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Boiler efficiency calculator | Most efficient boilers | Energy saving tips


Jo Alsop

Heating Hero

The Heating Hub

Condensing gas boilers are not A-rated out of the box. We explain all.


How efficient is my old boiler? Which are the most efficient boilers? How much will a new A-rated boiler reduce my heating bills?

Modern condensing boilers are labelled between 92-94% efficient ErP, aka A-rated. Boilers over 25 years old can be as little as 60% efficient, or G-rated. But it is not as simple as swapping out an old boiler for a new boiler to reduce your fuel bills by 30%. Condensing boilers are not A-rated out of the box. Most newly installed boilers run 10-25% under their A-label efficiencies because 99% of installers have not been trained to set condensing boilers up correctly.

In this guide we help you calculate the genuine potential energy savings from a new boiler and guide you on selecting a boiler with the most efficiency potential. You will need a competent installer to bring it together in the home and we guide you there too. There are lots of changes you can make to vastly improve the efficiency of your existing system without changing the boiler. We give you impartial advice on all things efficiency to get you the best heating system for your home.


  1. What is boiler efficiency?
  2. Why are new boilers more efficient?
  3. Warning label: why most new boilers do not reach their efficiency potential
  4. The role of a competent installer in efficiency
  5. What is the efficiency of my existing boiler
  6. Which are the most efficient boilers?
  7. Boiler tables (with another warning label)
  8. Energy saving calculator
  9. What can I do to improve efficiency?

1. What is boiler efficiency?

A boiler's 'energy efficiency' is the percentage of the total energy used by the boiler to provide useful heating. For a modern boiler with 94% efficiency, 94% of the energy used by the boiler goes to heating the home, only 6% is 'lost'/used to run itself. For a very old boiler with 60% efficiency, only 60% of the energy used by the boiler goes to heating the home, a whopping 40% is lost.

Boilers, like other domestic appliances, have efficiency ratings of A-G. Modern boilers have to be A-rated and show it on their literature. It is hard to determine the efficiency rating of older boilers because the ratings are retrospectively applied.

  • A - 90% and above
  • B - 86-90%
  • C - 82-86%
  • D - 78-82%
  • E - 74-78%
  • F - 70-74%
  • G - below 70%

2. Why are new boilers so much more efficient?

All modern boilers are ‘condensing’ boilers. This means they are able to recover heat that was previously lost via the flue to pre-heat the heating system (for the techies out there – this is via a second heat exchanger).

They are not A-rated out of the box however. The heat that previously escaped via the flue is captured by the second heat exchanger and it can only do this if it has turned back into water vapour (hence the term condensing). This requires the boiler to run at a much lower temperature. The lower the temperature, the more efficient the boiler. However most boilers are left at their factory settings (80degC) which means they rarely condense and most useful heat is lost via the flue.

3. Why most new boilers do not reach their efficiency potential

In order for modern boilers to run at their A-rated efficiencies, they need to run at lower temperatures. When a boiler is running in what is called ‘condensing mode’ it is producing heating water temperatures of 65 degC or less. Old systems ran at 80 degC

Despite 15 years of condensing boilers, 99% of installers have not been trained to set them up correctly, so most still run at 80 degC. If you are swapping out a poorly set up condensing boiler for another poorly set up condensing boiler, you will not benefit from any energy savings from your new boiler.

Why is this happening? A systematic under investment in training has left a skills gap amongst gas engineers who have not received the training to know how to specify and set up condensing gas boilers to run at their higher efficiencies. This is a huge industry failing that costs consumers £1,000s.

You can find out more about our work to correct the industry failings that cost consumers £1,000s on our Heating Heroes page.

4. The role of a competent installer in efficiency

In the majority of UK homes we find oversized, poorly configured boilers paired with basic heating controls, which includes many 'smart' controls if they do not speak the same language as the boiler. Despite 15 years of booming condensing boiler sales, no corresponding investment has been made to upskill installers in this new technology.

