what is a furnace plenum
GO. This calculator is for estimating capacity only, and does not give exact equipment capacities for your home. A supply air plenum is a sheet metal box that attaches to the outlet of a furnace or air handler and is used as a central manifold for other ductwork to connect to.As the air blows from the furnace or air handler, it flows through the supply plenum and any ductwork attached to it.he sizes of the supply plenums available here fit the furnaces and air handlers we sell. Each supply plenum consists of 5 sides that connect together for easy assembly.