Abstract: The control system maintains air temperature comfort in a zone of a commercial structure. Outside air is supplied into the zone for ventilation purposes and for cooling the zone to the desired temperature whenever possible to minimize the use of mechanical air cooling apparatus and thereby reducing energy consumption. Heating and cooling coils temperature condition the supplied air. A pneumatic control system selectively activates the heated, cooled or ventilated air supplied in the zone. A two-speed supply fan, controlled by the pneumatic control system, operates in a first state at only low speed to reduce energy consumption, or in a second state at automatic thermostat two-speed control (high or low). The fan in this second state operates at low speed except when maximum cooling is required and then switches back to low speed operation from high speed when the ambient temperature in the zone approaches the desired temperature.