Which boilers are most energy-efficient?

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What type of boiler is the most energy-efficient?

There are many types of boilers for UK homes, and it can be difficult to determine which boiler is the most energy-efficient. A good way to compare a selection of boilers on a level playing field is to look at how they perform on a heat exchanger test. This is a rigorous test that tests how quickly the units give out heat from one pan of water to the other. In this instance, more efficient boilers will have less thermal losses, meaning they will take longer for boiling water in one pan to reach the other.

The most efficient type of boiler is the condensing combi boiler. These boilers are fitted with an energy-saving condensing heat exchanger that can reduce greenhouse gas emissions by around 8 million tonnes every year (GPTES, 2021). The high efficiency comes in part from increased pressure within the heating system which raises thermal efficiency.

However, condensing boilers are the most expensive to install, costing roughly £270 more than a conventional boiler. It's best to think about how much it will cost to run your boiler over its lifetime before choosing one type of boiler over another. Early generation combi boilers were less efficient and needed replacing after around 12 years, whereas the latest condensing boilers last for up to 17 years.

SEDBUK efficiency rating

SEDBUK efficiency rating is a scale that is used to measure the efficiency of boilers. The higher your boiler's SEDBUK score, the more efficient it is at providing heat. For example, a boiler with a SEDBUK rating of 100 will provide heat more efficiently than one with 50.

SAP winter efficiency

SAP winter efficiency is a rating used to measure how well a boiler is at producing heat in winter. It is measured by heating up a cold cylindrical tank of water from 10°C to 50°C with the most efficient boilers achieving an SAP rating of 80% or above, and the least efficient achieving 55-60%.

SAP hot water efficiency

SAP hot water efficiency is a rating of how well a boiler can provide hot water and has an effect on how quickly it provides your water at the appropriate temperature. The most efficient boilers achieve 100% while the least efficient only get up to around 60%.

AFUE) rating

A good indication of a boiler's energy efficiency is its annual fuel utilization efficiency (AFUE) rating. This is a test done in a laboratory to determine how efficiently the boiler operates. A boiler's AFUE rating will tell you exactly how much of the energy it uses actually goes into heating your home, and not wasted through leaks or flue gases. The higher the AFUE rating, the more energy-efficient it is, but be aware that some boilers are measured using different scales in different countries. Also, boilers with low AFUE ratings are not necessarily inefficient if they have other features which make up for the loss of efficiency.

Some boilers will display their energy efficiency in terms of kilowatts per hour (kWh). This shows how much energy it uses to generate heat during the course of an hour. Generally, boilers with high watt ratings are more efficient than those with low watt ratings.

For more info and tips check out our guide to central heating systems

About the Author

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Heat Quick Direct are Vaillant accredited engineers

Heat Quick Direct is accredited by all the major gas boiler trade bodies, such as Gas Safe and we are approved G3 unvented hot water storage engineers approved by Worcester as accredited engineers and we are approved advanced Vaillant engineers.