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What is a condensing boiler?

What is a condensing boiler?

The term ‘condensing boiler’ refers to the fact that the boilers produce condense from time to time.

Condensing boilers use heat from exhaust gases that would normally be released into the atmosphere through the flue. To use this latent heat, the water vapour from the exhaust gas is turned into liquid condensate.

In order to make the most of the latent heat within the condensate, condensing boilers use a larger heat exchanger, or sometimes a secondary heat exchanger.

Due to this process, a condensing boiler is able to extract more heat from the fuel it uses than a standard efficiency boiler. It also means that less heat is lost through the flue gases.

Boiler efficiency

Boiler efficiency A-rated explained diagramWorcester’s Greenstar condensing boilers are at least 90% efficient, meaning that they turn 90% of the fuel they use into heat.

The official rating for boiler efficiency is the ‘SEDBUK’ scale. SEDBUK stands for ‘Seasonal Efficiency of a Domestic Boiler in the UK’.

Greenstar condensing boilers achieve the highest possible efficiency category – SEDBUK Band ‘A’.

What is SEDBUK?


SEDBUK efficiency grading chart The SEDBUK rating was developed under the UK Government’s ‘energy efficiency best practice programme’ with the co-operation of boiler manufacturers, including Worcester. It provides a basis for fair comparison of different models of boilers.

The SEDBUK rating is the average annual efficiency achieved in typical domestic situations. It takes into account sensible assumptions about climate, control, pattern of usage and other similar factors.

The rating is calculated from laboratory tests together with other important factors such as boiler type, fuel used, ignition type, UK climate, boiler water content and typical domestic usage patterns.

So, for estimating annual fuel running costs SEDBUK is a better guide than laboratory test results alone.

The boiler’s performance is scored, enabling the boiler to be placed in a banding system using a scale from ‘A’ to ‘G’. ‘A’ rated boilers are the most efficient.

SEDBUK was introduced in 1999 and has undergone a number of changes since then, the latest being from SEDBUK(2005) to SEDBUK(2009). Where boiler efficiency is quoted simply as ‘SEDBUK’ it should be assumed this refers to SEDBUK(2005).

Condensing boiler regulations

Building regulations that have come into force since 1st April 2005 state that any replacement or new gas or oil boiler must be a condensing boiler. Rare exceptions may apply.

Ask an installer about condensing boilers

Find out which is the best condensing boiler for you by asking your local Worcester Accredited Installer.

Worcester Accredited Installers specialise in Worcester central heating condensing boilers and can offer extended guarantees as well as advice on new installations and prices.

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