A water circulation system including a refrigerant cycle, comprising: an outdoor unit comprising a compressor (120, 120') for compressing the refrigerant, an outdoor heat exchanger for heat exchange between the refrigerant and outdoor air, and an expansion unit (102, 204, 152) for expanding the refrigerant; a water-heat exchange indoor unit including a first water-refrigerant heat exchanger for heat exchange between the refrigerant and water, the water-heat exchange indoor unit being connected to the compressor and the expansion unit to define the refrigerant circuit; a hot water supply unit for supplying the water passing through the first water-refrigerant heat exchanger; a heating unit for heating the indoor air using the water passing through the first water-refrigerant heat exchanger, characterized in that: the water circulation system further comprises: a second water-refrigerant heat exchanger that is provided in the outdoor unit and connected to a side of the refrigerant cycle to heat the refrigerant using the water flowing therein; a water heating unit that is fluidly connected to the second water-refrigerant heat exchanger to heat the water introduced into the second water-refrigerant heat exchanger; a refrigerant heating pipe connecting the refrigerant circuit to the second water-refrigerant heat exchanger, wherein the refrigerant heating pipe comprises: a first refrigerant pipe for guiding at least a part of the refrigerant flowing between the first water-refrigerant heat exchanger and the expansion unit (102, 204, 152) to the second water-refrigerant heat exchanger; a second refrigerant pipe for guiding at least a part of the refrigerant flowing between the first water-refrigerant heat exchanger and the compressor (120, 120') to the second water-refrigerant heat exchanger; and a third refrigerant pipe for guiding the refrigerant discharged from the second water-refrigerant heat exchanger to the compressor (120, 120'). an outdoor unit comprising a compressor (120, 120') for compressing the refrigerant, an outdoor heat exchanger for heat exchange between the refrigerant and outdoor air, and an expansion unit (102, 204, 152) for expanding the refrigerant; a water-heat exchange indoor unit including a first water-refrigerant heat exchanger for heat exchange between the refrigerant and water, the water-heat exchange indoor unit being connected to the compressor and the expansion unit to define the refrigerant circuit; a hot water supply unit for supplying the water passing through the first water-refrigerant heat exchanger; a heating unit for heating the indoor air using the water passing through the first water-refrigerant heat exchanger, the water circulation system further comprises: a second water-refrigerant heat exchanger that is provided in the outdoor unit and connected to a side of the refrigerant cycle to heat the refrigerant using the water flowing therein; a water heating unit that is fluidly connected to the second water-refrigerant heat exchanger to heat the water introduced into the second water-refrigerant heat exchanger; a refrigerant heating pipe connecting the refrigerant circuit to the second water-refrigerant heat exchanger, a second water-refrigerant heat exchanger that is provided in the outdoor unit and connected to a side of the refrigerant cycle to heat the refrigerant using the water flowing therein; a water heating unit that is fluidly connected to the second water-refrigerant heat exchanger to heat the water introduced into the second water-refrigerant heat exchanger; a refrigerant heating pipe connecting the refrigerant circuit to the second water-refrigerant heat exchanger, wherein the refrigerant heating pipe comprises: a first refrigerant pipe for guiding at least a part of the refrigerant flowing between the first water-refrigerant heat exchanger and the expansion unit (102, 204, 152) to the second water-refrigerant heat exchanger; a second refrigerant pipe for guiding at least a part of the refrigerant flowing between the first water-refrigerant heat exchanger and the compressor (120, 120') to the second water-refrigerant heat exchanger; and a third refrigerant pipe for guiding the refrigerant discharged from the second water-refrigerant heat exchanger to the compressor (120, 120'). a first refrigerant pipe for guiding at least a part of the refrigerant flowing between the first water-refrigerant heat exchanger and the expansion unit (102, 204, 152) to the second water-refrigerant heat exchanger; a second refrigerant pipe for guiding at least a part of the refrigerant flowing between the first water-refrigerant heat exchanger and the compressor (120, 120') to the second water-refrigerant heat exchanger; and a third refrigerant pipe for guiding the refrigerant discharged from the second water-refrigerant heat exchanger to the compressor (120, 120').