A split system is composed of an indoor and outdoor unit which work in tandem to cool your home. When the thermostat engages the AC, refrigerant begins to circulate among the coils. As the warm indoor air is drawn in through the return vents, it passes over the coils to remove the heat and humidity and disperse it outdoors. The resulting cooled air is then circulated through the supply vents, lowering the temperature and humidity of your indoor air.