603.4 Flexible Air Ducts — 2025 California Mechanical Code
California Mechanical Code (Title 24, Part 4) · 2025 edition · updated 2026-09-25 · California
CBSC: official editions, supplements and errata
Italicized text is a California amendment to the model code, as printed in the official publication.
Cite as: 2025 California Mechanical Code (Title 24, Part 4) § 603.4 · Effective January 1, 2026 · Official source: Building Standards Commission
Flexible air ducts shall comply with UL 181, and shall be installed in accordance with the manufacturer’s installation instructions and SMACNA
HVAC Duct Construction Standards – Metal and Flexible.
Flexible air duct installations shall comply with the following:
(1) Ducts shall be installed using the minimum required length to make the connection.
(2) Horizontal duct runs shall be supported at not more than 4 feet (1219 mm) intervals.
(3) Vertical risers shall be supported at not more than 6 feet (1829 mm) intervals.
(4) Sag between support hangers shall not exceed 1/2 inch (12.7 mm) per foot (305 mm) of support spacing.
(5) Supports shall be rigid and shall be not less than 1 1/2 inches (38 mm) wide at point of contact with the duct surface.
(6) Duct bends shall be not less than one duct diameter bend radius.
(7) Screws shall not penetrate the inner liner of non-metallic flexible ducts unless permitted in accordance with the manufacturer’s installation instructions.
(8) Fittings for attaching non-metallic ducts shall be beaded and have a collar length of not less than 2 inches (51 mm) for attaching the duct. Metal worm-gear clamps shall be used.
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138 2025 CALIFORNIA MECHANICAL CODE
DUCT SYSTEMS
Exception: Where attaching metallic ducts using screws in accordance with the manufacturer’s installation
instructions.
(9) Duct inner liner shall be installed at not less than 1 inch (25.4 mm) on the collar and past the bead prior to the application of the tape and mechanical fastener. Where mastic is used instead of tape, the mastic shall be applied in accordance the mastic manufacturer’s instructions.
(10)Duct outer vapor barriers shall be secured using two wraps of approved tape. A mechanical fastener shall be permitted to be used in place of, or in combination with, the tape.
(11)Flexible air ducts shall not penetrate a fire-resistancerated assembly or construction.
(12)The temperature of the air to be conveyed in a flexible air duct shall not exceed 250°F (121°C).
(13)Flexible air ducts shall be sealed in accordance with Section 603.9.
603.4.1 Length Limitation. [Not permitted for OSHPD 1, 2, 3, 4 & 5] Flexible air ducts shall be not more than 5 feet (1524 mm) in length and shall not be used in lieu of rigid elbows or fittings. Flexible air ducts shall be permitted to be used as an elbow at a terminal device.
Exception: Residential occupancies.
603.4.1.1 Flexible Ducts. [OSHPD 1, 2, 3, 4 & 5] In hospital building projects and all other health- care facilities, including clinics and correctional treatment centers, flexible ducts of not more than 10 feet (3048 mm) in length may be used to connect supply, return or exhaust-air terminal devices to rigid duct systems. Where constant volume, variable volume or mixing boxes are utilized, flexible duct of not more than 5 feet (1524 mm), may be used on the inlet side for alignment. An internal impervious liner shall be provided to isolate insulation material from conditioned air. Flexible duct is not permitted in cor- ridors where fire or smoke dampers are omitted per CBC 717.5.4 and the duct is required to be con- structed of steel not less than 0.019 inch (0.483 mm) in thickness.
603.4.2 Flexible Air Connectors. Flexible air connectors shall not be permitted.
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