Collective chimneys provides the opportunity of connecting more than one boiler to the same flue system, e.g heating systems connected to boilers in apartment buildings.
If the chimney is close to the facade then the best system is a collective chimney together with an individual duct for each boiler and another one which takes the air from the facade.
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 system, because the air intake and the gas removal are not in the same position, which means that there could be a pressure difference between both position.
The system 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 smoke in the chimney to enter boilers.
For over pressure the diameter will be smaller, but a security system to prevent the entering of gas from the chimney into the boilers which are not operative should be included.
Those chimneys must be double wall system (SEE) if they are installed outside the building and single walled (SVC) if they are exclusively for small courtyard without sharing with other facilities.
Collective, modular, double wall and insulated chimneys for appartment buildings
Collective moluar single wall chimney for chimney relining in appartment buildings