If there is no sheet metal duct; just the open ceiling or wall space, then you are required by law to use Plenum Rated cable. … The lack of wall or ceiling return air grates is usually a sure sign that you require Plenum Rated Cable. Computer Rooms typically use a raised flooring system for the server racks.

Do I need a plenum on my furnace?

A: In modern forced-air HVAC systems, you must have a plenum installed.

Are return air ducts required?

Ideally, every room except the kitchen and bath should have its own return air grille. At the very least, each level of the house should have one. Even so, many homes were built with just one or even no return ducts in an attempt to cut costs.