Air conditioning appliances defrost

An air conditioner unit defining a mutually-perpendicular vertical direction, lateral direction, and transverse direction, the air conditioner unit comprising:

• a housing defining an outdoor portion in fluid communication with an outdoor space and an indoor portion in fluid communication with an indoor space;
• an outdoor heat exchanger assembly disposed in the outdoor portion, the outdoor heat exchanger assembly comprising an outdoor heat exchanger and an outdoor fan, the outdoor heat exchanger comprising a first portion and a second portion;
• an indoor heat exchanger assembly disposed in the indoor portion, the indoor heat exchanger assembly comprising an indoor heat exchanger and an indoor fan;
• a compressor in fluid communication with the outdoor heat exchanger and the indoor heat exchanger to circulate a refrigerant between the outdoor heat exchanger and the indoor heat exchanger;
• and
• a controller configured for defrosting the first portion of the outdoor heat exchanger while operating the second portion of the outdoor heat exchanger in a heat pump mode.