In condensing technology, almost the entire energy content of fuel is utilised. As part of this, the flue gas is cooled to below the dew point. This gives rise to a liquid condensate. During this conversion, heat is released which can be additionally fed into the heating system. In contrast, older heating systems only utilise the net calorific value of the fuel. They therefore simply release the water vapour, the CO2 that occurs during combustion and other substances up the chimney and into the environment.