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Chapter 6 — DUCT SYSTEMS

603.13 Clearances. Duct clearances shall be in accordance

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 Section 603.13.1 through Section 603.13.6.

603.13.1 General. Duct clearances to combustible construction shall be in accordance with the listing and the manufacturer’s installation instructions. 603.13.2 Supply Air Ducts to Listed Furnaces. Supply air ducts connecting to listed central heating furnaces shall have the same minimum clearance to combustibles as required for the furnace supply plenum for a distance of not less than 3 feet (914 mm) from the supply plenum. Clearance shall not be required beyond the 3 feet (914 mm) distance. [NFPA 54:10.3.2.7] 603.13.3 Supply Air Ducts to Unlisted Furnaces. Supply air ducts connecting to unlisted central heating furnaces equipped with temperature limit controls with a maximum setting of 250°F (121°C) shall have a minimum clearance to combustibles of 6 inches (152 mm) for a distance of not less than 6 feet (1829 mm) from the furnace supply plenum. Clearance shall not be required beyond the 6 feet (1829 mm) distance. [NFPA 54:10.3.2.8] 603.13.4 Furnace Plenums and Air Ducts. A furnace plenum supplied as a part of the air-conditioning appliance shall be installed in accordance with the manufacturer’s instructions. Where a furnace plenum is not supplied with the appliance, any fabrication and installation instructions provided by the manufacturer shall be followed. The method of connecting supply and return ducts shall facilitate proper circulation of air. Where the air conditioner is installed within an enclosure, the installation shall comply with Section 604.4. [NFPA 54:10.2.5] 603.13.5 Duct Furnaces. Duct furnaces shall be installed in accordance with Section 905.0. 603.13.6 Fuel-Burning Kilns. A hood and duct serving a fuel-burning kiln shall have a clearance from combustible construction of not less than 18 inches (457 mm). This clearance shall be permitted to be reduced in accordance with Table 303.10.1.

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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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