An air conditioner and a method of operating an air conditioner are provided. The air conditioner may include at least one indoor device having an indoor heat exchanger that evaporates or condenses a refrigerant by heat exchange with indoor air; at least one outdoor device having a variable capacity compression device that sucks in and compresses a refrigerant of a refrigerant intake passage and discharges the refrigerant to a refrigerant discharge passage, a water-refrigerant heat exchanger that condenses or evaporates a refrigerant by heat exchange with heat source water, and an outdoor expansion device installed between the indoor heat exchanger of the at least one indoor device and the water-refrigerant heat exchanger; a heat source water flow path connected to the water-refrigerant heat exchanger; a pump installed on the heat source water flow path; and a variable flow valve installed on the heat source water flow path. An opening degree of the variable flow valve is varied depending on a pressure of at least one of the refrigerant discharge passage or the refrigerant intake passage.