5. What is the efficiency of my boiler?

Condensing boilers became mandatory in 2005. If your boiler is less than 15 years old then will have been labelled A-rated. However as the vast majority of condensing are still set up to run like a non-condensing boiler, so the efficiency is in line with old style boilers, as follows:

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

6. Which are the most efficient boilers?

All boilers must be A-rated with factory tested efficiencies of 92-94% (ErP) or higher. In practice this is rarely achieved. The boiler is just one part of the heating system and the whole system must work efficiently for the boiler to achieve 90% or more.

The boiler must be right and so must the installer. The best boiler is only as good as the installer that fits it, but we can give you some guidance as follows:

A The boiler must have a low minimum output

All boilers operate within a range, for example 3-30kW or 8-24kW. The lower the minimum end of the range, the more efficiently the boiler can operate all year round. All of the boilers featured on this page have low minimum output

B The boiler must be sized correctly

Most UK homes need just 6-8kW on a very cold day, less the rest of the year, but most boilers are much bigger. It is important that the maximum output of the boiler is reduced by the installer.

C The boiler must be paired with a compensation controls

Modern boilers can vary their output up and down to meet changing temperatures, which is very efficient, but only when paired with a control that speaks the same language. 

D The boiler must fitted by a competent installer

Get this wrong and everything else falls down. This is the most difficult aspect to achieve as 99% of installers have not be trained to setup condensing boilers correctly. You can access the Top 1% of UK installers on our website that have worked hard to rise above the industry standards and fit boilers correctly. Find out more about our Elite Network.


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7. Boiler tables

Warning label: you can pick 'the best' boiler but it will be an average boiler in the home if it is not specified and set up correctly by a competent installer.

Efficient combi boilers

Manufacturer/Model Minimum output Price Maximum output Litres per minute for hot water
Ravenheat HE80 3.6kW £649 25kW 10.2
Alpha Evoke 3.8kW £670 28kW 12.1
Intergas Xclusive 24 3.6kW £969 19kW 9.8
Ideal Vogue C26 4.0kW £1,005 19kW 10.6
Baxi EcoBlue Advance 4.9kW £1,020 19kW 9.8
Viessmann Vitoden 200 1.9kW £1,602 25kW 15.7
Worcester 8000 30 3.0kW £1,305 30kW 10.7
Vaillant ecoTEC Green iQ 835 3.9kW £1,651 26kW 14.5
Intergas Xtreme 3.6kW £1,269 30kW 13.3

Efficient heat only boilers

Manufacturer/Model Minimum output Price Maximum output
Glow Worm Energy 15 4.5kW £755 15kW
Ideal Logic Heat + 4.9kW £740 15kW
Vaillant ecoTEC Plus 15 4.5kW £880 15kW
Worcester Greenstar Ri 15i 5.0kW £830 15kW
Worcester Bosch 8000 3.0kW £1,275 30kW
Vaillant ecoFIT Pure 415 4.5kW £775 15kW

Efficient system boilers

Manufacturer/Model Minimum output Price Maximum output
Ravenheat CS 24 4.0kW £546 25kW
Ideal Vogue System 15 3.2kW £981 15kW
Vaillant ecoTEC plus 618 4.0kW £1,070 18kW
Worcester 8000 30kW 3.2kW £1,305 30kW
Viessmann Vitodens 200 19 1.9kW £1,470 19kW
Vaillant ecoTEC plus 615 3.2kW £925 15kW
Glow Worm Energy 12 4.5kW £690 12kW
Worcester Greenstar 9i 3.0kW £903 9kW
Worcester Greenstar 12i 3.1kW £900 12kW


8. Energy Calculator: What are my potential fuel savings with a new boiler?

Annual bills are around £750 - £1000. We know that installing a new A-rate condensing boiler will improve efficiencies by 14% - 34% if set up correctly (as per above).

