As a furnace operates and blows out air, it must at the same time return to it that same amount of air. A return air plenum is the ducting that is attached to a furnace for that return air to flow through, and is used as a central manifold for other return air ducting. Alpine's return air plenums are manufactured with a 1 inch filter rack as part of the plenum, and include one appropriately sized filter.
Watch the installation video, linked below, as a pro quickly assembles the supply air plenum and attaches it to the equipment, and shows you how to do the same. You'll have all your questions answered as you see how fast and extrememly easy it is to put together. Plenum assembles in about 5 minutes.
NOTE: Return air plenums are only available for purchase with equipment orders- such as furnaces, heat pumps or air conditioners - since their packaging requires they be delivered on a pallet. Also, these cold air plenums are for use with furnaces only, not air handlers. Air handlers require a type of return air plenum that attaches to the equipment bottom, and these return air plenums attach to the equipment side.