An air handler, or air handling unit (often abbreviated to AHU), is a device used to control and disperse air as part of a heating, ventilating, and air-conditioning (HVAC) system. An air handler is usually a large metal box holding a blower, heating or cooling elements, filter racks or chambers, sound attenuate systems, and dampers. Air handlers usually connect to a duct-work 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 duct-work.