Understanding boiler efficiency

When buying a boiler, boiler efficiency should be a key consideration. This is because the more efficient the boiler you purchase now, the more cost savings you are going to make on your energy bills in the future. 

If you are unaware of what boiler efficiency relates to, a simple way to understand it is that it is the percentage of the total energy used by the boiler to provide heating. For example, a 92% efficient boiler will use 92% of the supplied energy for heating your home but the remaining 8% of energy will be lost. To put this into context, for every £1 you spend on heating your home, only 8p is wasted.

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Is boiler efficiency really important?

Yes. According to the Energy Saving Trust:  “Heating accounts for about 55% of what you spend in a year on energy bills, so an efficient boiler makes a big difference.” 

With so many competing boiler manufacturers offering appliances to the UK market, boiler efficiency is a key point of difference to ensure that you are getting the best value for money when buying a boiler. This is not just for the initial cost of the boiler but also for your future energy bills. 

Since 1st April 2005, all new gas boilers installed into UK properties are required by law to be condensing boilers. Whilst since 1st April 2007, the same law applies to oil boilers too. With all boilers available for sale and installation in the UK needing to be condensing, boiler efficiency has become ever more important.

How does a condensing boiler improve efficiency?

Condensing technology achieves high combi boiler efficiency by condensing water vapours and recovering the latent heat of vaporisation.  The condensed vapour then leaves the boiler in liquid form through a condensate pipe into a water drain.  A modern condensing boiler has a much larger heat exchanger aimed at better heat recovery and sends cooler gases up the flue.  The heat exchangers cool flue gases to such an extent that it starts condensing the water vapour in the exhaust gases, making sure the boiler keeps condensing and keeps saving energy.

Older boiler efficiency 

If your existing boiler was installed prior to 1st April 2005, it is likely going to be a non-condensing boiler. This means that it won’t offer the same energy-saving abilities that a condensing boiler can offer. 

Non-condensing boilers waste the water vapour in exhaust gases that exit the flue. This means that they offer energy efficiency of around 70% to 80% when first installed. However, over time due to corrosion of internal components, the efficiency of a non-condensing boiler can drop to below 60%. 

In monetary terms, this means that for every £100 you are spending on heating, you will be losing at least £40.

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Understanding the efficiency of different boiler types

The below table uses SEDBUK ratings to illustrate the typical annual energy costs for heating your home using different types of boilers:

Type of BoilerEfficiencySemi-DetachedDetachedFlat
Modern Condensing Boiler90%-94%£740£1050£450
Non-Condensing Boiler75%-78%£850£1210£580
Old Back Boiler (Light Weight)60%-65%£1020£1450£670
Old Back Boiler (Heavy Weight)50%-55%£1210£1720£790

How is boiler efficiency calculated?

Seasonal Efficiency of Domestic Boilers in the UK (SEDBUK) is used to compare and classify boilers on energy efficiency.  Boiler efficiency is measured on how well the boiler can convert fuel to heat. There are two different types of SEDBUK ratings:

SEDBUK 2005 – Rates boiler efficiency from A to G.  A-rated boilers are above 90% efficient.

SEDBUK 2009 – This is a newer system that shows combi boiler efficiency as a percentage instead of letters.  All new boilers should be rated using Sedbuk 2009, but you may still see older models which were tested under the old SEDBUK 2005 system. A combi boiler which was A-rated 90% under SEDBUK 2005 will be 88% under the new SEDBUK 2009.  The difference is mainly down to differences in efficiencies between using a boiler to heat a home in the winter months and only using it for heating water in the summer.  Boiler manufacturers may still display the SEDBUK 2005 rating on their website and manuals because it gives them a higher figure for combi boiler efficiency.

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How can the efficiency of modern boilers be increased further?


Unlike older non-condensing boilers which use energy all the time, modern condensing boilers modulate energy usage when not needed.  For example, Viessmann’s Vitodens range modulates between 5kw and 30kw depending on the heating and hot water demand.  This saves you further money on energy bills.

Programmable thermostats

Programmable thermostats allow you to program your boiler to only work at set times.  So for example, if you work from 9-5 pm, you can program your hot water to switch off and heating to be turned down at 8:45 am and come back on at about 4:45 pm. This works even better with modern, condensing boilers because of their efficiency measures.

Smart controls

Using internet-enabled smart thermostat controls with your boiler isn’t just a gimmick, these devices can actually improve your boiler’s efficiency and in turn, save you money. These devices can learn about your heating usage to only heat your home when needed and enable you to control your heating whilst away from home using your smartphone or tablet. In addition to this, devices such as the tado° also offer functionality including open window detection, weather adaption and smart scheduling, our tado° page will tell you more about these features.

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