Abstract: An air exchange system is disclosed for use in a building having a plurality of stories each having a base floor, finished floor, finished ceiling and rough ceiling. The system comprises a plurality of main distribution ducts each adapted to be located in the space defined between the finished ceiling of a lower story and the finished floor of an upper story lying immediately above the lower story. The main ducts are located on alternate floors and adapted to supply sufficient air for the two adjacent floors. Each of the main ducts are provided with at least one upper duct which communicates with an upper story through the base floor and at least one lower duct which communicates with a lower story through the rough ceiling thereof. A variable air volume control unit is connected to each upper and lower duct controlling the volume of conditioned air entering a story of the building in order to control the temperature within the user space therein.