The air handler of an HVAC system uses a blower to remove air from the home, force it through a heat exchanger and distribute the conditioned air through a system of ducts back into the house. On most systems, the blower is a squirrel cage fan that simultaneously creates a vacuum to pull air out of the room and pressure to force it through the ducts.
Heat transfers through conduction, radiation and convection. Conduction occurs by placing a pan on the heating element of the stove. Radiation heat takes place with radiators and glowing electric heaters. However, the forced air system uses a modified form of convection heat. Convection currents normally rise off a hot surface. The air handler forces air through the hot elements to direct the convection currents to the desired area.
Central air conditioning, forced air furnaces and heat pumps all use air handlers. In all these systems, a single air handler is usually responsible for both heating and cooling. The air handler includes a filter that removes particulate material from the air as it passes through. When operating as an air conditioner, the circulation of air through the cold coils in the unit by the air handler lowers the temperature and reduces the humidity inside the building.