*A. Horizontal air handling unit (horizontal AHU)*

The horizontal air handling units place the supply air fan, cooling coil, heating coil, and humidifier in the casing horizontally (see **Figure 2**). This design requires a large floor area as AHU components are large in size. Horizontal AHU type are usually of high capacity and are usually placed in plant room, whereas small horizontal AHU type can be placed carefully on the roof [2].
