Abstract: A self-contained air conditioner cools a building's air using a water-cooled economizer coil and two refrigerant evaporators. The two evaporators are provided by two separate refrigeration circuits, each having its own water-cooled condenser. A water circuit interconnecting the economizer and the two condensers is reconfigureable to selectively operate the air conditioner in a normal mode or, if conditions permit, in an energy-saving economizer mode. In the normal mode, the water circuit conveys cooling water in series through both condensers while bypassing the economizer coil. In the economizer mode, the water flows in series through the economizer and one of the condensers while bypassing the other. The refrigerant circuit whose condenser is bypassed can thus be deactivated to save energy. The economizer coil and the condenser it bypasses have generally the same flow resistance to water flow.