Skip to content

Earlier editions: 20222019

Chapter 6 — DUCT SYSTEMS

602.2 Combustibles Within Ducts or Plenums.

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.

Materials exposed within ducts or plenums shall be noncombustible or shall have a flame spread index not to exceed 25 and a smoke-developed index not to exceed 50, where tested as a composite product in accordance with ASTM E84 or UL 723. Plastic piping installed in plenums shall be tested in accordance with all requirements of ASTM E84 or UL 723. Mounting methods, supports and sample sizes of materials for testing that are not specified in ASTM E84 or UL 723 shall be prohibited.

Exceptions:

(1) Return-air and outside-air ducts, plenums, or concealed spaces that serve a dwelling unit.

(2) Air filters in accordance with the requirements of Section 311.2.

(3) Water evaporation media in an evaporative cooler.

(4) Charcoal filters where protected with an approved fire suppression system.

(5) Products listed and labeled for installation within plenums in accordance with Section 602.2.1 through Section 602.2.4.

(6) Smoke detectors in accordance with the requirements of Section 609.0.

(7) Duct insulation, coverings, and linings and other supplementary materials installed in accordance with Section 605.0.

(8) Materials in a hazardous fabrication area including the areas above and below the fabrication area sharing a common air recirculation path with the fabrication area.

(9) Plastic water distribution piping listed and labeled for use in plenums in accordance with UL 2846 as having a peak optical density not greater than 0.50, an average optical density not greater than 0.15, and a flame spread distance not greater than 5 feet (1524 mm), and installed in accordance with its listing, shall be permitted.

602.2.1 Electrical. Electrical wiring in plenums shall comply with the California Electrical Code . Electrical wires and cables and optical fiber cables shall be listed and labeled for use in plenums and shall have a flame spread distance not exceeding 5 feet (1524 mm), an average optical density not exceeding 0.15, and a peak optical density not exceeding 0.5, where tested in accordance with NFPA 262.

602.2.2 Fire Sprinkler Piping. Nonmetallic fire sprinkler piping in plenums shall be listed and labeled for use in plenums and shall have a flame spread distance not exceeding 5 feet (1524 mm), an average optical density not exceeding 0.15 and, a peak optical density not exceeding 0.5, where tested in accordance with UL 1887.

602.2.3 Pneumatic Tubing. Nonmetallic pneumatic tubing in plenums shall be listed and labeled for use in plenums and shall have a flame spread distance not exceeding 5 feet (1524 mm), an average optical density not exceeding 0.15, and a peak optical density not exceeding 0.5, where tested in accordance with UL 1820.

602.2.4 Discrete Products in Plenums. Discrete plumbing, mechanical, and electrical products that are located in a plenum and have exposed combustible material shall be listed and labeled in accordance with UL

Get a plain-English answer with a citation back to this text.

Ask AI about this code
Export this section
▸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

GoCodebook provides public access, search, citation, multilingual explanation, and practical interpretation of legally adopted building regulations. It is not a substitute for the official ICC or California code publications.