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Chapter 5 — EXHAUST SYSTEMS

506.11 Clearance Reduction Methods.

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.

It shall be permitted to protect stored combustible material or combustible construction from ductwork in accordance with Table 506.11 and Section 506.11.1 through Section 506.11.6. In no case shall the clearance between the duct and the combustible surface be reduced below that allowed in Table 506.11. [NFPA 91:4.7.4, 4.7.4.1] 506.11.1 Spacers and Ties. Spacers and ties for protection materials shall be of noncombustible material and shall not be installed on the duct side of the protection system. [NFPA 91:4.7.4.2] 506.11.2 Wool Batts Insulation. Mineral wool batts (blanket or board) shall have a minimum density of 8 pounds per cubic feet (lb/ft 3 ) (128 kg/m 3 ) and have a minimum melting point of 1500°F (816°C). [NFPA 91:4.7.4.3] 506.11.3 Insulation Board. Insulation board used as a part of a clearance-reduction system shall meet the following criteria: (1) Have a thermal conductivity of 1 British thermal

unit inch per hour square foot degree Fahrenheit

[Btu•in/(h•ft 2 •°F)] [0.14 W/(m•K)] or less. (2) Be formed of noncombustible material. [NFPA

91:4.7.4.4] 506.11.4 Duct and Thermal Shield. With all clearance reduction systems, at least 1 inch (25.4 mm) clear space shall be provided between the duct and the thermal shield. [NFPA 91:4.7.4.5] 506.11.5 Thermal Shield and Combustible Sur- face. When using clearance reduction systems that include an air gap, at least 1 inch (25.4 mm) clear space shall be provided between the thermal shield and the combustible surface. [NFPA 91:4.7.4.6] 506.11.6 Reduced Clearance with Air Gaps. When using clearance reduction systems that include an air gap between the combustible surface and the selected means of protection, air circulation shall be provided by one of the methods in Section 506.11.6.1 through Section 506.11.6.3. [NFPA 91:4.7.4.7] 506.11.6.1 Air Circulation. Air circulation shall be permitted to be provided by leaving all edges of the protecting system open with at least a 1 inch (25.4 mm) air gap. [NFPA 91:4.7.4.7.1] 506.11.6.2 Single Flat Wall. If the means of protection is mounted on a single flat wall away from corners, air circulation shall be permitted to be provided by one of the following: (1) Leaving only the top and bottom edges open to

circulation by maintaining the 1 inch (25.4 mm) air gap.

2022 CALIFORNIA MECHANICAL CODE 95

EXHAUST SYSTEMS

TABLE 506.11 REDUCTION OF DUCT CLEARANCE WITH SPECIFIED FORMS OF PROTECTION

[NFPA 91: TABLE 4.7.4]
FORM OF PROTECTION* MAXIMUM ALLOWABLE REDUCTION IN CLEARANCE (percent) MAXIMUM ALLOWABLE REDUCTION IN CLEARANCE (percent)
FORM OF PROTECTION* AS WALL PROTECTOR OR
VERTICAL SURFACE		 AS CEILING PROTECTOR OR
HORIZONTAL SURFACE
31⁄2 inch thick masonry wall without ventilated air
space		 33 None
1⁄2 inch thick noncombustible insulation board over
1 inch glass fiber or mineral wool batts without
ventilated air space		 50 33
0.024 inch (24 gauge) sheet metal over 1 inch glass
fiber or mineral wool batts reinforced with wire, or
equivalent on rear face with at least a 1 inch air
gap
66		 66
31⁄2 inch (90 mm) thick masonry wall with at least
a 1 inch air gap		 66 None
0.024 inch (24 gauge) sheet metal with at least a 1
inch air gap		 66 50
1⁄2 inch thick noncombustible insulation board with
at least a 1 inch air gap		 66 50
0.024 inch (24 gauge) sheet metal with ventilated
air space over at least 0.024 inch (24 gauge) sheet
metal with at least a 1 inch air gap		 66 50
1 inch glass fiber or mineral wool batts sandwiched
between two sheets of 0.024 inch (24 guage) sheet
metal with at least a 1 inch air gap		 66 50

For SI units: 1 inch = 25.4 mm

  • Clearance reduction applied to and covering all combustible surfaces within the distance specified as required clearance with no protection in Table 506.10.4.

