Plenum
Also called: supply plenum, return plenum, plenum box, air handler plenum
The sheet metal or duct board box attached to the air handler or furnace that the ducts branch from: the supply plenum distributes conditioned air to the runs, the return plenum collects air coming back. Replacing one costs $400–$1,500; a cracked or unsealed plenum leaks more than any single duct.
The supply plenum sits on the outlet of the furnace or air handler, usually above or beside it, and every supply run leaves it through a take-off collar. The return plenum sits on the inlet side, behind the filter, and the return ducts feed into it. On a trunk-and-branch system a rectangular trunk extends from the supply plenum and the branches take off from the trunk instead.
Plenums are where the system’s air pressure is highest, so a gap there leaks more than the same gap at a boot. They’re also where replacements go wrong: reusing a plenum with rusted collars, cracked seams or a torn liner puts new ducts on a leaking box. A replacement quote should say whether the plenums are reused or replaced, and a like-for-like DIY job should inspect them first — seams sealed with mastic, collars tight, insulation intact.
“Plenum” also names the space above a dropped ceiling used as a return-air path in commercial buildings, which is why “plenum-rated” wiring exists. In a house it means the box.