Abstract: An air conditioner wherein supply air is first passed through the supply air passages of an air-to-air heat exchanger, then over a first refrigerant-to-air heat exchanger, into the space to be air conditioned, and return air from that space is passed throught the air-to-air heat exchanger, through a second refrigerant-to-air heat exchanger, and discharged to atmosphere, the refrigerant heat exchangers being the condenser and evaporator of a closed refrigeration system driven by a heat pump compressor.