High efficiency or condensing boilers use less gas to produce the same amount of heat as standard boilers. They do this by recovering and using heat that would otherwise be lost and wasted.
An extra large heat exchanger (or sometimes two heat exchangers) maximises heat transfer from the burner as well as recovering useful heat that would normally be lost with the flue gasses.
These can reduce carbon dioxide emissions by 12% (0.7 tonnes) annual average emissions, and can also reduce your heating bills by a third!
Boilers that are more than 15 years old should be replaced by an energy efficient boiler.1