Boiler Efficiency Calculator | Most Efficient Boilers | Energy Saving Tips

Quick guide summary

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

Household gas bills are commonly £750 - £1,000, depending on property size, age and heating system efficiency. A new A-rated boiler can potentially save you £100s per year, but it depends on the efficiency of your old boiler and new boiler. In this guide we show you the most efficient boilers available to buy and help you calculate your fuel bill savings for a new boiler in five simple steps. There is often little we can do to improve heat retention of our homes, but we can improve the efficiency of our heating systems and reduce our heating bills with smart thermostats and thermostatic radiator valves. This guides shows you how. If you need some extra help you can try a heating company near you, find a great local business on our network.

Overview of energy saving and boiler efficiency guide

Boiler efficiency savings

Energy Calculator: can you save £100s by installing a new boiler? Which are the most efficient boilers?

Payback periods for new boilers

Energy saver: how quickly can you recoup the cost of a new boiler via energy savings and no maintenance costs

Heating system efficiency

What else can you do? Other ways to reduce your heating bills.

How to use this Guide

How efficient is your boiler?

How much could you save by installing a new boiler?

what is the ‘payback’ period

Other energy saving measures

How efficient is my old boiler?

Modern condensing boilers are between 89-94% efficient. Boilers that are more than 20 years old can be as little as 60% efficient. What does that mean? 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.

What is the efficiency of your boiler? As a rough guide assume:

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

Replacing a boiler that is more than 10 years olds is therefore likely to improve your boiler efficiency by 14% - 34%.

Which are the most efficient boilers?

All boilers must be A-rated, however efficiencies still vary between 89% and 94%. The vast majority of A-rated condensing are 89% efficient. A handful are over 92%. We have researched boiler efficiencies from nine boilers manufacturers to bring you the most efficient boilers available to buy. The tables below are set out by boiler type: combi, heat only and system. They only feature boilers with efficiencies of 92% or over. The tables are in size order: smallest to largest. The higher efficiency models tend to be more expensive, so if you are looking for an entry level or budget model you will have to accept a slightly lower efficiency. You can review all boilers in the following guides: Combi Boilers Review; Heat Only Boilers Review; System Boilers Review.

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Most efficient Combi boilers



Heating output

Litres per minute 

for hot water

Worcestser/Greenstar 25i 94% £875 24kW 9.8
Worcester 25Si 94% £1,010 24kW 9.8
Worcester Greenstar Compact 28CDi 94% £1,045 24kW 10.0
Worcester Greenstar 30i 94% £990 24kW 11.5
Worcester 30Si 94% £1,135 24kW 11.5
Worcester Greenstar Compact 32CDi 94% £1,140 24kW 11.5
Worcester Greenstar Compact 36CDi 94% £1,285 24kW 12.9
Vaillant ecoTEC Green iQ 835 94% £1,651 25kW 14.5
Worcester Greenstar Classic 29CDi 92% £1,100 30kW 11.0
Worcester Greenstar Classic 34CDi 92% £1,225 30kW 12.0
Worcester Greenstar Classic 38CDi 92% £1,395 30kW 14.0
Worcester Greenstar Classic 42CDi 92% £1,665 30kW 15.0
Vaillant ecoTEC Green iQ 843 94% £1,979 33kW 17.8
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Most efficient Heat Only boilers



Heating output

Worcestser Greenstar Ri 24 92% £909 24kW
Worcester Greenstar Ri 27 92% £1,021 27kW
Worcester Greenstar Ri 30 93% £1,065 30kW
Worcester Greenstar Classic 30CDi 92% £1,141 30kW
Worcester Greenstar Classic 40CDi 92% £1,589 41kW
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Most efficient System boilers



Heating output

Worcestser/Greenstar System 12i 94% £879 12kW
Worcester Greenstar System 15i  94% £926 15kW
Ideal Vogue System 15 93% £944 15kW
Worcester Greenstar System 18i 94% £974 18kW
Ideal Vogue System 18 93% £1,015 18kW
Worcester Greenstar System 21i 94% £1,014 24kW
Worcester Greenstar System 24i 94% £1,035 24kW
Main System Eco Elite 24 92% £617 24kW
Ideal Vogue System 26 93% £1,169 26kW
Worcester Greenstar System 27i 92% £1,042 27kW
Main System Eco Elite 28 92% £680 28kW
Worcester Greenstar System 30i 92% £1,112 30kW
Worcester Greenstar System 30CDi 92% £1,132 30kW
Ideal Vogue System 32 93% £1,256 32kW
Worcester Greenstar System 35CDi 92% £1,245 34kW

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%.

Replacing a boiler that is only just over 10 years old will not greatly reduce your gas bills. 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 inefficienct and upgrading to a modern boiler would likely save the higher figure of around £340 per year.

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 70%

3) Look up the efficiency of your new boiler (see any one of our boiler guides, for example Combi Boiler Efficiency or  Heat Only Boiler Efficiency or System Boiler Efficiency), lets say 94%

4) Deduct your existing boiler efficiency of 70% from the new boiler efficiency of 94% to calculate the efficiency improvement, in this case 24%

5) Multiple your annual fuel bill by the efficiency improvement % to get your fuel saving figure: £800 x 0.24 = £192 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 £340 per annum will recoup its installation cost in 7.3 years, based fuel savings alone. This will be quicker for an LPG boiler because bottled gas is more expensive per kW hour.

This ‘payback’ period is much quicker if you factor in maintenance costs on the old boiler over the period. With many manufacturers now offering warranty periods of up to 10 years on their boilers, the new boiler has often paid for itself well within the period that the manufacturer will continue to repair it under warranty or guarantee.

Other energy saving measures

Boiler efficiency is of course important and may bring decent savings. However heating system efficiency is just as important and can cost a lot less to improve! 

Heating system improvements 

A heating system without any kind of heating control will waste energy being on when no one is home and heating rooms that are little used.

Installing thermostatic radiator valves (TRVs) and a central heating programmer will improve the efficiency of your heating system enormously.

Thermostatic radiator valves will allow you to turn some radiators down or off and a programmer will allow you to only have the heating on when you're home, for example mornings and evenings.. Homes without heating controls can save £100s by turning down or off the radiators in bedrooms for example and restricting the times the heating is on, mornings and evenings for example.

The total cost for 7 TRVs can be as little £100 in parts and perhaps £150 in labour. The cost of a programmable room stat 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!

If you have a reasonably new boiler (less than 10 years) and it is reliable, then it is first worth considering heating system efficiency measures before considering a new boiler. For more on system efficiency read our Guide to All Heating Controls.

Other energy saving measures

More sophisticated heating controls, including Smart thermostats, smart radiator valves and weather compensated controls, offer further efficiencies and fuel savings. Read our Guide on Smart Thermostats and Smart Heating Controls to find out more.

What do you want to do next?

Use our calculator to get an estimate of boiler installation cost


Guide to All Heating Controls


When to Repair or Replace your boiler


Other useful guides

Book a survey with a local heating company


Guide to boiler warranties


Boiler Manufacturer Smart controls V Nest & Hive