Abstract: An air conditioning system is provided with indoor and outdoor air. Since the moisture content of the outdoor air is the major factor in determining the resulting humidity of the indoor space to which conditioned air is supplied, outdoor air is cooled to the greatest extent consistent with the total air cooling needed, before cooling is applied to the indoor air. Cooling is provided by a heat exchanger, through which a cooling medium, such as chilled water or a refrigerant, is circulated. Indoor air not cooled by passing through the heat exchanger is directed, by means of a face and bypass damper, through a bypass duct extending around the heat exchanger from an input mixing plenum at the input to the heat exchanger to an output mixing plenum at the output of the heat exchanger. In a first version, a second face and bypass damper is provided to direct air from the input mixing plenum to pass through the heat exchanger or through the bypass duct.