These types of boilers are characterized by a burner that takes the air directly from the boiler room itself and pressurizes the combustion chamber to overcome the boiler load losses, although the operation of extracting fumes in most of the cases is by natural draught. In the event that the operation of the installation is a forced draught, a silicone gasket must be included in the joints of the chimney to ensure its correct operation in excess pressure.

The burners can be gas or diesel, which does not affect the characteristics of the chimney, except in the case of gasoil with high sulphur content that requires the use of stainless steel AISI 316L in its interior wall. It is often confused that these boilers generate superheated steam instead of hot water with condensate in the exhaust fumes.

Regarding the operating principle of the smoke circuit, these boilers do not differ from conventional boilers, although the exit temperatures of the fumes are usually somewhat higher in steam boilers than in hot water boilers, since the temperature of the boiler steam generated is greater than that of the hot water. So, in these steam boilers there is no special specification regarding the use of AISI 316L steel due to the fact that they are steam boilers.

Suitable systems for this application are: