An HVAC plenum is a thin box with five sides made of galvanized sheet metal. It connects to the outlet of a furnace or air handler and is a key component of any HVAC system.
An HVAC plenum is a square or rectangular box that connects the furnace or air handler on one side to the duct-work that carries conditioned air throughout a structure on the other. Openings up to 12 inches in diameter are sealed with three metal screws, while openings larger than 12 inches in diameter require five screws for solid connections.The connections are then sealed with mastic, an adhesive, or special tape. Plenums are available in different sizes to accommodate different sized HVAC systems.