiency.

Final Thoughts on the Most Efficient Boilers in 2024

The most efficient boilers in 2024 are A-rated and feature high ErP ratings of 90% and above. You can save thousands of pounds every year by replacing your old boiler with a new, efficient boiler. The most efficient boilers are also environmentally friendly since they release fewer emissions, helping contribute to a healthier, cleaner environment.

Have you heard about the gas boiler ban, and wondering what the alternatives to gas boilers are? Read our complete guide to replacing your gas boiler with a heat pump.

Sources and References