Zoned control system

In a thermal system having a central plant feeding a plurality of adjustable zone regulators for bringing their respective zones to corresponding target temperatures, a controller comprising:

• a plurality of temperature sensors distributed one to a zone, each of said sensors being operable to produce a zone signal signifying zone temperature;
• a plurality of zone actuators, each having a zone control terminal and each, in response to a signal at its zone control terminal, being operable to adjust a corresponding one of said zone regulators;
• and
• a control means coupled to each of the temperature sensors and to the zone control terminal of each of said zone actuators for calculating the zone temperature errors with respect to their respective target temperatures and for starting said central plant response to a predetermined function of the cumulative zone temperature errors exceeding a given limit, each of said zone temperature errors being at least producible at above said given limit and at a plurality of non-zero values smaller in magnitude than that of said given limit so that the zone temperature errors may be insufficient individually but sufficient cumulatively to start said central plant, said control means being operable to apply corresponding signals to each zone control terminal with a timing determined by its corresponding temperature error.