Abstract: A control system for space heating and cooling units which includes modulated damper means for controlling entry of outside air. Temperature sensitive resistances sense the outside air temperature, the temperature in the space, and the temperature of return air. When cooling is required, the damper can be opened. When the return air temperature is greater than the outside air temperature, the outside air damper is modulated in opening direction when the space temperature rises above a minimum comfortable space air temperature so that the outside air is used for cooling. When the space temperature falls to a temperature below the minimum comfortable space air temperature, the outside air damper can be fully closed.