HVAC Duct Work delivers the conditioned air to the spaces. The Air Handling Duct channels this conditioned air to spaces where it is needed and then a return duct channels the air back to the air handling unit duct for re-conditioning. The return ductwork has a negative pressure while the supply ductwork has a positive pressure. In commercial systems and tight residential homes it is necessary to add fresh air to this conditioned air so the air remains fresh and healthy for the occupants of the structure. This is required by the mechanical code and often this air comes through ductwork from the outside.
Depending on the type of building and type of occupancy will depend on what the mechanical code says about the quantity or amount of fresh air ventilation required. Natural ventilation is applicable to this where there is no fan to force the air into the space or building where it is required to have fresh air introduced into the spaces which people will be occupying. It is important to note that when fresh is forced into the building with a fan that another opening or duct be used for relief air ventilation to avoid over pressurizing the building.