Most U.S. residences use central ducted systems to deliver cooling and heating. Ducted systems consist of a network of ducts with a blower that delivers conditioned air to different indoor spaces. An outdoor condensing unit cools refrigerant, which then flows to a refrigerant coil mounted in the duct. If the outdoor unit is a heat pump, the indoor coil also transfers heat to the duct during the heating season. More commonly, however, the ducted airflow passes through a furnace that heats the air.