Communal Flue Systems provide the opportunity of connecting more than one boiler to the same flue system, e.g heating systems with wall hung boilers in apartment buildings.

If the chimney is close to the facade then the best system is a Communal Flue System connected to each boiler and a separate individual air intake ducting. -

In such cases legislation allows a maximum of 5 boilers connected to the same collective chimney with a maximum of 2 boilers per floor.

This is considered an unbalanced flue system, because the air intake and the gas exhaust are not in the same height, which means that there could be a pressure difference between both positions.

A CFS for just exhaust can be designed for over pressure or negative pressure. For over pressure the diameter of the chimney is smaller, but there must be safety system (non-return valves) to prevent the exhaust of one boiler entering to another boiler at a different level.

Those chimneys must be Twin wall isolated as they are outdoor (SEE) if they are installed outside the building and single walled (SVC) if they are exclusively for small courtyard without sharing with other facilities.