Ducted return systems are preferable because ducts provide the most thermally and acoustically efficient path for air to return to central units. Ideally, every room or space should have a return air register ducted into the return system. This makes it unnecessary to resort to such expedients as placing grilles in doors, under-cutting doors so return air can flow under them, or (in worst cases) simply leaving doors open and suffering the resulting lack of privacy.

It is often important that return ducts be acoustically treated to prevent central equipment noise from reaching occupied spaces. Fibrous glass ducts, insulated flexible ducts, and sheet metal ducts with fibrous glass duct liners can all be used to provide acceptable noise control. Duct wraps provide no acoustical benefit.

Thermal insulation is also important on return systems because the temperature differential between the ambient air and the return air in the duct is often large.