An HVAC duct is a passageway that distributes heated or cooled air through a building. It is part of the heating, ventilation, and air conditioning system. The duct carries air from the furnace or air handler to the rooms, and from the rooms back to the unit. It is made of sheet metal, fiberglass, or flexible plastic. It is sealed to prevent leakage and insulated to prevent heat loss. The duct is the circulatory system of the building.

Ducts come in several types. A trunk duct is the main line. A branch duct serves a single room. A flexible duct is a corrugated tube that can be routed around obstacles. A rigid duct is made of sheet metal and is more efficient. The duct is sized based on the air volume and the static pressure. Too small, and the system cannot deliver enough air. Too large, and the air moves too slowly and does not mix. The duct must be balanced so that each room gets the right amount of air.

Duct installation is a trade. The sheet metal worker fabricates and installs the duct. The duct must be sealed with mastic or tape. It must be insulated in unconditioned spaces. It must be supported at regular intervals. It must not be crushed or kinked. Leaky ducts waste energy and reduce comfort. The duct is hidden above ceilings and inside walls. It is rarely seen. But it is essential. Without it, the air conditioning would not reach the rooms. The building would be uncomfortable and inefficient.