Big savings with 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.