Abstract: A solid state control circuit operative for connection to existing low-voltage thermostat terminals of a central, forced-air, air-conditioning system having a compressor and an indoor blower (referred to herein as a "fan") and gas-fired or electrical heating elements. Automatic means is provided for starting the blower simultaneously when the compressor or electric heating elements are turned on, but the blower continues to run by a timer for a predetermined timed period after the compressor is stopped, during cooling and heat pump heating, and is stopped simultaneously when the heating elements are turned off during electric heating. The control circuit is automatically inoperative with gas heat, since the blower in gas heating systems is controlled by a separate plenum thermostat which introduces start and stop delays independent of the room thermostat.