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Witten by Bernard Morgan (Engineer) & Adam Apperley (Marketing). Published: January 4, 2019. Updated: June 24, 2024

Energy Efficient Combi Boilers

According to the Energy Saving Trust, switching to a high-efficiency new boiler could save almost 7 million tonnes of CO2 a year. It could also save you money on your bills, up to 40%. Below is a breakdown of the most energy-efficient combi boilers available today.

What makes new boilers efficient?

There are a few things that make boilers efficient. The main one is whether it condenses or not.

All new boilers in the UK are now condensing boilers. This change was made in 2005 with new building regulations. They also need to be 90%+ efficient.

This means that all boilers, at the time of sale and installation, are at least 90% efficient. This means for every £1 you spend on gas for your heating or hot water in a combi boiler only 10p is wasted.

For comparison, it’s estimated that some back boilers are only 50% efficient. This means 50p from every £1 is wasted. If your existing boiler is over 17-20 years old, it won’t be a condensing boiler.

What does condensing mean, and how to condensing boilers work?

An old non-condensing boiler wastes heat through hot exhaust. This is through the flue which either goes through the wall or through the roof.

A new condensing boiler recycles that hot gas through the heat exchanger. This essentially preheats the water returning from your radiators. This means the burner won’t have to work as hard, saving you money by saving energy.

The diagram below explains condensing boilers in more detail:

How a condensing boiler works. Energy efficient combi boilers.

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How can energy-efficient combi boilers save you money?

As technology evolves, the focus has been on making more energy-efficient boilers. With a non-condensing, low-efficiency boiler, you are using more fuel than you need to. Think of the efficiency percentage as the part of each £1 that is actually used for heat or hot water.

Plus, if your boiler keeps breaking down, you have to spend money on repairs and more fuel. So, are you wasting your money?

Energy efficiency means using less fuel to give the same, if not better, results.  Older non-condensing boilers were about 85% efficient when installed. Over years of use, this efficiency will have dropped to between 50% and 70% efficient. This will be due to corrosion, heat stress and mechanical stresses.

We’ll assume an old non-condensing boiler started at 85% efficiency. That meant for every £100 that you spent on energy, you lost £15. At the time, this was above average for a new boiler. Over ten years, that boiler would have lost an additional 10% in efficiency. This brings wasted spend up to £25. After another ten to fifteen years, that boiler is most likely 50% efficient – £50 wasted from £100.

Condensing boilers start at 90% energy efficiency and can now be as high as 98% efficient. This is due to the condensing technology, programmable thermostats and smart technology. Programmable thermostats (or stats) allow you to set the temperature in the house. But they also allow you to do this for certain times of the day. This allows you to turn the heat down whilst out at work, for example. They can also be set manually.

Using the previous calculation, a 98% efficient boiler would only waste £2 from every £100. 98% efficiency is rare in a boiler; the Viessmann Vitodens 200-W is the one we would recommend. But, it is more expensive than other boilers of similar power. It does this by recycling outside air and by operating in condensing mode all the time.

In summary, upgrading your boiler can translate into saving £590+ per year on your energy bills. Over 5 years that is £2,950 saved. And over ten years it amounts to £5,900.  That’s enough money to buy a car or holiday. And the new boiler has most likely paid itself off.

Most Energy Efficient Combi Boilers UK 2024

All combi boilers manufactured since 2005 are condensing boilers. All boilers made today, no matter the type or brand, are condensing boilers.

Let’s look at the most efficient combi boilers in 2023 and beyond.

Viessman Vitodens 200-W Review - Super Efficient Boilers

1. Viessmann Vitodens 200-W Combi

> Energy efficiency: 98%
> Approx. price (boiler only): £1,600 inc. VAT
> Approx. price (boiler + installation): £3,200 inc. VAT

This is one of the most energy efficient boilers someone can buy in the UK right now. It is available in combi and system versions. It’s also hydrogen-ready and can burn LPG without extra hardware.

Viessmann focused on commercial heating before moving into domestic gas boilers. They work with many of the biggest stadiums, corporate buildings, and royal palaces.

Key features of the Vitodens 200-W:

  • Up to 98% of the fuel used is converted into heat
  • Modulation range of 1:17 (industry-leading modulation)
  • At only 34db, one of the quietest boilers available in the UK
  • Patented stainless steel Inox-Radial heat exchanger
  • Easy to operate ‘Vitotronic’ control unit with 5-inch colour touchscreen
  • Control via app and internet (with optional Vitoconnect 100 WLAN adaptor)
  • Environmentally responsible MatriX cylinder burner
  • Lambda Pro combustion controller automatically adjusts to different gas types
  • No LPG conversion kit required
  • Compact dimensions, only 450 mm wide
  • Also available as a system boiler (type B2HB)
  • System boiler: Cascade with up to six boilers totalling 594 kW
  • Optional remote fault notifications for Viessmann-trained installers
  • Warranty: up to 10 years

Read our 200-W review    //    (Back to top ^)

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2. ATAG iC35+ Economiser Plus 35 Combi

> Energy Efficiency: 94%
Approx. Price (Boiler + Installation): £2,600 inc. VAT

ATAG are a manufacturer from the Netherlands. Their unique stainless steel heat exchanger ranks them at number 2 on this list.  Their primary heat exchanger condenses twice. This is how they can achieve a high energy efficiency figure of 94% and above.

