An air handling unit (AHU) handles and conditions the air, controls it to a required state, and provides motive force to transport it. An AHU is the primary equipment of the air system in a central air-conditioning system. Horizontal AHUs have their fan, coils, and filters installed at the same level. In vertical units, the supply fan is installed at a level higher than coils and filters. Factory-fabricated units are standard in construction and layout, low in cost, of higher quality, and fast in installation. Field built-up units or custom-built units are more flexible in construction, layout and dimensions than factory-built standardized units. A make-up AHU, also called a primary-air unit, is used to condition outdoor air entirely and is a once-through unit. A packaged unit is a self-contained air conditioner. It conditions the air and provides it with motive force and is equipped with its own heating and cooling sources.