A heat source apparatus comprising: a compressor configured to compress refrigerant; an air-cooled heat exchanger configured to exchange heat between air and the refrigerant; a load-side heat exchanger configured to exchange heat between a heat medium flowing through a load, and the refrigerant;a main expansion valve connected between the air-cooled heat exchanger and the load-side heat exchanger; a power failure detection unit configured to detect stoppage of power supply from a power supply source; a valve control unit configured to control to change the main expansion valve to a state of being fully closed when the stoppage of power supply is detected by the power failure detection unit; and a refrigerant flow control circuit connected in parallel to the main expansion valve, and configured to regulate a flow rate of the refrigerant by storing the refrigerant or causing outflow of the refrigerant, the refrigerant flow control circuit comprising a refrigerant tank configured to store the refrigerant; a first sub-expansion valve, located between the refrigerant tank and the air-cooled heat exchanger, configured to regulate the flow rate of the refrigerant and to be in a state of being fully closed during a cooling operation in which the load-side heat exchanger serves as an evaporator, and a second sub-expansion valve, located between the refrigerant tank and the load-side heat exchanger, configured to regulate the flow rate of the refrigerant and to be in a state of being open during the cooling operation which are connected in series with one another, during the cooling operation, the valve control unit being configured to control to maintain the state of being fully closed of the first sub-expansion valve and the state of being open of the second sub-expansion valve when the stoppage of power supply is detected by the power failure detection unit. a compressor configured to compress refrigerant; an air-cooled heat exchanger configured to exchange heat between air and the refrigerant; a load-side heat exchanger configured to exchange heat between a heat medium flowing through a load, and the refrigerant;a main expansion valve connected between the air-cooled heat exchanger and the load-side heat exchanger; a power failure detection unit configured to detect stoppage of power supply from a power supply source; a valve control unit configured to control to change the main expansion valve to a state of being fully closed when the stoppage of power supply is detected by the power failure detection unit; and a refrigerant flow control circuit connected in parallel to the main expansion valve, and configured to regulate a flow rate of the refrigerant by storing the refrigerant or causing outflow of the refrigerant, the refrigerant flow control circuit comprising a refrigerant tank configured to store the refrigerant; a first sub-expansion valve, located between the refrigerant tank and the air-cooled heat exchanger, configured to regulate the flow rate of the refrigerant and to be in a state of being fully closed during a cooling operation in which the load-side heat exchanger serves as an evaporator, and a second sub-expansion valve, located between the refrigerant tank and the load-side heat exchanger, configured to regulate the flow rate of the refrigerant and to be in a state of being open during the cooling operation which are connected in series with one another, a refrigerant tank configured to store the refrigerant; a first sub-expansion valve, located between the refrigerant tank and the air-cooled heat exchanger, configured to regulate the flow rate of the refrigerant and to be in a state of being fully closed during a cooling operation in which the load-side heat exchanger serves as an evaporator, and a second sub-expansion valve, located between the refrigerant tank and the load-side heat exchanger, configured to regulate the flow rate of the refrigerant and to be in a state of being open during the cooling operation which are connected in series with one another, during the cooling operation, the valve control unit being configured to control to maintain the state of being fully closed of the first sub-expansion valve and the state of being open of the second sub-expansion valve when the stoppage of power supply is detected by the power failure detection unit.