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The Most Efficient Combi Boilers in 2020

Did you know that your heating bill accounts for around 55% of your annual energy costs?

Choosing an energy efficient combi boiler will make sure you pay considerably less and still keep your home warm and cosy.

With so many boilers on the market it can be tricky to make the best choice for your property size and needs.

Here at HomeSage, we want to make choosing your next combi boiler easy, so we’ve developed our own unique scoring system so you can make an informed choice.

We’ve put together this guide to give you a detailed overview of energy efficient combi boilers, and it includes:

most efficient combi boiler
Credit: Worcester Bosch
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What is a combi boiler?

A combi boiler is the most cost and energy efficient way to heat your home and provide you with hot water.

It draws water directly from the mains supply, heats it internally using a heat exchanger, and pipes it around your property through radiators for central heating, or through the hot water taps

A combi boiler doesn’t require any further water storage tanks like a traditional boiler nor is it a large unit with its own internal storage tank as system boilers have. 

It is usually a fairly compact, wall-hung unit that offers maximum energy efficiency because it makes best use of the energy created by the boiler itself resulting in energy ratings of ‘A’ or ‘B’.

Older boilers often ‘lose’ energy just by heating the water.

For example, if a boiler has an energy efficiency rating of 60%, 40% is ‘lost’ energy that is simply used by the boiler to do its job.

Combi boilers have lower amounts of ‘lost’ energy so it will cost you less to heat your home.

This is why they are the boiler of choice for many UK properties. 

We examine combi boilers in more depth in our dedicated combi boiler article, which you can find here!

Choosing the best right size combi boiler for your home

A boiler’s output is measured in kilowatts (kW) and it tells you how much it will cost to provide you with heat and hot water. 

It’s important to choose the correct output size for your property because it needs to produce enough power for your needs but shouldn’t be too big to avoid unnecessary over-payments on your energy bill.

The equation to work out exactly how much your boiler will cost to run is as follows: multiply the boiler output by the price of a unit of energy.

For example, a 28kW gas combi boiler would cost £1.12 (28kW x 0.04p).

You can find out an approximate gauge of the size of output you may require in the table below.

Property sizeBathroomsRadiatorsBedroomsBoiler output rate
Small1Up to 121 or 2Up to 27kw
Medium1 or 212 to 182, 3, or 428-34kw
Large2 or more19 or more4 and above35kw+

When you’re considering your new combi boiler, it’s important to take the age of your property into account.

This is because older properties tend to have less efficient insulation systems so it will cost more to keep your house warm at an optimum temperature. 

For example, a pre-1900 house is likely to have poor insulation so a small 2 bed and 1 bath property will likely require a 24kW boiler, whereas the same size property built after 2000 will only require a 20kW output.

Similarly, a large 1950s 3-bed house with 2 bathrooms may need a boiler with a 28kW output whereas a new build of the same size will operate to the same level using 24kW of power.

Another important element to consider with combi boilers is the water flow rate.

This refers to the amount of water that can be produced in one minute.

For example, if two people are using the hot tap at one time, the pressure may reduce by half for each of the users if the flow rate is too low.

A boiler’s flow rate is also affected by the flow rate in your local area.

So even if you have a boiler with a flow rate of 16 lpm, if your water mains can only produce 10 lpm, that is the maximum amount you can get when you turn on the taps.

Combi boilers are generally much smaller than traditional or system boilers due to their lack of water storage tank

This makes them an excellent choice for a smaller property where there is a need to maximise space because they can often be installed under the stairs or within a kitchen cabinet. 

Because combi boilers are a self-contained unit, their warranties are usually longer.

Compared to heat only or system boilers that tend to have either an extra water tank, more pipework, and a greater number of internal components, combi boilers house relatively straightforward engineering which results in them benefitting from more generous guarantees

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The 10 most-energy efficient gas-powered combi boilers on the market right now

Here at HomeSage, we want to help you make an informed choice when it comes to choosing your new energy efficient combi boiler.

There are thousands of different combi boiler models available to purchase so we thought we would help narrow down your choices.

We’ve created our own original online boiler scoring system that helps you decide on your boiler upgrade at a glance. 

We rank boilers according to their output, energy efficiency, price, length of warranty, customer reviews on Trustpilot, and price so you can see which boiler will suit your needs and budget.

