An air circulation and conditioning system for a building in which the supply and return ducts are coincident and vent to a room so as to form air circulation in the room including an upward return air current surrounded by a downward supply current. An array of vents may be provided, each with a return duct opening and supply duct opening to create multiple air currents in the room. A wireway may be provided for co-locating electronics and communications network along the ductwork. A modular design air handling unit with multiple compatible modules providing different air treatment, purification, and monitoring functions may be provided. A method for retrofitting an existing single duct system into a dual duct system is provided.
