408.1 General — 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) § 408.1 · Effective January 1, 2026 · Official source: Building Standards Commission
Filter efficiencies shall be certified by the manufacturer and shall be based on ASHRAE 52.2 Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size when specifically set forth in these standards.
408.1.1 A filter gauge shall be installed across each fil- ter bank serving central air systems. The gauge shall be red lined or a filter alarm light installed to signal when the recommended maximum static pressure drop has been reached.
408.1.2 Central air-handling systems are defined as any unit requiring duct work on the supply or inlet side that serve more than one room.
408.1.3 Filter banks shall be visually inspected for torn media and bypass in filter frames by means of a flashlight or equivalent, both with fans in operation and stopped. Tears in media and bypass in filter frames shall be elim- inated in accordance with the manufacturer’s directions and the requirements of the enforcing agency prior to commencing operation of the system.
408.1.4 Central air-handling systems shall be maintained in a reasonably clean condition during construction and shall be cleaned as necessary prior to replacement of tem- porary filter used during construction to ensure that clean air will be delivered to the occupied spaces.
408.1.5 Filter bank No. 1 shall be located upstream of the air-conditioning equipment. Filter bank No. 2 and additional filter banks shall be located downstream of the supply fan and all cooling and humidification equipment with efficiencies as indicated in Table 4-B or Table 4-C.
Exception: Humidifiers for local room humidity control may be installed in the supply air duct downstream of the final filter bank where designs are specifically approved by the enforcing agency. Humidification shall be in accor- dance with 320.1.1.
408.1.6 Filter bank No. 2 and additional filter bank media shall be rigid or supported (noncollapsing type) and shall operate on the principles of impingement, straining, and diffusion. Filter bank No. 2 shall have sealing interface surfaces.
408.1.7 HEPA filtration shall be provided in the air ter- minal device where required by Table 4-B.
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