Key features of the ATAG iC Economiser Plus 35:

  • Compact dimensions
  • Warranty of up to 12 years standard and 18 years extended
  • Instant hot water with pre-heat facility
  • Built-in passive flue gas heat recovery
  • Comes with an outside weather compensator as standard
  • Achieves ErP ‘A’ rating for heating and hot water

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3. Viessmann Vitodens 100-W Combi

Viessmann Vitodens 100-W

> Energy efficiency: 94%
> Approx. price (boiler only): £1,400 inc. VAT
Approx. price (boiler + installation): £2,800 inc. VAT

This is one of Viessmann’s best-selling boilers in the UK and Europe.  It’s a solid, highly efficient mid-level boiler. It sits between the entry-level Vitodens 050-W and the high-end Vitodens 200-W. The Vitodens 100-W is a great value option for most people.

Key features of the Viessmann Vitodens 100-W Combi:

  • Compact design and low weight
  • Quiet operation thanks to low fan speed
  • Available in outputs from 4.7 to 35 kW
  • High quality stainless steel Inox-Radial heat exchanger
  • Class A rated for energy efficiency
  • Low CO2 emissions for reduced environmental impact
  • User-friendly Backlit LCD touchscreen for easy operation
  • Optional room thermostats available
  • Up to 10-year extended warranty available if installed by Boilerhut

Read our in-depth Viessmann Vitodens 100-W review >

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4. Vaillant ecoTEC exclusive with Green iQ combi

The most efficient combi boilers; Vaillant Ecotec Plus with Green IQ Combi Boiler - A Boilerhut Guide

> Energy Efficiency: 94%
Approx. Price (Boiler Only): £1,400 inc. VAT
Approx. Price (Boiler + Installation): £3,000 inc. VAT

Vaillant is a great brand that produces some highly efficient boilers. This ecoTEC boiler from Vaillant has its efficiency merits being 94% efficient. Vaillant also claims 85% of its parts are recyclable at the end of its life.

There are a couple of reasons we’re unable to place them higher on this list. They don’t use a stainless steel heat exchanger in this model. And this unit is not LPG-compatible.

Key features of the Vaillant ecoTEC Exclusive Combi:

  • Modulation range: 1:10
  • Quiet operation
  • Flow rate of 17.8 litres per min for instant hot water
  • ErP A rated for both hot water and heating efficiency
  • Dimensions in mm (43kW): W 720 x H 440 x D 474
  • Warranty: 5 years standard, up to 10 years extended.

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5. Worcester Bosch Greenstar 8000 Life

> Energy Efficiency: 94%
> Approx. Price (Boiler Only): £1,300 inc. VAT
> Approx. Price (Boiler + Installation): £2,700 inc. VAT

Worcester Bosch makes a range of energy efficient gas boilers.  They have been the UK market leader for boilers for many years.

Like other brands, they come with a good warranty and modulation range. Their parts are widely available, and servicing is easy as there are so many Worcester Bosch-trained engineers. Their after-sales service is also ranked very highly by customers and engineers.

However, we have placed them low on this list for one reason: they use an aluminium heat exchanger. This is across all their boiler models. And includes this Greenstar 8000, which we would consider their flagship model. For more information on aluminium vs stainless steel heat exchangers, please see below.

Key features of the Worcester Bosch Greenstar 8000 Life:

  • Modulation range of 1:10
  • Low energy pump
  • Frost protection
  • Available with power up to 50kW
  • Wide choice of controls enables greater comfort and economy
  • Instantaneous hot water, always available
  • Inbuilt keyless filling link, making it easy to refill your system
  • Warranty: up to 12 years

Read our Worcester Bosch Greenstar 8000 Life review    //    (Back to top ^)

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Heat exchangers: aluminium vs stainless steel

When looking at the efficiency of a boiler, we like to look at its performance over many years. Not just when they’re new. This is where the heat exchangers come in. If you want to learn more about them you can read our guide. Below we’ll summarise the differences and why we like stainless steel.

Aluminium heat exchangers are not bad products. They do what they need to do, and when they’re new they are highly efficient. But, over 10 years, they can lose 10% of their efficiency. This doesn’t sound that bad, until you consider stainless steel.

Stainless steel heat exchangers are more robust than aluminium. They can take a wider range of water acidity or alkalinity and do not need chemicals in the heating system. They do start their lives lower in efficiency than aluminium, but they do not have the drop-off over time. In fact, they often get slightly more efficient.

Aluminium is a better conductor of heat than stainless steel. This is why it can be more efficient when a boiler is new. But once an aluminium heat exchanger has been in service for around 600 hours (about 30 days of 24/7 use) efficiency drops. You can see the comparison in the graph below:

energy efficient combi boilers, heat exchanger comparison

As you can see in the graph, aluminium can drastically drop-off once it gets to about 1,500 hours of service. It is because of this that gas bills can rise after a few years. We’ve had customers with aluminium-based boilers ask us to quote for Viessmann or Alpha boilers after only 6 or 7 years.

Energy Efficient Combi Boilers Installed by Boilerhut

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