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Earlier editions: 20222019

Chapter 6 — DUCT SYSTEMS

603.4 Flexible Air Ducts. Flexible air ducts shall comply

California Mechanical Code (Title 24, Part 4) · 2025 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-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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▸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. 601.1 Applicability. Ducts and plenums that are portions of
    2. 601.2 Sizing Requirements. Duct systems shall be sized
    3. 602.1 General. Materials used for duct systems shall comply wi…
    4. 602.2 Combustibles Within Ducts or Plenums. Materials exposed …
    5. 602.3 Tall Wood (Mass Timber) Buildings. Duct systems installe…
    6. 602.4 Metallic. Ducts, plenums, or fittings of metal shall
    7. 602.5 Nonmetallic Ducts. Nonmetallic ducts shall comply
    8. 602.6 Vibration Isolators. Vibration isolation connectors
    9. 602.7 Corridors. Corridors shall not be used to convey air to
    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. 603.4 Flexible Air Ducts. Flexible air ducts shall comply
    14. 603.5 Plastic Ducts. Plastic air ducts and fittings shall be
    15. 603.6 Protection of Ducts. Ducts installed in locations
    16. 603.7 Support of Ducts. Ducts shall be supported in
    17. 603.8 Protection Against Flood Damage. In flood hazard areas, …
    18. 603.9 Joints and Seams of Ducts. Joints and seams for
    19. 603.10 Cross Contamination. Exhaust ducts that convey
    20. 603.11 Underground Installation. Ducts installed underground s…
    21. 603.12 Air Dispersion Systems. Where installed, air dispersion…
    22. 603.13 Clearances. Duct clearances shall be in accordance
    23. 604.1 Furnace Plenums and Air Ducts. Furnace plenums
    24. 604.2 Supplied as a Part of Furnace. A furnace plenum
    25. 604.3 Not Supplied with the Furnace. Where a furnace
    26. 604.4 Return Air. Where a furnace is installed so supply
    27. 605.1 General. Air ducts conveying air at temperatures
    28. 605.2 [OSHPD 1, 3 (Surgical Clinics), 4 & 5] Thermal
    29. 605.3 [OSHPD 1, 2, 4 & 5] Thermal or acoustical lining
    30. 606.1 Smoke Dampers. Smoke dampers shall comply
    31. 606.2 Fire Dampers. Fire dampers shall comply with UL
    32. 606.3 Ceiling Radiation Dampers. Ceiling radiation
    33. 606.4 Combination Fire/Smoke Dampers. Combination fire/smoke d…
    34. 606.5 Corridor Dampers. Corridor dampers shall comply
    35. 606.6 Periodic Testing and Inspection. Testing and
    36. 606.7 Multiple Arrangements. Where size requires the
    37. 606.8 Access and Identification. Fire and smoke
    38. 606.9 Freedom from Interference. Dampers shall be
    39. 606.10 Temperature Classification of Operating Ele-
    40. 607.1 General. Perforated ceilings shall be permitted to be
    41. 607.2 Requirements. Ventilating ceilings shall comply
    42. 608.1 General. An under-floor space shall be permitted to
    43. 608.2 Dwelling Units. The use of under-floor space shall
    44. 608.3 Enclosed. Such spaces shall be cleaned of all loose
    45. 608.4 Flammable Materials. The enclosing material of
    46. 608.5 Access. Access shall be through an opening in the
    47. 608.6 Automatic Control. A furnace supplying warm air
    48. 608.7 Temperature Limit. A furnace supplying warm air to
    49. 608.8 Noncombustible Receptacle. A noncombustible
    50. 608.9 Floor Registers. Floor registers shall be designed
    51. 608.10 Exterior Wall and Interior Stud Partitions.
    52. 608.11 Wall Register. Each wall register shall be connected
    53. 608.12 Distance from Combustible. A duct complying
    54. 608.13 Vapor Barrier. The entire ground surface of the
    55. 608.14 Prohibited. Fuel gas lines and plumbing waste
    56. 609.1 Air-Moving Systems and Smoke Detectors.
    57. 609.2 Air-Moving Systems and Smoke Detectors in
  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 — GEOTHERMAL ENERGY SYSTEMS AND AMBIENT TEMPERATURE…
  18. Chapter 18 — REFERENCED STANDARDS
  19. Appendix A — RESIDENTIAL PLAN EXAMINER REVIEW FORM FOR HVAC SY…
  20. Appendix B — PROCEDURES TO BE FOLLOWED TO PLACE GAS EQUIPMENT …
  21. Appendix C — INSTALLATION AND TESTING OF OIL (LIQUID) FUEL-FIR…
  22. Appendix D — FUEL SUPPLY: MANUFACTURED/MOBILE HOME PARKS AND R…
  23. Appendix E — SUSTAINABLE PRACTICES
  24. Appendix F — SIZING OF VENTING SYSTEMS AND OUTDOOR COMBUSTION …
  25. Appendix G — EXAMPLE CALCULATION OF OUTDOOR AIR RATE
  26. Appendix H — PROFESSIONAL QUALIFICATIONS
  27. Appendix I — INDOOR HORTICULTURAL FACILITIES
  28. Appendix J — CLEAN AIR DELIVERY

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