Abstract: Generally there is provided for placement in the supply and return air ducts of a building ventilation system, a unique flow control unit having an orifice, static pressure sensors on opposite sides of the orifice, and sensor controlled dampers to regulate flow through the unit. In a further aspect, a remote static pressure sensor may be located at a strategic position in the ducts and arranged to control the dampers in the flow control units to alter the flow through the unit in response to system demand. In another aspect there is described a terminal box of similar design for controlling flow into and out of a room. This box provides center dampers on both supply and return ducts and pressure sensors on both sides of the center dampers. Under control of the room thermostat, both center dampers are adjusted under common control. The pressure sensors on either side of the center dampers are connected to control end dampers upstream and downstream of the unit in reponse to a chosen pressure differential.