An air conditioning system that includes at least one indoor unit that uses water as a working fluid, an outdoor unit that uses a refrigerant as a working fluid, the outdoor unit including a compressor compressing the refrigerant and an outdoor heat exchanger for heat-exchange with the refrigerant, and a heat collection unit connecting the at least one indoor unit to the outdoor unit, the heat collection unit including at least one heat exchange part for heat-exchanging water supplied from the at least one indoor unit with the refrigerant supplied from the outdoor unit.