Gas boilers are divided into two groups; flued and hermetic gas boilers. Flued gas boilers take the ambient oxygen and release it to outside, while hermetic gas boilers have two double-wall flues which are directed towards the atmosphere and which take in the air they need from the outside and exhaust it to the outside. There are two types of hermetic gas boilers; regular hermetic gas boilers and condensing hermetic gas boilers.
Flues of condensing gas boilers open up to the atmosphere, as well. What is noteworthy for such gas boilers is that water transits to solid form when it is evaporated. Adding the energy derived from the combustion of natural gas to the gas boiler, the energy reaches a level beyond maximum. Gas does not burn twice in that type of gas boilers. As a result of revolving the produced water vapor two times around the heat-exchanger, the energy within the water is withdrawn and the water vapor, energy of which is withdrawn and which has transformed into water, is exhausted. Condensing hermetic gas boilers are economically more efficient than regular hermetic gas boilers.