A hot water system comprising: a compressor; a four-way valve for switching the direction of the refrigerating cycle during defrosting operation; a water heat exchanger for exchanging heat between water and a refrigerant; a first expansion valve for controlling the flow rate of the refrigerant to reduce the pressure; and an air heat exchanger for exchanging heat between air and the refrigerant, which are connected in that order by a pipeline to form a refrigerating cycle for circulating the refrigerant, thereby heating water; the hot water system further comprising: a bypath for redirecting the refrigerant from a portion of the pipeline in upstream of the first expansion valve to the inlet of the compressor during defrosting operation; characterized by a solenoid valve disposed in the bypath, for switching; a receiver disposed between the first expansion valve and the air heat exchanger; and a second expansion valve disposed between the receiver and the air heat exchanger, wherein the bypath is connected to a portion of the pipeline between the first expansion valve which is downstream of the receiver during defrosting operation and the receiver, wherein, there is a difference in internal volume between the water heat exchanger 3 and the air heat exchanger 5, and wherein when the solenoid valve is opened to redirect part of the refrigerant passing through the water heat exchanger acting as an evaporator into the bypath during defrosting operation, the amount of the refrigerant passing through the bypath is controlled by changing the degree of opening of the first expansion valve. a compressor; a four-way valve for switching the direction of the refrigerating cycle during defrosting operation; a water heat exchanger for exchanging heat between water and a refrigerant; a first expansion valve for controlling the flow rate of the refrigerant to reduce the pressure; and an air heat exchanger for exchanging heat between air and the refrigerant, which are connected in that order by a pipeline to form a refrigerating cycle for circulating the refrigerant, thereby heating water; the hot water system further comprising: a bypath for redirecting the refrigerant from a portion of the pipeline in upstream of the first expansion valve to the inlet of the compressor during defrosting operation; characterized by a solenoid valve disposed in the bypath, for switching; a receiver disposed between the first expansion valve and the air heat exchanger; and a second expansion valve disposed between the receiver and the air heat exchanger, wherein the bypath is connected to a portion of the pipeline between the first expansion valve which is downstream of the receiver during defrosting operation and the receiver, wherein, there is a difference in internal volume between the water heat exchanger 3 and the air heat exchanger 5, and wherein when the solenoid valve is opened to redirect part of the refrigerant passing through the water heat exchanger acting as an evaporator into the bypath during defrosting operation, the amount of the refrigerant passing through the bypath is controlled by changing the degree of opening of the first expansion valve. a bypath for redirecting the refrigerant from a portion of the pipeline in upstream of the first expansion valve to the inlet of the compressor during defrosting operation; characterized by a solenoid valve disposed in the bypath, for switching; a receiver disposed between the first expansion valve and the air heat exchanger; and a second expansion valve disposed between the receiver and the air heat exchanger, wherein the bypath is connected to a portion of the pipeline between the first expansion valve which is downstream of the receiver during defrosting operation and the receiver, wherein, there is a difference in internal volume between the water heat exchanger 3 and the air heat exchanger 5, and wherein when the solenoid valve is opened to redirect part of the refrigerant passing through the water heat exchanger acting as an evaporator into the bypath during defrosting operation, the amount of the refrigerant passing through the bypath is controlled by changing the degree of opening of the first expansion valve.