Air conditioner and operation method of the same

An air conditioner includes: an indoor unit (1) having an indoor heat exchanger (30) to evaporate or condense a refrigerant by heat exchange with indoor air; an outdoor unit (0) having a variable capacity compression part (2) for sucking and compressing a refrigerant of a refrigerant intake passage (4) and discharging the refrigerant to a refrigerant discharge passage (5), a water-refrigerant heat exchanger (20) for condensing or evaporating a refrigerant by heat exchange with heat source water, and an outdoor expansion device (40)installed between the indoor heat exchanger (30) and the water-refrigerant heat exchanger (20); a heat source water flow path (80) connected to the water-refrigerant heat exchanger; a pump (90) installed on the heat source water flow path; and a variable flow valve (100) installed on the heat source water flow path and capable of regulating the opening degree. The opening degree of the variable flow valve is varied depending on the pressure of the refrigerant discharge passage during a cooling operation, and the opening degree of the variable flow valve is varied depending on the pressure of the refrigerant intake passage during a heating operation. Thus, the air conditioner can be operated while maximizing the cycle of the refrigerant, and the reliability of the outdoor unit can be improved.