What is the name of the first duct section that connects a furnace to the branch ducts?

The correct answer is the "Plenum." In HVAC systems, a plenum is used as a chamber that connects the furnace to the ductwork. It serves as a central hub for distributing conditioned air to various branch ducts. The plenum is crucial because it allows for an even distribution of airflow throughout the duct system, ensuring that each area of a building receives the appropriate amount of heated or cooled air.

The plenum typically sits directly on top of the furnace, collecting the output air from the unit before it is distributed into the supply ducts that lead to different rooms. This function makes the plenum essential for the overall efficiency and effectiveness of the heating system.

Other terms such as trunk line, return air duct, and supply duct refer to different components of the HVAC system. The trunk line refers to the main duct running from the plenum to various branches, while the return air duct is used to draw air back into the system for reconditioning. Supply ducts carry the conditioned air to the living spaces, but they do not connect directly to the furnace in the same way that the plenum does. Hence, the plenum is the correct term for the first duct section that connects to a furnace.