Since the working principle of the heat pump device is the same as that of compression refrigeration, in order to give full play to its efficiency in a small air conditioner, the same set of equipment is used for air conditioning cooling in summer or heating in winter. When heating in winter, the evaporator and condenser in the air conditioner are switched through a reversing valve, as shown in the "Heat Pump System Schematic" on the left. It can be seen from the figure that when the air conditioner is cooling in the summer, it operates according to the refrigeration working condition. The high-pressure steam discharged from the compressor enters the condenser through the reversing valve (also known as the four-way valve), and the refrigerant steam is condensed into liquid. It enters the evaporator through the throttling device and absorbs heat in the evaporator to cool the indoor air. The evaporated refrigerant vapor is sucked into the compressor after passing through the reversing valve. This repeats itself to realize the refrigeration cycle. When heating in winter, first turn the reversing valve to the heat pump working position, so the high-pressure refrigerant vapor discharged from the compressor flows into the indoor evaporator (used as a condenser) through the reversing valve. The latent heat released when the refrigerant vapor condenses, Heating the indoor air to achieve the purpose of indoor heating. The condensed liquid refrigerant flows through the throttling device in the reverse direction and enters the condenser (used as an evaporator). It absorbs external heat and evaporates. The evaporated steam is compressed after passing through the reversing valve. The machine inhales and completes the heating cycle. In this way, the heat in the outside air (or circulating water) is "pumped" into the room with a higher temperature, so it is called a "heat pump". For a split heat pump air conditioner, during summer cooling, the outdoor unit is used as the condenser and the indoor unit is used as the evaporator. During operation, the indoor heat is transported to the outdoors. In winter, the indoor unit is used as the condenser and the outdoor unit is used as the evaporator, so that the outdoor heat is transported indoors. This is usually achieved through a four-way reversing valve. There is a four-way reversing valve in the heat pump air conditioner. Under cooling conditions, the indoor heat exchanger is the evaporator, and the outdoor heat exchanger (the thing that breathes out hot air outside in summer) is the condenser. During heating in winter, the four-way reversing valve switches to change the flow direction of the refrigerant. At this time, the indoor heat exchanger is the condenser, and the outdoor heat exchanger (the thing that breathes out cold air in winter) is the evaporator. Since the cold wind blows out in winter, the heat exchanger will frost. Therefore, when the frost reaches a certain level, the four-way reversing valve switches again, and the air conditioner changes to summer cooling mode. The outdoor heat exchanger receives heat and defrosts. After completion, the four-way valve switches to the heating state. During defrost, in order to prevent cold air from blowing into the room, the fan of the indoor unit stops running. (Of course, this reverse defrost has a certain impact on comfort, so there are methods such as hot gas bypass defrost and thermal storage defrost that do not require switching working conditions)