On this list, boilers are listed firstly by energy efficiency followed by the HomeSage total score which takes the above features into account.

As you can see, Ferroli comes out on top with an impressive 98% efficiency rating, however their total score is considerably lower due to the lack of customer reviews on Trustpilot.

Make and Model NameEfficiency RatingParts and Labour WarrantyOutput kWPrice fromTOTAL
Ferroli Energy Top W125 Boiler98.00%60116£1,00579.57%
Ferroli Energy Top W60 Boiler98.00%2465£59078.09%
Ferroli Energy Top W80 Boiler98.00%2475£67577.88%
Worcester Bosch Greenstar 25i Boiler94.00%6025£95591.41%
Worcester Bosch Greenstar 28CDi Compact Boiler94.00%6028£98091.35%
Worcester Bosch Greenstar 25Si Compact Boiler94.00%6025£1,05591.16%
Worcester Bosch Greenstar 30i Boiler94.00%6030£1,05591.16%
Worcester Bosch Greenstar 32CDi Compact Boiler94.00%6032£1,09091.08%
Worcester Bosch Greenstar 30Si Compact Boiler94.00%6030£1,14090.95%
Worcester Bosch Greenstar 8000 Life 30 Boiler94.00%6030£1,17590.87%
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The most efficient gas-powered combi boilers for a small property

For a studio apartment or 1-2 bedroom flat with 1 bathroom, a gas combi boiler with an output of up to 27kW should suffice to meet your needs.

As of June 2020, there are 78 boilers in this category available in the UK and the average price is £1,035.

Worcester Bosch Greenstar 25i Boiler (91.41%)

Make and ModelWorcester Bosch Greenstar 25i Boiler
TypeCombi
PowerGas
LocationInternal
Condensing?Condensing
Efficiency Rating94.00%
Band RatingA
LPG Compatible?Yes
Parts and Labour Warranty in months60
Heat Exchange Warranty in months60
Output kW25
MountingWall Mounted
Flow Rate in litres per second9
Price from£955
Price to£995
ERP HeatA
ERP WaterA
TOTAL91.41%

The Worcester Bosch Greenstar 25i boiler has been designed with a focus on energy efficiency and user-friendly controls.

This is by far one of their most popular boilers.

It measures 710mm x 400mm x 330mm and it has a simple, white casing with a concealed control panel

This range of boilers are available with a warranty of up to 10 years providing you follow the required terms and conditions of installation and servicing. 

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Worcester Bosch Greenstar 25Si Compact Boiler (91.16%)

Make and ModelWorcester Bosch Greenstar 25Si Compact Boiler
TypeCombi
PowerGas
LocationInternal
Condensing?Condensing
Efficiency Rating94.00%
Band RatingA
LPG Compatible?Yes
Parts and Labour Warranty in months60
Heat Exchange Warranty in months60
Output kW25
MountingWall Mounted
Flow Rate in litres per second8.9
Price from£1,055
Price to£1,095
ERP HeatA
ERP WaterA
TOTAL91.16%

A very similar model in function to the Greenstar 25i, the Worcester Bosch Greenstar 25Si Compact features a 25kW output and a flow rate of 8.9.

It is housed in a comparatively smaller white shell measuring just 690mm x 390mm x 280mm hence the compact name.

In fact, it’s so compact that it can be installed discreetly and comfortably behind a kitchen unit door.

The warranty for this boiler is up to 10 years so long as it is installed by a Worcester Bosch accredited engineer. 

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Vaillant ecoTEC plus 825 Boiler (87.07%)

Make and ModelVaillant ecoTEC plus 825 Boiler
TypeCombi
PowerGas
LocationInternal
Condensing?Condensing
Efficiency Rating94.00%
Band RatingA
LPG Compatible?Yes
Parts and Labour Warranty in months60
Heat Exchange Warranty in months60
Output kW20.4
MountingWall Mounted
Flow Rate in litres per second10.5
Price from£1,130
Price to£1,175
ERP HeatA
ERP WaterA
TOTAL87.07%

Coming in at a slightly higher price point, the Vaillant ecoTEC plus 825 combi boiler has a high energy efficiency rate of 94% meaning only a mere 6% of energy is lost. 

