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Chapter 6 — DUCT SYSTEMS

603.4 Flexible Air Ducts. Flexible air ducts shall comply

California Mechanical Code (Title 24, Part 4) · 2022 edition · updated 2026-09-10 · California

Italicized text is a California amendment to the model code, as printed in the official publication.

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-metal lic 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. 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-resistance rated 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 Sec tion 603.9.

603.4.1 Length Limitation. [Not permitted for OSHPD 1, 1R, 2, 3, 4 & 5] Flexible air ducts shall be not more than 5 feet (1524 mm) in length and shall not be

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DUCT SYSTEMS

and moisture content shall be controlled so that the material is not adversely affected. All gypsum products shall have a mold or mildew resistant surface. For the purpose of this section, gypsum products shall not be exposed in supply ducts. 602.4.3 Air Dispersion Systems. Air dispersion systems shall be listed and labeled in accordance with UL 2518. 602.4.4 Other Materials. Flexible and rigid ducts, plenums, or fittings for use in heating, ventilation, and air conditioning systems of other nonmetallic materials listed and labeled to UL 181 shall be permitted. Exception: Plastic ducts shall comply with Section 603.5.

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▸Contents — California Mechanical Code (Title 24, Part 4)
California Mechanical Code (Title 24, Part 4)
  1. Chapter 1 — GENERAL CODE PROVISIONS
  2. Chapter 2 — DEFINITIONS
  3. Chapter 3 — GENERAL REGULATIONS
  4. Chapter 4 — VENTILATION AIR
  5. Chapter 5 — EXHAUST SYSTEMS
  6. ▸Chapter 6 — DUCT SYSTEMS
    Overview
    1. 602.3 Metallic. Ducts, plenums, or fittings of metal shall com…
    2. 602.4 Nonmetallic Ducts. Nonmetallic ducts shall comply
    3. 601.1 Applicability. Ducts and plenums that are portions of
    4. 601.2 Sizing Requirements. Duct systems shall be sized
    5. 602.1 General. Materials used for duct systems shall comply wi…
    6. 602.2 Combustibles Within Ducts or Plenums. Materials exposed …
    7. 603.4 Flexible Air Ducts. Flexible air ducts shall comply
    8. 602.5 Vibration Isolators. Vibration isolation connectors
    9. 602.6 Corridors. Corridors shall not be used to convey air
    10. 603.1 General. Air ducts shall be installed in accordance
    11. 603.2 Under Floor or Crawl Space. Air ducts installed
    12. 603.3 Metal Ducts. Ducts shall be supported at each change
    13. 603.8 Protection Against Flood Damage. In flood hazard areas, …
    14. 603.9 Joints and Seams of Ducts. Joints and seams for
    15. 603.5 Plastic Ducts. Plastic air ducts and fittings shall be
    16. 603.6 Protection of Ducts. Ducts installed in locations
    17. 603.7 Support of Ducts. Ducts shall be supported in
    18. 603.12 Air Dispersion Systems. Where installed, air dispersion…
    19. 603.13 Clearances. Duct clearances shall be in accordance
    20. 604.1 Furnace Plenums and Air Ducts. Furnace plenums
    21. 604.2 Supplied as a Part of Furnace. A furnace plenum
    22. 604.3 Not Supplied with the Furnace. Where a furnace
    23. 603.10 Cross Contamination. Exhaust ducts that convey
    24. 603.11 Underground Installation. Ducts installed underground s…
    25. 604.4 Return Air. Where a furnace is installed so supply
    26. 605.1 General. Air ducts conveying air at temperatures
    27. 605.2 [OSHPD 1, 1R, 2, 3 (Surgical Clinics), 4 & 5] Ther-
    28. 605.3 [OSHPD 1, 1R, 2, 4 & 5] Thermal or acoustical lining
    29. 606.1 Smoke Dampers. Smoke dampers shall comply
    30. 606.2 Fire Dampers. Fire dampers shall comply with UL
    31. 606.3 Ceiling Radiation Dampers. Ceiling radiation
    32. 606.4 Multiple Arrangements. Where size requires the
    33. 606.5 Access and Identification. Fire and smoke
    34. 608.1 General. An under-floor space shall be permitted to
    35. 608.2 Dwelling Units. The use of under-floor space shall
    36. 608.3 Enclosed. Such spaces shall be cleaned of all loose
    37. 608.4 Flammable Materials. The enclosing material of
    38. 608.5 Access. Access shall be through an opening in the
    39. 608.6 Automatic Control. A furnace supplying warm air
    40. 608.7 Temperature Limit. A furnace supplying warm air to
    41. 608.8 Noncombustible Receptacle. A noncombustible
    42. 608.9 Floor Registers. Floor registers shall be designed
    43. 608.10 Exterior Wall and Interior Stud Partitions.
    44. 606.6 Freedom from Interference. Dampers shall be
    45. 606.7 Temperature Classification of Operating Ele-
    46. 606.8 When the automatic activation of a smoke damper or a
    47. 607.1 General. Perforated ceilings shall be permitted to be
    48. 607.2 Requirements. Ventilating ceilings shall comply
    49. 608.11 Wall Register. Each wall register shall be connected
    50. 608.12 Distance from Combustible. A duct complying
    51. 608.13 Vapor Barrier. The entire ground surface of the
    52. 608.14 Prohibited. Fuel gas lines and plumbing waste
    53. 609.1 Air-Moving Systems and Smoke Detectors.
  7. Chapter 7 — COMBUSTION AIR
  8. Chapter 8 — CHIMNEYS AND VENTS
  9. Chapter 9 — INSTALLATION OF SPECIFIC APPLIANCES
  10. Chapter 10 — BOILERS AND PRESSURE VESSELS
  11. Chapter 11 — REFRIGERATION
  12. Chapter 12 — HYDRONICS
  13. Chapter 13 — FUEL GAS PIPING
  14. Chapter 14 — PROCESS PIPING
  15. Chapter 15 — SOLAR ENERGY SYSTEMS
  16. Chapter 16 — STATIONARY POWER PLANTS
  17. Chapter 17 — REFERENCED STANDARDS
  18. Appendix A — RESIDENTIAL PLAN EXAMINER REVIEW FORM FOR HVAC SY…
  19. Appendix B — PROCEDURES TO BE FOLLOWED TO PLACE GAS EQUIPMENT …
  20. Appendix C — INSTALLATION AND TESTING OF OIL (LIQUID) FUEL-FIR…
  21. Appendix D — FUEL SUPPLY: MANUFACTURED/MOBILE HOME PARKS AND R…
  22. Appendix E — SUSTAINABLE PRACTICES
  23. Appendix F — GEOTHERMAL ENERGY SYSTEMS
  24. Appendix G — SIZING OF VENTING SYSTEMS AND OUTDOOR COMBUSTION …
  25. Appendix H — EXAMPLE CALCULATION OF OUTDOOR AIR RATE

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