An air conditioner and a method of operating an air conditioner are provided. The air conditioner may include a heat pump having a water-refrigerant heat exchanger that condenses or evaporates a refrigerant by heat exchange with heat source water; a heat source water flow path connected to the water-refrigerant heat exchanger; a pump installed on the heat source water flow path; a variable flow valve installed on the heat source water flow path; and a variable flow valve controller that controls an opening degree of the variable flow valve. The variable flow valve controller may include a heat source water minimum flow manipulator that manipulates a minimum flow rate of the heat source water and regulates the opening degree of the variable flow valve according to the manipulation of the heat source water minimum flow manipulator. Accordingly, a user or installation personnel may selectively regulate power consumption and efficiency as desired.