In view of traditional heat sources, heat is transferred from exhaust gases to heating water in the primary heat exchanger where cooling to a certain temperature (on average approximately 120°C) occurs. Thus obtained heat is referred to as a sensible heat. Combustion products furthermore contain a certain proportion of heat – so called latent / condensation heat. It is related to the heat connected with water vapour resulting from gas combustion.
In conventional boilers, combustion gases are vented into the chimney without any further use. The construction of condensing boilers due to a large surface area of the heat exchanger (or two heat exchangers) allows the use of the condensation heat. After the primary transfer of the heat from exhaust gases, they are subsequently cooled to a temperature below the dew point of exhaust gases. As the temperature reaches the level, water vapour of exhaust gases is condensed and the heat is additionally conveyed to the heating system. The primary heat is thus used more efficiently than in conventional boilers.
