An air handler, also called an air handling unit or AHU, is used to circulate and condition indoor air. Air handling units include filters, fans, heating and cooling coils, air control dampers and filters. Smaller air handler units simply consist of an air filter, a coil and a blower.
The air handling unit pulls air from the building and first passes it through the air filter, which sieves out dirt, dust and other particles before entering the air handler. This helps keep the other parts clean and working efficiently. The air handler then blows the filtered air through the heating or cooling coils back into the house as warm or cool air.