Boiler Efficiency Calculator | Most Efficient Boilers | Energy Saving Tips
Overview of energy saving and boiler efficiency guide
How to use this Guide
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.
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%
What is the efficiency of your boiler? As a rough guide for non condensing boilers assume:
- Over 25 years old: 60% efficient
- 20 years old: 70% efficient
- 15 years old 80% efficient
- 10+ years old 85% efficient
If you have a condensing boiler (available in the last 15 years or so) Replacing a boiler that is more than 10 years olds is therefore likely to improve your boiler efficiency by 14% - 34%.
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%
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.
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.
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 15-21kW 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. For more on accurately sizing your boiler see our Guide to boiler sizing.
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 benefit of thermostatic radiator valves (TRVs) is that they will shut down rooms as the TRV temperature is met. However they do not signal the boiler to reduce its output accordingly and the boiler will continue to output at a higher temperature than is required by the remaining radiators. The result is that water enters the remaining radiators at 70 degrees and leaves at say 60 degrees. The return temperature of water back to the boiler is then above the threshold for the boiler to operate in condensing mode.
The solution is to use IMI Hydronics self-balancing radiator valves, as they 1) ensure the right amount of heat enters each radiator, i.e. they will not overheat it; and 2) signal the boiler to reduce its output as rooms reach their desired temperature. The remaining radiators do not then overheat and the return temperature is 50 degrees. They are growing in popularity amongst installers and save up 16% efficiency on heating bills.
Thermostatic radiator controls
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. However, they can impact boiler efficiencies - see section above on balanced systems.The total cost for 7 TRVs can be as little £100 in parts and perhaps £150 in labour. The cost of IMI TRVs a little more than good quality standard TRVs but half the price of smart TRVs.
Smart controls and compensation controls
Boiler Plus is a new regulation that came into force in April 2018 (click here for more on Boiler Plus). 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 effective are what are knows 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!
For more on thermostatic controls read our Guide to Heating Controls.