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When buying a boiler, the primary requirement should be that it is a gas condensing boiler which reduces CO2 and nitrous oxide emissions.  Whilst these are usually double the price of a non-condensing boiler, the substantial energy savings will translate into lower energy bills and the extra cost can therefore be made up within about five years in many cases.  That represents a large additional financial saving over the average boiler lifespan of 20 years.  New boilers come with energy efficiency labels banded from A to G.