The purpose of classifing air-conditioning or heating, ventilating, air-conditioning and refrigerating systems is to distinguish one type from another so that an optimum air-conditioning system can be selected according to the requirements. Proper classification of air-conditioning systems also will provide a background for using knowledge-based expert systems to help the designer to select an air-conditioning system and its subsystems. A room air conditioner is the sole factory-fabricated self-contained equipment used in the room air-conditioning system. It is often mounted on or under the window sill or on a window frame. An air-conditioning cycle for a four-pipe fan-coil system with outdoor ventilation air delivered to the suction side of the fan coil. A part of the space cooling and dehumidifying load is usually taken care by the conditioned outdoor ventilation air from the make-up air handling unit. A space conditioning system has the benefit of a separate demand-based outdoor ventilation air system. A Water-source heat pumps system incorporates heat recovery automatically.