An air-conditioning apparatus includes a refrigerant circuit in which pipes sequentially connect a compressor, a flow switching device, a heat source side heat exchanger, an expansion device, a load side heat exchanger, and the flow switching device, and configured to perform a cooling operation and a heating operation switched by the flow switching device, an oil separator configured to separate refrigerating machine oil from refrigerant discharged from the compressor, a first bypass passage in which fluid flowing out of the oil separator flows, an auxiliary heat exchanger configured to cool the fluid, a first flow control device configured to control passing of the fluid, a second bypass passage in which liquid refrigerant or two-phase gas-liquid refrigerant flowing through one of the pipes connecting the heat source side heat exchanger and the expansion device flows, and a second flow control device configured to control passing of refrigerant.

Disclosed are embodiments of heat pump systems and methods for operating such systems in the heating mode while a heat pump operating parameter reaches or is equal to a predetermined operating value at which it is not advisable to use conventional heat pump systems for heating an enclosed structure. In addition to ancillary components, the heat pump systems includes indoor, outdoor and auxiliary heat exchangers, reversing and flow diverting valves, compressors, refrigerant superheat controllers, and auxiliary energy sources. During the heating mode of operation, while the heat pump operating parameter reaches or is equal to the predetermined operating value, the flow of liquid refrigerant through the outdoor heat exchangers is inhibited and is diverted to flow through the auxiliary heat exchangers. Thermal energy from the auxiliary energy source is used for evaporating the liquid refrigerant flowing through the auxiliary heat exchangers.

An air conditioning device includes a gas furnace unit, a heat-pump heat source unit, and one or more utilization units. The gas furnace unit includes an air blower and a first heat exchanger. The utilization unit includes a second heat exchanger. The air conditioning device has a first operation mode in which a heating operation is performed using the gas furnace unit, a second operation mode in which a heating operation is performed using the heat-pump heat source unit, and a third operation mode in which a cooling operation is performed using the heat-pump heat source unit, and performs refrigerant discharge control for temporarily moving a refrigerant on the heat-pump heat source unit side to the second heat exchanger side of the utilization unit at the time of starting of a compressor provided in the heat-pump heat source unit when the first operation mode is switched to the second operation mode.