An air handler, or air handling unit (often abbreviated to AHU), is a device used to regulate and circulate air as part of a heating, ventilating, and air-conditioning (HVAC) system. An air handler is usually a large metal box containing a blower, furnace or A/C elements, filter racks or chambers, sound attenuators, and dampers. Air handlers usually connect to a ductwork ventilation system that distributes the conditioned air through the building and returns it to the AHU. Sometimes AHUs discharge (supply) and admit (return) air directly to and from the space served without ductwork Small air handlers, for local use, are called terminal units, and may only include an air filter, coil, and blower; these simple terminal units are called blower coils or fan coil units. A larger air handler that conditions 100% outside air, and no recirculated air, is known as a makeup air unit (MAU) or fresh air handling unit (FAHU). An air handler designed for outdoor use, typically on roofs, is known as a packaged unit (PU), heating and air conditioning unit (HCU), or rooftop unit (RTU). Construction The air handler is normally constructed around a framing system with metal infill panels as required to suit the configuration of the components. In its simplest form the frame may be made from metal channels or sections, with single skin metal infill panels. The metalwork is normally galvanized for long term protection. For outdoor units some form of weatherproof lid and additional sealing around joints is provided. Larger air handlers will be manufactured from a square section steel framing system with double skinned and insulated infill panels. Such constructions reduce heat loss or heat gain from the air handler, as well as providing acoustic attenuation. Larger air handlers may be several meters long and are manufactured in a sectional manner and therefore, for strength and rigidity, steel section base rails are provided under the unit. Where supply and extract air is required in equal proportions for a balanced ventilation system, it is common for the supply and extract air handlers to be joined together, either in a side-by-side or a stacked configuration. Air handling units types There are six factors for air handlers classifications and determine types of them, based on: Application (air handling unit usage) Air flow control (CAV or VAV air handlers) Zone control (single zone or multi zone air handlers) Fan location (draw-through or blow-through) Direction