The plenum, or trunk line, is what comes directly off of the air handler and is what the supply tubes connect to. Plenum can be used in several different ways. In most installations, round metal plenum is used in 7", 9" or 10 " diameters, depending on the size of the system. This is then wrapped in an insulation sleeve to prevent thermal loss and most importantly to stop condensation.
Unico, Inc. also manufactures our own plenum. This is a fiberglass square plenum that features ship lap connections, making installation a breeze. We manufacture the plenum in 6' sections available in 6.5", 8.5" and 9.5" square dimensions, depending on system size. In addition to the 6' straight sections, we also offer a full range of components that includes elbows, tees, reducers and end caps.
A lesser used option is to use rectangular plenum. This option is typically done in installations where their is no room for round or fiberglass plenum and the installing contractor needs to make their own plenum. To view sizing requirements, see below for Bulletin 30-45.