The dimensions of this boiler are a cosy 720mm x 440mm x 338mm.

It comes with a standard 5-year warranty which can be extended for free to 7-years if installed by a Vaillant engineer. 

This is a slick-looking boiler with a hidden control panel. 

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Vaillant ecoTEC plus 832 Boiler (86.83%)

Make and ModelVaillant ecoTEC plus 832 Boiler
TypeCombi
PowerGas
LocationInternal
Condensing?Condensing
Efficiency Rating94.00%
Band RatingA
LPG Compatible?Yes
Parts and Labour Warranty in months60
Heat Exchange Warranty in months60
Output kW25.7
MountingWall Mounted
Flow Rate in litres per second13
Price from£1,225
Price to£1,275
ERP HeatA
ERP WaterA
TOTAL86.83%

The next step up from the ecoTECH Plus 825 is the Vaillant ecoTEC plus 832 boiler, which again is slightly more expensive than similar spec models from other manufacturers.

It measures 720mm x 440mm x 338mm.

The boiler comes with a standard 5-year warranty with options to extend.

Customers also benefit from built-in two stage frost protection for extra peace of mind over the winter.

It also boasts a step-by-step setup wizard for easy installation.

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Ideal Logic Max C24 Boiler (86.43%)

Make and ModelIdeal Logic Max C24 Boiler
TypeCombi
PowerGas
LocationInternal
Condensing?Condensing
Efficiency Rating94.00%
Band RatingA
LPG Compatible?No
Parts and Labour Warranty in months120
Heat Exchange Warranty in months120
Output kW24.2
MountingWall Mounted
Flow Rate in litres per second9.9
Price from£950
Price to£985
ERP HeatA
ERP WaterA
TOTAL86.43%

A huge 10-year warranty delivers extra peace of mind for new owners of the Ideal Logic Max C24.

It measures 700mm x 395mm x 278mm and has a low lift weight of 33.7kg so installation should be stress-free.

This boiler rates relatively highly due to its affordable price tag and its high energy efficiency rating of ‘A’. 

It features a large digital display and the white casing is hard-wearing and scratch resistant

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The most energy-efficient combi boilers for a medium sized property

If you’re considering a new combi boiler for a property of 3-4 bedrooms with 1-2 bathrooms, you will likely need a boiler with an output of between 28-34kW.

There are a wide range of specifications to choose from in 2020, the average price is £933 and there are 80 models available to purchase in the UK.

Worcester Bosch Greenstar 28CDi Compact Boiler (91.35%)

Make and ModelWorcester Bosch Greenstar 28CDi Compact Boiler
TypeCombi
PowerGas
LocationInternal
Condensing?Condensing
Efficiency Rating94.00%
Band RatingA
LPG Compatible?Yes
Parts and Labour Warranty in months60
Heat Exchange Warranty in months60
Output kW28
MountingWall Mounted
Flow Rate in litres per second10
Price from£980
Price to£1,020
ERP HeatA
ERP WaterA
TOTAL91.35%


This is a compact boiler measuring in at 690mm x 390mm x 280mm but it has an output of 28kW and a good flow rate of 10 litres per minute.

It would be beneficial to a property that is short on space but still has fairly high hot water and heating requirements. 

The Greenstar 28CDi Compact has simple user controls for your convenience.

If it is installed by a Worcester Bosch Accredited engineer, you can upgrade the 5-year warranty to 10-years for extra reassurance. 

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Worcester Bosch Greenstar 30i Boiler (91.16%)

Make and ModelWorcester Bosch Greenstar 30i Boiler
TypeCombi
PowerGas
LocationInternal
Condensing?Condensing
Efficiency Rating94.00%
Band RatingA
LPG Compatible?Yes
Parts and Labour Warranty in months60
Heat Exchange Warranty in months60
Output kW30
MountingWall Mounted
Flow Rate in litres per second10.8
Price from£1,055
Price to£1,095
ERP HeatA
ERP WaterA
TOTAL91.16%


Another popular Greenstar model, the Worcester Bosch Greenstar 30i is one of the more powerful boilers in the range and is also a fairly recent addition to their catalogue.

Although compact in size at 690mm x 390mm x 280mm, it would fit in most locations within a mid-sized home.

