Enthalpy is the total amount of heat energy contained within one pound of air.

When a coil cools the air, it is cooling both the air and the water vapor contained in the air as humidity. The higher the humidity, the more water vapor is in the air. Since humid air takes more energy to cool than dry air, it would make sense to monitor both the temperature and the humidity of the outside air when we consider the economizer control.

Conventional economizer control uses only the temperatures of the outside air and return air to determine when to minimize the damper position. The total energy content of the return air may or may not be higher than the outside air.