Condensing boilers are designed to extract the heat from exhaust gases which are usually expelled through flues in standard boilers. So they not only utilise the heat from the initial burning of fuel but also the heat that would usually be lost making them a lot more efficient.
Condensing boilers capitalise on this extra heat source by incorporating an extra heat exchanger which pre-heats the water in the boiler system using the latent heat from water vapour. This water vapour is created during the burning of fuel, these boilers then condense this vapour back into a warm liquid and the heat from this process is used to pre-heat the water entering the central heating system, the condensed water is then drained away. Because the water now entering the central heating system is already warm, the boiler does not need to use as much energy to heat it to the temperature required by the system, thus making it a more efficient process.
There are many myths about the reputation of condensing boilers that have been proved false by the Building Research Establishment (BRE), which is the UK’s major researcher for the building trade. They found that:
Due to their efficiency, which makes them better for the environment, since 2005 all new gas central heating boilers that are fitted in the UK must be high efficiency condensing boilers unless there are exceptional circumstances.