The air handler is the indoor part of the HVAC unit that contains a blower and coils for heating and cooling. In a HVAC system that uses a fossil fuel, the air handler contains the heat exchanger, which is a firebox that separates combustion byproducts from the circulating air.
The air handler connects to ductwork to circulate the air in a home. A return air vent, usually found in the ceiling, collects warm air from the room and provides a passage that connects it to the air handler. In the air handler, the fan forces the air through the cooling coils. As the air cools, moisture collects on the coils and drips to a collection tray, which has a drain to empty it. The system distributes cooled air to the home through a set of ducts and registers.
As air passes through the air handler, a filter removes particulate matter. Some filters remove only the largest particles to protect the cooling coils. Filters that are more efficient also help to reduce dust, pollen and other allergens from the air in the house. Filters become dirty and require changing on a regular basis. Most manufacturers recommend changing them at least once every three months, but when using high-efficiency filters, the occupant should change them more frequently.