Abstract: This invention is directed to a variable air volume cooling system for a building which utilized changes in the diameter of the belt going around a two piece driver pulley and a follower two piece driven pulley to effect changes in fan speed and, accordingly, the air volume supplied. A control means to operate a servo-motor connected to the driver pulley provides for separating the two piece driven pulley to slow the fan, driving the two pieces of the driven pulley together to speed up the fan, and for maintaining the same speed should a desired temperature range be obtained. The control compares the temperature of the area to be cooled with the speed of the fan to produce its outputs. Deadband operation is provided to eliminate toggling and other excesses of output reversals.