PURPOSE: To enable a refrigerant pressure for a water side heat exchanger to be kept at a stable condition irrespective of a temperature of water by a method wherein a flow rate control valve is installed at a water circuit of the water side heat exchanger, and a water amount communicating with the water side heat exchanger is controlled in response to the output of the pressure sensor.

CONSTITUTION: When a cooling operation is carried out, refrigerant gas of high pressure compressed by a compressor 1 is flowed to a water side heat exchanger 3, resulting in that the gas is cooled by water fed into the water side heat exchanger 3 through a water circuit 7 and liquified. In turn, the water is heated up to hot water, and supplied for heating. When a temperature of the hot water is low, a high pressure in an output side refrigerant passage of the compressor 1 is lowered and this lowered pressure is detected by a pressure sensor 9 and the detected signal is converted into an electrical signal. A controller 10 receives this electrical signal and controls a changing-over of a flow rate control valve 8, decreases an amount of hot water flowing in the water side heat exchanger 3 and keeps a refrigerant pressure within the water side heat exchanger 3. In turn, when a temperature of the hot water in the water side heat exchanger 3 is increased, the pressure sensor 9 detects an increased pressure of refrigerant and then the controller 10 controls the flow rate control valve 8 in its operation in response to the detected output, increases the amount of hot water flowing in the water side heat exchanger 3 and further keeps a refrigerant pressure of the water side heat exchanger 3 constant.