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Improve your EPC Score Part 1 - Boilers

Providing British homes & businesses with the vast majority of our heating & hot water, boilers are an incredibly significant factor in both commercial and domestic EPC results - ignore them at your peril


Most of us don't think about our boilers at home or in the office until they suddenly pack up on a wintry morning. By their nature they are squirrelled away devices found lurking under the stairs or secreted in innocuous cupboards, in the workplace they are often larger beasts sometimes inhabiting whole dedicated rooms in basements with octopean arms reaching out pumping entire buildings full of the warmth and hot water they generate.


Artist impression of london EPC result on gas boiler

The Importance of Boilers in EPC ratings

Very often the boiler is the single largest power consumer in a building and as such its efficiency is a major factor in the overall final EPC score.

The difference between an inefficient boiler and a newer more efficient one can be as much as 40 points on an EPC - as much as two whole rating bands difference.

A legacy gas boiler may run as low as 65% efficiency meaning £3.50 of every £10 worth of gas energy paid to the supplier never even makes it to the radiators or taps. When this is compared to a modern gas combi-boiler which can run in excess of 90% efficiency, the running cost savings alone are clear to see. In terms of domestic property sales (and indeed some commercial ones) the boiler's relative age may lower the achievable asking price for the property - if the boiler is seen to be outdated the implied cost burden of replacement is placed on the prospective purchaser.

How to check the efficiency of your gas boiler

In the UK the vast majority of dwellings are heated by natural Gas - 74% (2021 National Census) whilst no similarly definitive figure exists for commercial buildings we can assume that a majority follow this trend particularly in SMEs based outside of purpose-built office accomodation. As we expand on this series of EPC efficiencies we will have a separate dedicated post addressing heat pumps and other electical boiler systems.


Identification marks of various gas boilers
Three Examples of Gas Boiler ID points

The three examples shown above are typical gas boilers all found in commercial buildings in London - though they could equally have been found in domestic settings. From an EPC assessors perspective the one shown on the left is the ideal scenario - make and model clearly evident with supporting paperwork even present allowing for the efficiency to be looked up on the spot. The middle image shows the increasing use of ID stickers on boilers with a QR code linking directly to the manufacturer's website for that boiler model, again enormously helpful. The last image shows the fallback option of looking at the (often complex) mess of serial numbers found on the underside of most boilers, if you have to resort to this you should try and find a GC (Gas Council) number if at all possible. If its not marked as such it will always be in the following 2-3-2 number format:- 00-000-00. This can then be looked up online although in our experience it's not present on 100% of boilers.

Efficiency ratings - ErP and SEDBUK

Once you've tracked down your boilers specifications datasheet you want to find the corresponding ErP or SEDBUK rating, it should be shown as a number and this can be taken as the percentage efficiency of the boiler overall.


ErP efficiency on a boiler datasheet

ErP stands for the EU Energy-related products directive and was introduced to incorporate Environmental design and attest to operational performance and has largely superceded SEDBUK (Seasonal Efficiency of Domestic Boilers in the UK) since 2015. Confusingly however manufacturers still routinely add SEDBUK data to their product specifications and its useful as its always given a numerical rating, ErP is sometimes only given an energy efficiency letter A-G (akin to those seen on an EPC certificate).

Heating system controls & EPC results

Along with the boiler itself its important to not ignore how the energy output is controlled - there's little point in having the World's most efficient boiler if its heating rooms with nobody in them or keeping an office warm over the Christmas holiday. The EPC software assumes poor patterns of human behaviour in regard to turning systems on and off unless there are systems present which prevent natural laziness and wastage.


Thermostatic Radiator Valves (TRVs), zoned heating control, zone valves on the boiler and smart thermostats all have a role to play in improving both domestic and commercial EPCs. They are worthwhile investments as they lead to lower boiler use and therefore lower energy spend and emissions. Having wargamed various scenarios in both domestic and non-domestic EPCs they can make a saving of around 6 points, perhaps enough for a rating band jump and vital at the top end of the scale.

Easy boiler wins

If you're struggling to find the specifications of your boiler online - that in it itself may be a red flag for its replacement. This is on the simple asumption that the energy efficiency of boilers has only become a priority in the last ten to fourteen years, their perceived expected lifecycle is around 10 years and manufacturers have no desire to put obsolete specifications sheets online. Not online = not energy efficient.


Another straightforward win as far as EPC results are concerned is to consider replacing gas systems with electric only boilers, whilst this opens a can of worms of opinions (which we will fully dive into in a forthcoming post) the fact remains that EPCs are weighted by design and intent towards electic only systems. On commercial EPC assessments I'm occasionally asked to estimate a building's score the presence of a gas burning system automatically reduces its maximum score to a 'C' in my eyes.


Should you succeed in locating your gas boiler's efficiency and want to make a rough calculation of its wastage take 60% of your gas bill (the average proportional boiler use - adjust accordingly if no other gas appliances) and deduct from that the percentage wastage of your boiler. This helps visualise ongoing loss and the result may make it clear that a boiler upgrade isn't just a good idea but crucial.

When in doubt - contact us

We really encourage everyone to get to grips with the energy use going on around them at home and in the workplace. The high-energy cost environment we have entered makes this a major overhead for families and businesses alike. Should you wish to discuss your boiler and how it may influence your EPC or perhaps you are considering an upgrade already and want to know what impact it might have on your building's score please do get in touch.




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