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 operational characteristics of the installation is a forced draft (pressure jet), a silicone gasket must be included in the joints of the chimney to ensure its correct operation in excess pressure mode. 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 inner wall.

The chimneys should always be double-wall both inside the building and outside.

Suitable systems for this application are: