An air handler, also known as an “air handling unit” (AHU), is the part of your heating, ventilating, and air conditioning (HVAC) system that circulates and conditions air throughout your home. It typically consists of a large metal box that contains a blower fan, heating and cooling coils, and an air filter. The air handler circulates air through the coils and filter before distributing it back into the building through ductwork.