(2) Leaving the top and both side edges open to cir culation by maintaining the 1 inch (25.4 mm) air gap. [NFPA 91:4.7.4.7.2] 506.11.6.3 Thermal Shielding. Thermal shielding that covers two walls in a corner shall be permitted to be open at the top and bottom edges with at least 1 inch (25.4 mm) air gap. [NFPA 91:4.7.4.7.3]

Part II - Commercial Hoods and Kitchen

Ventilation.

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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
    Overview
    1. 501.1 Applicability. This chapter includes requirements for
    2. 502.1 Exhaust Opening Protection. Exhaust openings
    3. 502.2 Termination of Exhaust Ducts. Exhaust ducts
    4. 503.1 General. Motors and fans shall be sized to provide
    5. 503.2 Fans. Parts of fans in contact with explosive or flammab…
    6. 504.1 General. Where not specified in this chapter, exhaust
    7. 504.2 Independent Exhaust Systems. Single or combined mechanic…
    8. 504.3 Domestic Range. Ducts used for domestic kitchen
    9. 504.4 Clothes Dryers. A clothes dryer exhaust duct shall not
    10. 504.5 Heat (Energy) Recovery Ventilators. Heat
    11. 504.6 Gypsum Wallboard Ducts. Bathroom and laundry
    12. 505.1 General. A mechanical ventilation or exhaust system
    13. 505.2 Incompatible Materials. Incompatible materials
    14. 505.3 Flammability Limit. Unless the circumstances stipulated …
    15. 505.4 Air-Moving Devices. Air-moving devices shall be
    16. 505.5 Generating Flames, Sparks, or Hot Materials.
    17. 505.6 Fire Dampers. Fire dampers shall be permitted to be
    18. 505.7 Fire Detection and Alarm Systems. Unless the
    19. 505.8 Product-Conveying Ducts Classification. Product-conveyin…
    20. 505.9 Minimum Velocities and Circulation. The velocity and cir…
    21. 505.10 Makeup Air. Makeup air shall be provided to replenish a…
    22. 505.11 Hoods and Enclosures. Hoods and enclosures shall
    23. 505.12 Pharmacies – Compounding Area of Par-
    24. 506.1 Materials. Materials used in product-conveying duct
    25. 506.2 Construction. Ducts used for conveying products
    26. 506.3 Penetrations. Exhaust ducts shall not pass through
    27. 506.4 Condensate. Joints in duct construction shall be liquidt…
    28. 506.5 Fittings. Fittings in Class 2, 3, and 4 systems shall be
    29. 506.6 Explosion Venting. Ducts conveying explosive dusts
    30. 506.7 Supports. Supports shall be of noncombustible materials,…
    31. 506.8 Fire Protection. Sprinklers or other fire-protection
    32. 506.9 Protection from Physical Damage. Ducts
    33. 506.10 Duct Clearances. Unless the conditions stipulated
    34. 506.11 Clearance Reduction Methods. It shall be permitted to p…
    35. 507.1 Type I Hood Exhaust System. Exhaust systems
    36. 507.2 Exhaust System. Cooking equipment used in
    37. 507.3 Listed Devices. Penetrations shall be sealed with
    38. 507.4 Clearance. Where enclosures are not required, hoods,
    39. 507.5 Drawings. A drawing(s) of the exhaust system installatio…
    40. 507.6 Notification of Change. If required by the Authority Hav…
    41. 507.7 Pharmaceutical Compounding Exhaust Dis-
    42. 508.1 Where Required. Type I hoods shall be installed at
    43. 508.2 Listed Type I Hood Assemblies. Listed hood
    44. 508.3 Construction of Type I Hoods. The hood or that
    45. 508.4 Supports. Hoods shall be secured in place [OSHPD
    46. 508.5 Hood Size. Hoods shall be sized in accordance with
    47. 508.6 Solid-Fuel Hood Assemblies. Where solid-fuel
    48. 508.7 Exhaust Outlets. An exhaust outlet within an
    49. 509.1 Grease Removal Devices. Listed grease filters or
    50. 509.2 Installation. The distance between the grease removal
    51. 509.3 Solid-Fuel Grease Removal Devices. Where
    52. 510.1 General. Ducts shall not pass through fire walls.
    53. 510.2 Clearance. Clearance between ducts and combustible
    54. 510.3 Openings. Openings shall be provided at the sides or
    55. 510.4 Listed Grease Ducts. Listed grease ducts shall be
