Abstract: An environmental control system for use in a greenhouse or another structure which requires the control of an ambient condition regulating element in response to a plurality of sensed parameters. The system includes a plurality of sensor elements and actuator elements, which communicate with a central control through communication interface units. In an illustrated embodiment the interface units communicate over previously installed AC power lines by frequency shift keyed signals. The central control unit maintains a protocol where, in response to operator inputs, addreses are assigned to different peripheral control elements to configure the system. The system is configured by assigning sets of equipment to separate zones of the structure which are individually controlled. All peripheral control elements are addressed in one zone before sequencing to control the next zone.