The purpose of this Technical Bulletin is to clarify construction requirements for plenums used as return air for forced air units (FAUs). These requirements apply to air duct penetrations through the fire-rated wall separating a dwelling unit from a private garage.  Such ducts within the private garage shall be constructed of minimum 0.019-inch thick (26 gauge) galvanized steel, and shall not have openings into the private garage, per California Building Code (CBC) Section 406.3.2.2, and California Residential Code (CRC) Section R302.5.2.

I. Penetration of Enclosed Spaces Under Stairways

Penetrations of the fire-rated wall separating a dwelling unit from a private garage may occur in the enclosed space under a stairway, where no access is provided and where such space is used as a return air plenum for FAUs. The plenum space under the stairs is not considered to be usable space and need not comply with the requirements of CBC Section 1011.7.3, nor CRC Section R302.7.  See Figure 1.

Figure 1

II. Penetration in Other Locations

Penetration of the fire-rated wall separating a dwelling unit from a private garage may occur wherever construction is as shown in Figure 2 and the return air grille opens directly into the return air plenum under a forced air unit.

Figure 2

III. Protection of Plumbing and Electrical Wiring

If any wall spaces adjacent to the plenum contain plumbing or electrical wiring, they must be separated from the plenum by 1/2-inch gypsum wallboard, except when plenum-rated plumbing piping, electrical wiring, and metallic piping are used.