    56. 510.5 Other Grease Ducts. Other grease ducts shall comply with…
    57. 510.6 Exterior Installations. The exterior portion of the
    58. 510.7 Interior Installations. In all buildings more than one
    59. 510.8 Underground Installations. Grease ducts installed
    60. 510.9 Termination of Type I Hood Exhaust System.
    61. 510.10 Solid-Fuel Duct Systems. Where solid-fuel cooking equip…
    62. 511.1 Exhaust Fans for Commercial Cooking Opera-
    63. 511.2 Airflow. The air velocity through a duct shall be not
    64. 511.3 Makeup Air. The makeup air quantity shall prevent
    65. 511.4 Common Duct (Manifold) Systems. Master
    66. 511.5 Solid-Fuel Air Movement Requirements. Where
    67. 512.1 Dampers. Dampers shall not be installed in exhaust
    68. 512.2 Electrical Equipment. Wiring systems of any type
    69. 512.3 Other Equipment. Fume incinerators, thermal recovery uni…
    70. 512.4 Solid-Fuel Auxiliary Equipment. Where solid fuel
    71. 513.1 General. Fire-extinguishing equipment for the protection…
    72. 513.2 Types of Equipment. Fire-extinguishing equipment
    73. 513.3 Simultaneous Operation. Fixed pipe extinguishing
    74. 513.4 Fuel and Electric Power Shutoff. Upon activation of
    75. 513.5 Manual Activation. All systems shall have both automatic…
    76. 514.2 Inspection, Testing, and Maintenance. Maintenance of the…
    77. 513.6 System Annunciation. Upon activation of an automatic fir…
    78. 513.7 Special Design and Application. Hoods containing
    79. 513.8 Review and Certification. Where required, complete
    80. 513.9 Installation Requirements. Installation of systems
    81. 513.10 Portable Fire Extinguishers. Portable fire extinguisher…
    82. 513.11 Maintenance. Portable fire extinguishers shall be
    83. 513.12 Solid-Fuel Fire-Extinguishing Equipment.
    84. 514.1 Operating Procedures. Exhaust systems shall be
    85. 514.3 Inspection for Grease Buildup. The entire exhaust
    86. 514.4 Cleaning of Exhaust Systems. If, upon inspection,
    87. 514.5 Cooking Equipment Maintenance. Inspection and
    88. 515.1 Cooking Equipment. Cooking equipment shall be
    89. 515.2 Operating Controls. Deep-fat fryers shall be equipped
    90. 516.1 General Requirements. Recirculating systems containing o…
    91. 516.2 Design Restrictions. All recirculating systems shall
    92. 516.3 Interlocks. The recirculating system shall be provided
    93. 516.4 Location and Application Restrictions. The
    94. 516.5 Additional Fire Safety Requirements. In addition to the …
    95. 516.6 Use and Maintenance. Automatic or manual covers on cooki…
    96. 517.1 Venting Application. Venting requirements of solidfuel c…
    97. 517.2 Location of Appliances. Every appliance shall be
    98. 517.3 Hoods for Solid-Fuel Cooking. Hoods shall be sized
    99. 517.4 Exhaust Systems for Solid-Fuel Cooking. Where
    100. 517.5 Grease Removal Devices for Solid-Fuel Cook-
    101. 517.6 Air Movement for Solid-Fuel Cooking. Exhaust
    102. 517.7 Fire-Extinguishing Equipment for Solid-Fuel
    103. 517.8 Other Safety Requirements. Metal-fabricated solidfuel co…
    104. 518.1 General. Downdraft appliance ventilation system
    105. 518.2 Ventilation System. The downdraft appliance ventilation …
    106. 518.3 Fire-Extinguishing Equipment. For fire-extinguishing equ…
    107. 518.4 Airflow Switch or Transducer. An airflow switch or
    108. 518.5 Surface Materials. Any surface located directly
    109. 519.1 Where Required. Type II hoods shall be installed
    110. 519.2 Construction of Type II Hoods. Type II hoods
    111. 519.3 Dishwashing Appliances. The net airflow for Type
    112. 519.4 Type II Exhaust Duct Systems. Ducts and
    113. 519.5 Termination of Type II Hood Exhaust System.
    114. 519.6 Makeup Air. Makeup air shall be provided in accordance w…
  6. Chapter 6 — DUCT SYSTEMS
  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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