There is disclosed an air conditioning control system for a building which has zone temperature sensing means for generating a plurality of DC analog signals proportional to the deviation of the air temperature in a controlled zone from first and second set point temperatures. The signal generated by the deviation of the controlled zone air temperature below the lowermost set point temperature is applied to the controller of the air heating facility of the air condition system while the signal generated by deviation of the controlled zone air temperature above the highest set point temperature is applied to the control means for the air cooling facility of the air conditioning system. Ventilation damper means are provided in the air circulation facilities to control the proportion of return and fresh air supplied to the air conditioning system and the signal generated by the zone temperature sensing means when the controlled zone air temperature deviates to above the lowermost set point temperature is applied to the controller on the ventilation damper means whereby the proportion of fresh and return air is controlled in response thereto. In a typical embodiment the control system is employed with an air conditioning system including a plurality of air heating stages housed in a heating deck and a plurality of air cooling stages housed in a cooling deck. The air conditioning system is commonly employed in a building having a plurality of zones of controlled air temperature with air circulation facilities which include the necessary ducts to recycle zone air through the air conditioning system and blend therewith a controlled proportion of fresh or outside air. Individual zone damper means are placed in each of the air supply ducts from the heating and cooling decks to supply air from each of these decks in proportion to the demands of the individual zones. The zone air temperature sensing means, which is associated with each of the controlled zones, comprises a thermostatic device having two set point temperatures, typically a heating and ventilating set point temperature of about 68.degree. and a cooling set point temperature of about 78.degree. F.
The controls for the air conditioning system employed for the multiple zones are operative to select the control signal of the zone having the greatest heating demand and to apply this signal to the heating facility of the system. When a plurality of heating stages are employed the signal of maximum demand is applied to the stages in a progressive fashion to provide a staged or successive activation of the stages in response to a falling temperature and a phased deactivation of the stages in response to a rising temperature as the temperature approaches the lower set point temperature. The controls of the system are also operative to open the ventilation air damper of the air circulation facilities whereby outside air can be introduced into the cooling deck when the outside air is at a temperature close to or below the maximum set point temperature. There is a band of uncontrolled air temperature between the lower set point and upper set point temperatures through which the controlled zones are permitted to drift without activation of the heating or cooling facilities. Should the temperature of the warmest controlled zone, however, exceed the maximum set point temperature by a preset differential, the air cooling facility of the air conditioning system is activated. When a plurality of stages of air cooling are provided, these are activated in a progressive or staged fashion in response to a rising temperature of the warmest controlled zone and are successively deactivated as the control zone temperature approaches the maximum set point temperature.