Abstract: An economizer system for air conditioning uses a space thermostat to determine a demand for cooling and an outside air controller to determine whether cooling shall be mechanical or natural. A mixed air controller senses the temperature of the mixture of outside air and recirculated air and adjusts the relative proportions as desired by means of a controlled bidirectional damper, moving at a rate far less than the response time of the mixed air controller. This slow bidirectional damper movement in response to the mixed air control sensors causes the dampers to modulate in an inexpensive and reliable manner.