The low energy pump supplied with this range of boilers means it uses a minimal amount of electricity so you are saving money at every turn.

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Worcester Bosch Greenstar 32CDi Compact Boiler (91.08%)

Make and ModelWorcester Bosch Greenstar 32CDi Compact Boiler
TypeCombi
PowerGas
LocationInternal
Condensing?Condensing
Efficiency Rating94.00%
Band RatingA
LPG Compatible?Yes
Parts and Labour Warranty in months60
Heat Exchange Warranty in months60
Output kW32
MountingWall Mounted
Flow Rate in litres per second11.5
Price from£1,090
Price to£1,135
ERP HeatA
ERP WaterA
TOTAL91.08%


The mid-range model for the compact Greenstar boilers of a higher output is the Worcester Bosch Greenstar 32CDi Compact boiler.

It boasts a grade ‘A’ SEDBUK rating so you know your annual energy bill will be exceptionally low compared to lesser effective models.

The boiler measures 690mm x 390mm x 280mm and it is housed in a traditional-style white gloss case.

This boiler can achieve an a+ ErP package efficiency rating when you install it alongside the Bosch EasyControl system

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Worcester Bosch Greenstar 30Si Compact Boiler (90.95%)

Make and ModelWorcester Bosch Greenstar 30Si Compact Boiler
TypeCombi
PowerGas
LocationInternal
Condensing?Condensing
Efficiency Rating94.00%
Band RatingA
LPG Compatible?Yes
Parts and Labour Warranty in months60
Heat Exchange Warranty in months60
Output kW30
MountingWall Mounted
Flow Rate in litres per second10.7
Price from£1,140
Price to£1,185
ERP HeatA
ERP WaterA
TOTAL90.95%


This wall-hung combi boiler measures in at just 690mm x 390mm x 280mm.

It could easily be installed in a cupboard or kitchen unit, subject to installer confirmation, to maximise space in your property.

It benefits from grade ‘A’ energy efficiency with only 6% lost energy meaning you should see a considerable drop in your heating bill.

It has a wide range of controls and settings for your comfort and all accessed through the user-friendly display panel. 

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Worcester Bosch Greenstar 8000 Life 30 Boiler (90.87%)

Make and ModelWorcester Bosch Greenstar 8000 Life 30 Boiler
TypeCombi
PowerGas
LocationInternal
Condensing?Condensing
Efficiency Rating94.00%
Band RatingA
LPG Compatible?Yes
Parts and Labour Warranty in months60
Heat Exchange Warranty in months60
Output kW30
MountingWall Mounted
Flow Rate in litres per second12.3
Price from£1,175
Price to£1,225
ERP HeatA
ERP WaterA
TOTAL90.87%


The Worcester Bosch Greenstar 8000 Life 30 is one of the latest range of boilers from market leader Worcester Bosch. 

This model measures 824.95mm x 440mm x 364.6mm and it is available in a stunning gloss white finish.

It has a unique and striking touchscreen control panel that can also be linked to your smartphone so you can control your heating even when you’re not at home.

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Conventional boilers to consider for a larger property

A relatively smaller range of boilers are available for semi-detached and detached properties that have 4+ bedrooms and 3+ bathrooms – currently there are 48 on the market.

The average price of a boiler for this property size in 2020 is £1,357.

Ferroli Energy Top W125 Boiler

Make and ModelFerroli Energy Top W125 Boiler
TypeCombi
PowerGas
LocationInternal
Condensing?Condensing
Efficiency Rating98.00%
Band RatingA
LPG Compatible?No
Parts and Labour Warranty in months60
Heat Exchange Warranty in months60
Output kW116
MountingWall Mounted
Flow Rate in litres per second12.38
Price from£1,005
Price to£1,050
ERP HeatA
ERP WaterN/A
TOTAL79.57%

The first Ferroli in our boiler chart is the Energy Top W125 with a staggering 98% energy efficiency rating

This range of combis boast aluminium silica alloy heat exchangers that eliminate debris-related corrosion for extra boiler longevity.

It is a wall-hung unit and measures 945mm x 430mm x 278mm which places it at the taller end of the scale.

The warranty for this boiler is a standard 5-years. 

