Should I replace my boiler?

Should I replace my boiler with a new energy efficient model?

This is a question we get all the time from customers. In this blog I intend to show you how to run the numbers to see if it is worthwhile you splashing the cash on a new shiny energy efficient boiler!

You will need two bits of information to allow you to do this. Firstly, the amount of gas in kWh you use per year. Your utility provider should be able to provide you with this figure, or it might even be on your gas bill (although to be honest the energy companies make these a bit of a bugger to read!).

The other thing you need to do is to determine the energy efficiency of your existing boiler. This is a relatively simple process provided you can find the make and model number of your boiler (this is normally on the instructions or on the boiler casing somewhere).

Once you have this – click on the link below:

>>> Check the efficiency of your boiler – click here <<<

sedbukNext to your boiler you will see the SAP2009 annual efficiency rating, which provides you with the efficiency rating of your boiler.

You may be surprised to hear that the European Union are still messing about trying to establish some Europe-wide benchmarking mechanism to measure the efficiency of boilers, therefore we have to refer to the SEDBUK boiler efficiency ratings (which are much more sensible anyway because they simply go A-G rather than have A+, A++ and A+++).

The table can be found on the right, and your boiler should sit somewhere on that scale. So now when someone mentions replacing a ‘G’ rated boiler you actually know what he or she is taking about.

Should I replace my boiler now?

So now you know the energy efficiency rating (both number and banded letter) and the usage, you can do so simple maths to see what kind of financial savings you would make by installing a new boiler. I am going to take you through a worked example using the following 3 assumptions:

    • Yearly gas usage: 15,000kWh (slightly lower than the national average)
    • Old boiler: Baxi Combi Instant 80e – D rated running at 78.6% efficiency
    • New boiler: Baxi Avanta Plus 39c Combi – A rated running at 90.8% efficiency

See below if the numbers stack up for you!

Step 1: Calculate how much useful heat units my current boiler produces per year

So my current boiler will turn 15,000kWh of gas into 11790 units of useful heat (simply calculated by multiplying 15,000 by 0.786).

Step 2: Find out how many equivalent kWh of gas my new boiler would have to burn to produce the same number of units of useful heat

11,790 divided by 0.908 = 12,984 kWh of gas.

Step 3: Calculate how much less gas you are using by installing new boiler

15,000 minus 12,984 = 2016 kWh less units of gas needed

Step 4: Multiply the units of gas saved by the average cost of gas (4.5p / kWH)

2016 kWh x £0.045 = £90.72 per year saving

Now an average boiler costs about £2,500 to buy and the estimated lifespan of a boiler is 12 – 15 years. So even if you were to replace this today, you are only looking at saving £1,088 over 15 years from replacing it, based on this energy usage.

What if you used 27,000kWh of gas – does that make it worthwhile replacing the boiler?

Old boiler produces 21,222 units of useful heat

New boiler needs 23,372 kWh of gas to produce the same amount of heat

New boiler saves 3627kWh of gas per year.

3627 kWh x 0.045 = £164.24 per year saving

Remember an average boiler costs £2500 to replace, and replacing a D-rated boiler with the A rated boiler detailed above would provide about £1970.88 of savings over the next 12 years (without factoring in any increases in energy prices over that time). You can begin to see that actually it may be worth considering!

The issue is more pronounced however if you are having to pay a yearly sum to keep your boiler alive. Obviously if you are paying £200 a year to get a plumber out to essentially put a plaster on  your boiler to get it through another winter then this it becomes an even more attractive proposition.

The key though is to run this calculations for yourself to see if it is worth doing.

In the final table below you can see that I have taken the average efficiency for each band and run the calculations on the average yearly gas usage in the uk – 17,000kWh, so you should be able to see that from upgrading from a band ‘D’ boiler to a band ‘A’ will save you £85 per year will upgrading from a band ‘G’ to a band ‘A’ will save you £170 per year.


Hopefully the blog above has given you the means to decide whether it is worthwhile replacing your boiler with a new more energy efficient one. As I mentioned earlier though we have not taken into account any fuel price rises in this, so if gas continues to rise at 10% a year then the savings will increase significantly.

If you want to give yourself a sense of how much a boiler replacement costs, you may be interested in reading this blog.

If you are considering replacing your  boiler, fill in the form below and we will send one of our partnered gas safe engineers to quote for the job.