PDFWAC 51-51-1600
Chapter 16—Duct systems.
M1601.1.1 Above-ground duct systems. Above-ground duct systems shall conform to the following:
1. Equipment connected to duct systems shall be designed to limit discharge air temperature to a maximum of 250°F (121°C).
2. Factory-made ducts shall be listed and labeled in accordance with UL 181 and installed in accordance with the manufacturer's instructions.
3. Fibrous duct construction shall conform to the SMACNA Fibrous Glass Duct Construction Standards or NAIMA Fibrous Glass Duct Construction Standards.
4. Field-fabricated and shop-fabricated metal and flexible duct constructions shall conform to the SMACNA HVAC Duct Construction Standards—Metal and Flexible, except as allowed by Table M1601.1.1. Galvanized steel shall conform to ASTM A 653.
5. Use of gypsum products to construct return air ducts or plenums is permitted, provided that the air temperature does not exceed 125°F (52°C) and exposed surfaces are not subject to condensation.
6. Duct systems shall be constructed of materials having a flame spread index not greater than 200.
7. Stud wall cavities and the spaces between solid floor joists shall not be used as a duct or an air plenum in new construction. For existing systems, stud wall cavities and the spaces between solid floor joists to be used as air plenums shall comply with the following:
7.1. These cavities or spaces shall not be used as a plenum for supply air.
7.2. These cavities or spaces shall not be part of a required fire-resistance-rated assembly.
7.3. Stud wall cavities shall not convey air from more than one floor level.
7.4. Stud wall cavities and joist-space plenums shall be isolated from adjacent concealed spaces by tight-fitting fire blocking in accordance with Section R302.11. Fireblocking materials used for isolation shall comply with Section R302.11.1.
7.5. Stud wall cavities in the outside walls of building envelope assemblies shall not be utilized as air plenums.
[Statutory Authority: RCW 19.27.031 and 19.27.074. WSR 23-23-104, § 51-51-1600, filed 11/15/23, effective 3/16/24; WSR 16-03-025, § 51-51-1600, filed 1/11/16, effective 7/1/16. Statutory Authority: RCW 19.27.031 and chapters 19.27 and 34.05 RCW. WSR 13-04-068, § 51-51-1600, filed 2/1/13, effective 7/1/13. Statutory Authority: RCW 19.27.031 and 19.27.074. WSR 10-03-098, § 51-51-1600, filed 1/20/10, effective 7/1/10.]