Abstract: A room air conditioner mounted on an external building wall, comprising an indoor unit, an outdoor unit and an interface unit. The indoor unit includes an indoor cabinet having a suction opening and outlet port for indoor air, an indoor heat exchanger, and an indoor fan. The outdoor unit includes an outdoor cabinet having a suction opening and outlet port for fresh air, an outdoor heat exchanger, an outdoor fan and a fan motor which engages the outdoor and indoor fans. The interface unit is mounted in the external building wall and includes a fan drive shaft engaged at the first end by the fan motor in the outdoor cabinet and engaging the indoor fan.