Abstract: A control system for use in central heating and air-conditioning installations to control the volumetric flow rate of conditioned air through a supply duct to an enclosed space to be serviced. The system is adapted to maintain a desired ambiant temperature in the enclosed space by regulating the circulation of conditioned air in accordance with the demand generated by prevailing conditions. An adjustable restrictor valve defining an orifice is located in the supply duct and sensors are provided to continuously measure the orifice diameter and the pressure drop across the orifice. Electronic circuitry is provided to generate a flow rate voltage signal representative of the product of the orifice diameter and the pressure drop across the orifice and to compare the flow rate voltage level with a fixed or temperature controlled reference voltage to obtain either an "increase" or "decrease" control voltage signal.