“Save as much as £300 off your heating bills”

This is a carrot dangled in front of people to try to persuade them to buy a new gas boiler. Now I’m not going advise whether or not you should scrap your gas boiler and get a new one, but I am going to try to help you calculate how much gas you’re going to save, to help you make up your mind.


If the word “calculate” strikes you with dread, be assured there’s nothing complicated, and in any case, there’s a site where you can simply enter numbers, and you can also download my simple spreadsheet that does the working out. Don’t be prey to those who sell you things by relying on you not to check the figures.


The saving on heating bills comes from burning less gas to provide your heating and hot water, because modern (condensing) boilers are more efficient than old, non-condensing boilers, which for one thing, lost a significant amount of energy in the form of water vapour disappearing up the flue.


The more inefficient your current boiler, and the worse-insulated and large your house, the more money you’ll save in getting a new boiler.


What do we mean by efficiency? In a technical context, it’s the amount of something we get out (that’s useful), divided by the amount we have to put in to get it (“output divided by input”). Multiply by 100, and we have efficiency expressed as a percentage. So for a gas boiler, it’s the amount of useful heat we get out, divided by the amount of heat that burning the gas could theoretically have produced.


The efficiency of a modern boiler is given as a percentage, when tested in a standardised way to arrive at the so-called SEDBUK figure (Seasonal Efficiency of Domestic Boilers in the UK). You can learn a bit more and look at the boiler database here:



Note this paragraph from the site: “Boiler efficiency tests are subject to measurement uncertainty. Consequently small differences in the efficiency values calculated from them are not significant and should not be relied on when comparing boilers. Statistical analysis suggests that if two boilers have SEDBUK values 3 percentage points apart then there is 95% confidence that the boiler with the higher value is more efficient.”


In other words, don’t base your boiler choice on tiny differences in stated efficiencies.


The database shows that we can take the efficiency of a modern boiler as around 90%.


The site also contains an easy-to-use calculator which allows you to estimate your gas costs:

http://www.boilers.org.uk/index.htm   I recommend you try it.


The Which site also contains some useful information:



The efficiency of your current boiler can be found here: http://www.homeheatingguide.co.uk/efficiency-tables.html


These figures can be used to compare new with old (but not without question – see the Appendix below).


As an example of a relatively old non-condensing boiler, I looked on the site among the Potterton range and found the Profile 40e with a relatively low stated efficiency of 71.8% (let’s call it 72%).


Another way of looking at boiler efficiency is to turn the figures upside down. Useful heat is what we want. To get one unit of useful heat out of a boiler we have to burn 100/efficiency(%) units of gas. In the case of the old Profile 40e, that’s 100/72 = 1.39 units of gas burned. With a new boiler, to get the same heat out, we only need to burn 100/90 = 1.11 units of gas. The ratio between them is 1.11/1.39 = 0.8. (We could just have taken the ratio of efficiencies: 72/90 = 0.8.)


So, if our current annual gas cost is £600, the cost with the new boiler (ignoring the complexity of split tariffs) will be £600 x 0.8 = £480, a saving of £120.


To achieve a saving of £300 per year in changing from a 72% efficient Profile 40e to a new 90% efficient boiler, we would currently have to be burning gas costing £1500 per year.


(Note that in studying costs and looking at your old gas bills, ignore standing charges and try to extract what you’re charged for the gas itself.)


Sorry if you feel mesmerised by the numbers but I hope with the information above and the spreadsheet, I’ve equipped you to check how much you could save on your current gas bill by getting a new, more efficient boiler.


Notice the word “current” – if gas becomes more expensive relative to the general cost of living, future savings will be greater.


There are plenty of other factors to weigh up in deciding whether to go for a new boiler, including maintainability of the existing boiler and the installation cost of a new one but these are outside the scope of this discussion on burning gas. You may also prefer to burn less gas for altruistic, environmental reasons, and there are other ways of saving gas, including home insulation and improved heating controls.


Other references:





Efficiency figures for old boilers obtained from the Installation & Service Manual.

For many older boilers, the Manual won’t have a SEDBUK efficiency figure, or indeed may not have an efficiency figure at all. However, there will be data on input and output powers, as in this table extracted from the Potterton Profile 40e manual:


Dividing input by output gives a measure of efficiency. The poorest figure for this boiler is 75.6% (divide 11.63 by 8.79 and multiply by 100), obtained at the lowest power. This is 3.8% higher than the efficiency figure given in the website above. So this simple method based on the manual figures will underestimate the savings obtained by changing to a high efficiency boiler.


Some discrepancies between the website and figures obtained from the manufacturer’s manual are large and puzzling, as in the case of the Glow-worm Hideaway 60B. The website gives the seasonal efficiency as 65%, whereas the calculation from the manual (output 14.65kW, input 18.59kW) obtains 78.8%.


As so often in life, getting to the facts, or being confident you’ve arrived at them, is not always straightforward.