Abstract: In this thermal system there is a central plant feeding a plurality of adjustable zone regulators which bring their respective zones to corresponding target temperatures. The system has a controller including a plurality of temperature sensors. The sensors are distributed one to a zone. Each of the sensors can produce a zone signal signifying zone temperature. The controller also has a plurality of zone actuators, each having a zone control terminal. Each of the actuators can, in response to a signal at its zone control terminal, adjust a corresponding one of the zone regulators. Also included is a control subsystem that is coupled to each of the temperature sensors and to the zone control terminal of each of the zone actuators. The control subsystem can start the central plant in response to a predetermined function of the zone temperature errors with respect to their respective target temperatures exceeding a given limit.