Ducts are a system of tubes that connect the HVAC system with the air vents in your home. These tubes are usually made of rigid metal like aluminum or galvanized steel. How they function is as follows: the HVAC units in your home, such as an air conditioner or furnace, pull air in through an intake vent, heat or cool the air depending on the thermostat setting, and then force that air to blow through the ducts and air vents to reach every room in your home.