The air handler is the indoor portion of a mini-split system that circulates and filters the air within the room. Different styles are available, such as the MRCOOL Olympus 4th Gen Wall AH. For those who prefer a hidden look, the Slim Duct Multi-Zone air handlers are installed within the ceiling or crawlspace. These units house the evaporator coil and the blower fan responsible for air movement. Proper placement of the air handler is crucial for even air distribution. They are designed to operate quietly so they don't disturb your sleep or work.