Economizers are designed to provide free air conditioning when outside conditions are appropriate. When the outside air is cool and dry enough, the economizer automatically opens to circulate this cool aire, thereby eliminating the need to run the air conditioning compressor. This enables the building owner to lower his electrical costs and increase the life of the equipment. If the outside air becomes too warm or humid, the enthalpy control automatically closes the fresh air damper and the compressor begins operation. The economizer is fully modulating and therefore will automatically mix the fresh air and the return air proportionally to get maximum energy savings.