Published on 5th April 2018

How Is Boiler Efficiency Measured

The cost of heating your home is often the largest portion of your household bills, especially during the winter. In fact, during the colder months heating costs can constitute up to 60% of your monthly outgoings.

With such a significant cost associated with keeping warm in the winter, it is no surprise that the single best piece of advice for reducing your bills at home is to reduce the cost of heating it.

For those with an older boiler, that saving can come through increasing boiler efficiency.


Strictly speaking there are two main ways how boiler efficiency is measured – combustion efficiency and thermal efficiency.

To work out combustion efficiency, you would have to look at unburnt fuel quantity at the exhaust and also at surplus oxygen levels. Generally, you get a better combustion efficiency on gas boilers than oil boilers because solid fuel is more difficult to convert to heat.

However, the boiler ratings that we see on the box come off the back of a different efficiency measurement method – thermal efficiency. It is the rate at which the heat exchanger converts the energy into usable heat. The formula to calculate thermal efficiency is as follows:

η=(Energy Output)/(Energy Input) x 100

Total energy output is divided by energy input and multiplied by 100 to get percentage. This is the number that you see advertised by the boiler manufacturers. A decent A-rated boiler will have its thermal efficiency in the region of 90% – 93%.


How energy efficient is your boiler depends on several factors including age of your boiler, type and manufacturer. A brand-new condensing combi boiler made by Worcester Bosch or Vaillant is going to be significantly more efficient than a 2005 Baxi boiler.

A high efficiency A-rated boiler can convert up to 92% of consumed energy into usable heat. That means only 8% of the energy is wasted. If, however, you have an old inefficient boiler, you will be wasting 20% to 35% of energy and that makes a huge difference. Depending on your energy consumption behaviour, your gas central heating system probably accounts for at least 50% of your yearly energy bill. According to Energy Saving Trust, it’s nearer the 55% mark.

Now we have to be careful with the claims we make about how much money a new boiler can save you? Probably, the payback time is not going to be as quick as some TV ads make you believe, however, new boilers do pay for themselves relatively quickly besides there are other benefits of getting yourself an efficient A-rated boiler – you no longer need to worry about the old inefficient boiler breaking down constantly and you do the right thing for the environment.

The latter is quite an important factor, both from the moral aspect and a legislative point of view. According to the EU regulations, everyone will need to hit certain energy efficiency goals. After Brexit, when the UK leaves the EU, we will not be legally bound by these regulations, however, it is certain the UK is going to establish its own environmental targets that might even surpass the EU ones. As a guideline, it was planned to achieve a 20% reduction in greenhouse gas emissions across the EU compared to the 1990 figures.


For those with an old, inefficient boiler, the benefits of installing a modern A-rated energy efficient boiler are significant.

If you have an old gas boiler with a G efficiency rating, then upgrading to a modern condensing boiler with an A rating could save you upwards of £350 per year on your heating bill.

By way of a case study, consider this situation:

You have an old, free standing G-rated gas boiler with a 55% efficiency rating. It has no separate room controls and has to heat an entire detached house. The estimated cost of running this boiler for one year is around £1,705.

Replacing this inefficient boiler with a new, A-rated boiler rated at 90% efficiency could heat the same home for a year for just £1,053.

That’s an annual saving of £652, roughly £54 per month!

With integrated heating controls and smart home technology, the effective efficiency of these boilers can be increased further, saving you even more money.

However, the reduction in your heating bills will be much lower when replacing a moderately efficient boiler to a highly efficient boiler. In these cases, the best way to save on energy costs is to integrate smart controls.

Being aware of what the efficiency labels on your boiler mean is crucial in helping you to decide if getting a new boiler would work for you. So let’s take a look at the different rating systems for boiler efficiency that you might encounter.


Different companies seem to be operating with different figures. This could be down to several factors – they might be “tweaking” it to entice the people, or they may simply have used different methods for calculating the savings. This boiler efficiency savings table is based on a mix of data provided by consumers and other sources.

Even the lowest saving – £20 per month easily covers the popular boiler replacement £17 per month boiler finance deal and depending on circumstances you can end up paying less than that per month.

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SEDBUK 2005 & 2009 BOILER EFFICIENCY Sedbuk, which stands for Seasonal Efficiency of Domestic Boilers in the UK, is one of the primary systems used to judge new boiler efficiency in Britain.

There are two commonly found rating systems:

Sedbuk 2005, the older of the two, uses an A-G rating system, where boilers with 90% efficiency or higher attain an A grade. It has fallen out of use as of late and was replaced in 2009 with a new system.

This new system, Sedbuk 2009, ditched the letter grading in favour of a straight percentage. This was to differentiate itself from the EU rating labels that still use a letter grading system.

New boilers in the UK should be rated with Sedbuk 2009, but older boilers may have been tested under the 2005 system. Many boiler manufacturers also like to prominently display their Sedbuk 2005 rating as the efficiency ratings are slightly higher on the older system.

This difference is mostly to do with efficiency differences between heating the whole home in winter and just heating the water during the summer.

ERP BOILER RATINGS The European system introduced in 2015 will be used for all modern boilers, but its uses when judging boiler efficiency from a consumer standpoint are limited.

This is because nearly all boilers installed in the UK will get an A rating on the ErP system.

Therefore, those who want the best energy efficiency for their boiler should pay more attention to the Sedbuk 2009 rating instead.