Replacing a boiler that is only just over 10 years old will not greatly reduce your gas bills if at all. If it is reliable, with low maintenance costs, then there seems little point replacing it. However poorly designed boilers with high maintenance costs would probably be worth replacing.

A 20+ year old boiler on the other hand will be very inefficient& and upgrading to a modern boiler would likely save the higher figure of around £340 per year but only if set up correctly.

Calculate your own energy bill saving:

1) Take your annual heating bill for last year, as an example lets say it is £800.

2) Estimate the efficiency of your current boiler based on its age, for example 75%

3) Estimate the efficient of your new boiler. NB unless you have a highly competent installer your efficiency improvement is likely to be 10%.

4) Deduct your existing boiler efficiency of 75% from the new boiler efficiency of 85% to calculate the efficiency improvement, in this case 10%

5) Multiply your annual fuel bill by the efficiency improvement % to get your fuel saving figure: £800 x 0.10 = £80 saving per annum

What is the ‘payback’ period for a new boiler

A ‘payback’ period is the number of years it will take to recoup the upfront cost of a new boiler via fuel bill savings and reduced maintenance costs.

The cost of a new boiler installation commonly varies from as little as £1,400 for a straightforward ‘swap’ of an entry level, heat-only boiler. To perhaps £4,000 for a large, premium quality combi boiler, with everything in-between. Most new boiler installations cost between £2,000 - £3,000.

At £2,500 say, a new boiler that brings an energy saving of £40 per annum will not recoup its value.

9. Getting the highest efficiencies out of your existing systems

Boiler efficiency is of course important and may bring decent savings. However heating system efficiency is more important and can cost a lot less to achieve. If you have a reliable condensing boiler, then it is worth considering heating system efficiency measures before considering a new boiler. You can use us to help you with this.

The importance of a correctly sized boiler

It is common for boilers to be oversized 'just in case'. Over-sizing however tends to be around 10kW greater than the heat requirement of the property. For example, a standard 3-bed period terrace with double glazing and loft insulation will have a heat requirement of around 6-8kW. Rule of thumb sizing would specify a 15-24kW boiler.

Over-sizing the boiler will lead to it 'cycling' on and off. It is the heating equivalent of being stuck in town centre traffic versus cruising along a motorway. Like your car, your boiler will burn more fuel and the components will be put under greater stress, accelerating wear and tear. It will also rarely operate in what is known as 'condensing mode' which is when can be 90% efficient. A competent engineer can size and adjust your boiler to run more efficiently.

The importance of a truly balanced system

System balancing takes place as part of the commissioning when a new boiler is installed. The process is to ensure that all radiators get hot and give off the right amount of heat. Balancing involves restricting the flow to radiators nearer to the boiler in order to push the hot water along to radiators that are further away. The aim is for the water to enter the radiator at 70 degrees and leave the radiator at 50 degrees to return to the boiler, i.e. 20 degrees of heat is imparted to the room via the radiator. When the return temperature to the boiler is 50 degrees or ideally lower the boiler can operate in what is knows as 'condensing mode' which is when it can achieve higher efficiencies. A competent engineer can balance your heating system to improve efficiency.

Smart controls and compensation controls

Boiler Plus is a new regulation that came into force in April 2018. It requires all new combination boilers to be fitted with an advanced heating measure, commonly one of three forms advanced heating controls. New heat-only and system boilers only require a thermostat and programmer at present. However all existing boilers would benefit enormously from an advanced control.

The most efficient are what are known as 'compensation controls'. These reduce the output of the boiler according to outside temperatures and help it run for longer periods at lower outputs, which is far more efficient.

The cost of a compensation control will be about £140 in parts and £60 - £150 (as a very rough guide) in labour depending on whether the engineer needs to run a wire to the boiler or not; some programmers will work remotely without a wire. They need to speak the same language as the boiler and that is where it gets a bit tricky. We can help you with this, contact us for help.


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