Wall units are an economic, relatively simple tool to condition air in a limited or single-zone space. Physically, wall units resemble window air conditioners, but are a more permanent installation. Wall units are mounted through a wall, so removing them when the cooling season ends is not desirable. There are a number of types. Some are cooling only, and some generate both cooling and heating. Virtually all are self-contained, single cabinet systems with the controls and evaporator on the internal facing side, and the condenser and condensate drain on the external facing side.