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Ferroli Energy Top W60 Boiler

Make and ModelFerroli Energy Top W60 Boiler
TypeCombi
PowerGas
LocationInternal
Condensing?Condensing
Efficiency Rating98.00%
Band RatingA
LPG Compatible?No
Parts and Labour Warranty in months24
Heat Exchange Warranty in months24
Output kW65
MountingWall Mounted
Flow Rate in litres per second6.97
Price from£590
Price to£610
ERP HeatA
ERP WaterN/A
TOTAL78.09%

A lower output model from Ferroli is the Energy Top W60 boiler which only comes with a standard 2-year warranty

The dimensions for this model are 945mm x 440mm x 278mm and it is a wall-hung unit. 

As with all the Ferroli boilers in our chart, it boasts a very impressive 98% energy efficiency score. 

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Ferroli Energy Top W80 Boiler

Make and ModelFerroli Energy Top W80 Boiler
TypeCombi
PowerGas
LocationInternal
Condensing?Condensing
Efficiency Rating98.00%
Band RatingA
LPG Compatible?No
Parts and Labour Warranty in months24
Heat Exchange Warranty in months24
Output kW75
MountingWall Mounted
Flow Rate in litres per second7.94
Price from£675
Price to£705
ERP HeatA
ERP WaterN/A
TOTAL77.88%

This model uses the latest Ferolli technology and design.

This ground-breaking unit features an air and gas premix metal fibre burner complete with a variable fan speed for optimum performance. 

The dimensions of this boiler are 945mm x 430mm x 278mm as with the other Ferolli models featured in the chart.

It has a digital control panel and it can be controlled using outdoor weather compensation for even lower energy bills.

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Worcester Bosch Greenstar 8000 Life 35 Boiler

Make and ModelWorcester Bosch Greenstar 8000 Life 35 Boiler
TypeCombi
PowerGas
LocationInternal
Condensing?Condensing
Efficiency Rating94.00%
Band RatingA
LPG Compatible?Yes
Parts and Labour Warranty in months60
Heat Exchange Warranty in months60
Output kW35
MountingWall Mounted
Flow Rate in litres per second14.3
Price from£1,290
Price to£1,340
ERP HeatA
ERP WaterA
TOTAL90.59%

The Worcester Bosch Greenstar 8000 Life 35 is a powerful machine that doesn’t compromise on fashion or function.

Owners benefit from a high output, flow rate and an eye-catching white gloss finish.

It measures 824.95mm x 440mm x 364.6mm and has a reduced weight from the model it was based on – the award-winning Greenstar CDi Classic. 

You can link this boiler up to your smart device to control your heating requirements on the go.

It also boasts the Greenstar Intelligent Filling System which can eliminate the need for you to manually refill your boiler. 

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Worcester Bosch Greenstar 36CDi Compact Boiler

Make and ModelWorcester Bosch Greenstar 36CDi Compact Boiler
TypeCombi
PowerGas
LocationInternal
Condensing?Condensing
Efficiency Rating94.00%
Band RatingA
LPG Compatible?Yes
Parts and Labour Warranty in months60
Heat Exchange Warranty in months60
Output kW36
MountingWall Mounted
Flow Rate in litres per second12.9
Price from£1,345
Price to£1,395
ERP HeatA
ERP WaterA
TOTAL90.45%

A powerful boiler in a compact size, the Worcester Bosch Greenstar 36CDi Compact measures a mere 690mm x 390mm x 280mm yet houses technology for an output of 36kW.

This is the largest output size in the Greenstar compact range.

It comes with a warranty of up to 10-years providing it is installed by a Worcester Bosch Accredited engineer and serviced annually

Much like the other models in the range, this boiler features a user-friendly control panel which is hidden by a flip-down cover. 

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Save up to 54% on your boiler installation with HomeSage

If you’re considering upgrading your boiler to a more energy efficient model, you can save up to 54% of the cost of the supply and the installation simply by filling in our online form.

We contact three or four local, reliable and Gas Safe regulated engineers with your requirements who will get in touch with you directly with a free quote to supply and install an energy efficient combi boiler on your property.

Our service is completely free for you and it is no obligation.

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Andy

Andy is one of the HomeSage founders and an occasional content contributor. His love for boilers is only just outweighed by his love for football.