A refrigeration cycle apparatus comprising a refrigerant circuit in which a compressor , a refrigerant flow switching device , an air-side heat exchanger , an expansion valve , a water-side heat exchanger , and an accumulator are connected by refrigerant pipes and refrigerant is circulated, wherein the refrigerant pipes include a high-pressure pipe that connects a discharge side of the compressor with the water-side heat exchanger in a heating operation and a low-pressure pipe that connects the air-side heat exchanger with a suction side of the compressor in the heating operation, characterized in that G/A is less than or equal to a predetermined threshold X , where G is a difference in volume between the air-side heat exchanger and the water-side heat exchanger , and A is a volume of the refrigerant circuit, and where: (i) an outer diameter of the high-pressure pipe is 25.4 mm, and an outer diameter of the low-pressure pipe is D mm, the threshold X is expressed by the following equation: x = 0.0000001479D 3 - 0.0000245654D 2 + 0.0000786935D + 0.2018219300; or (ii) an outer diameter of the high-pressure pipe is 28.6 mm, and an outer diameter of the low-pressure pipe is D mm, the threshold X is expressed by the following equation: x = 0.0000001432D3 - 0.0000239347D2 + 0.0000774572D + 0.1991706771; or (iii) an outer diameter of the high-pressure pipe is 31.75 mm, and an outer diameter of the low-pressure pipe is D mm, the threshold X is expressed by the following equation: x = 0.0000001376D3 - 0.0000231712D2 + 0.0000758817D + 0.1959228337. wherein the refrigerant pipes include a high-pressure pipe that connects a discharge side of the compressor with the water-side heat exchanger in a heating operation and a low-pressure pipe that connects the air-side heat exchanger with a suction side of the compressor in the heating operation, characterized in that G/A is less than or equal to a predetermined threshold X , where G is a difference in volume between the air-side heat exchanger and the water-side heat exchanger , and A is a volume of the refrigerant circuit, and where: (i) an outer diameter of the high-pressure pipe is 25.4 mm, and an outer diameter of the low-pressure pipe is D mm, the threshold X is expressed by the following equation: x = 0.0000001479D 3 - 0.0000245654D 2 + 0.0000786935D + 0.2018219300; or (ii) an outer diameter of the high-pressure pipe is 28.6 mm, and an outer diameter of the low-pressure pipe is D mm, the threshold X is expressed by the following equation: x = 0.0000001432D3 - 0.0000239347D2 + 0.0000774572D + 0.1991706771; or (iii) an outer diameter of the high-pressure pipe is 31.75 mm, and an outer diameter of the low-pressure pipe is D mm, the threshold X is expressed by the following equation: x = 0.0000001376D3 - 0.0000231712D2 + 0.0000758817D + 0.1959228337. (i) an outer diameter of the high-pressure pipe is 25.4 mm, and an outer diameter of the low-pressure pipe is D mm, the threshold X is expressed by the following equation: x = 0.0000001479D 3 - 0.0000245654D 2 + 0.0000786935D + 0.2018219300; or (ii) an outer diameter of the high-pressure pipe is 28.6 mm, and an outer diameter of the low-pressure pipe is D mm, the threshold X is expressed by the following equation: x = 0.0000001432D3 - 0.0000239347D2 + 0.0000774572D + 0.1991706771; or (iii) an outer diameter of the high-pressure pipe is 31.75 mm, and an outer diameter of the low-pressure pipe is D mm, the threshold X is expressed by the following equation: x = 0.0000001376D3 - 0.0000231712D2 + 0.0000758817D + 0.1959228337. <EOS>