18-28-510.5.6 Clearances.

   The minimum clearance between hoods and combustible construction shall be the clearance required by the duct system.

18-28-510.5.7 Ducts.

   Hazardous exhaust duct systems shall extend directly to the exterior of the building and shall not extend into or through ducts and plenums.

18-28-510.6 Penetrations.

   Penetrations of structure elements by a hazardous exhaust system shall conform to the building code.

18-28-510.7 Suppression required.

   Ducts shall be protected with an approved automatic fire suppression system installed in accordance with the building code.

   Exception: An approved automatic fire suppression system shall not be required in ducts conveying materials, fumes, mists and vapors that are nonflammable and noncombustible.

18-28-510.8 Duct construction.

   Ducts utilized to convey hazardous exhaust shall be constructed of approved G90 galvanized sheet steel, with a minimum nominal thickness as specified in Table 18-28-510.8.

   Nonmetallic ducts utilized in systems exhausting nonflammable corrosive fumes or vapors shall be listed and labeled. Nonmetallic duct shall have a flame spread index of 25 or less and a smoke-developed index of 50 or less, when tested in accordance with ASTM E 84. Ducts shall be approved for installation in such an exhaust system. Where the products being exhausted are detrimental to the duct material, the ducts shall be constructed of alternative materials that are compatible with the exhaust.

Table 18-28-510.8
Minimum Duct Thickness

 

Diameter of Duct or Maximum Side Dimension	Minimum Nominal Thickness
	Nonabrasive Materials	Nonabrasive/ Abrasive Materials	Abrasive Materials
0 – 8 inches	0.028 inch (No. 24 Gauge)	0.034 inch (No. 22 Gauge)	0.040 inch (No. 20 Gauge)
9 – 18 inches	0.034 inch (No. 22 Gauge)	0.040 inch (No. 20 Gauge)	0.052 inch (No. 18 Gauge)
19 – 30 inches	0.040 inch (No. 20 Gauge)	0.052 inch (No. 18 Gauge)	0.064 inch (No. 16 Gauge)
Over 30 inches	0.052 inch (No. 18 Gauge)	0.064 inch (No. 16 Gauge)	0.079 inch (No. 15 Gauge)

 

For SI: 1 inch = 25.4 mm.

(Amend Coun. J. 11-9-16, p. 36266, § 36)

18-28-510.8.1 Duct joints.

   Ducts shall be made tight with lap joints having a minimum lap of 1 inch (25 mm).