A method of designing an air-conditioning apparatus, the air-conditioning apparatus including a refrigerant circuit in which a compressor, a heat-source-side heat exchanger, an expansion device, and a refrigerant flow path in at least one intermediate heat exchanger configured to cause heat exchange between a refrigerant and a heat medium are connected by a refrigerant pipe, the refrigerant circuit being configured to cause the refrigerant to circulate therethrough, and a heat medium circuit in which a pump, a use-side heat exchanger, and a heat-medium flow path in the at least one intermediate heat exchanger are connected by a heat-medium conveying pipe, the heat medium circuit being configured to cause the heat medium to circulate therethrough, wherein the at least one intermediate heat exchanger comprises a plurality of intermediate heat exchangers, the plurality of intermediate heat exchangers are operative in a heating only operation mode in which all of the intermediate heat exchangers serve as condensers, a cooling only operation mode in which all of the intermediate heat exchangers serve as evaporators, and a cooling and heating mixed operation mode in which some of the intermediate heat exchangers serve as condensers and others of the intermediate heat exchangers serve as evaporators, and characterized in that the method comprises the steps of: acquiring a capacity of the use-side heat exchanger in cooling operation; acquiring a capacity of the use-side heat exchanger in heating operation; and setting an inside diameter of the heat-medium conveying pipe connected to the use-side heat exchanger to a value within both a first selection range of inside diameters which varies with the capacity of the use-side heat exchanger connected to the heat-medium conveying pipe in cooling operation and a second selection range of inside diameters which varies with the capacity of the use-side heat exchanger connected to the heat-medium conveying pipe in heating operation, where first and second selection range are different from each other, and wherein each of the selection ranges of inside diameters has a lower limit above which a flow speed of the heat medium flowing in the heat-medium conveying pipe is a flow speed at which an oxide film is not separated, and each of the selection ranges of inside diameters has an upper limit below which the flow speed of the heat medium flowing in the heat-medium conveying pipe is a flow speed at which corrosive products do not accumulate. acquiring a capacity of the use-side heat exchanger in cooling operation; acquiring a capacity of the use-side heat exchanger in heating operation; and setting an inside diameter of the heat-medium conveying pipe connected to the use-side heat exchanger to a value within both a first selection range of inside diameters which varies with the capacity of the use-side heat exchanger connected to the heat-medium conveying pipe in cooling operation and a second selection range of inside diameters which varies with the capacity of the use-side heat exchanger connected to the heat-medium conveying pipe in heating operation, where first and second selection range are different from each other, and wherein each of the selection ranges of inside diameters has a lower limit above which a flow speed of the heat medium flowing in the heat-medium conveying pipe is a flow speed at which an oxide film is not separated, and each of the selection ranges of inside diameters has an upper limit below which the flow speed of the heat medium flowing in the heat-medium conveying pipe is a flow speed at which corrosive products do not accumulate.