CMC · California Mechanical Code
What are the Type II hood and exhaust duct construction and termination requirements?
Type II hoods are required over non‑grease equipment that produces steam or heat; they must be built of specified metal thicknesses with tight seams, use rigid metallic ducts supported per Chapter 6 and sealed if under positive pressure, and must terminate outdoors following general termination rules — rooftop outlets must be directed away from roof surfaces within 40 inches and moisture-bearing exhausts (like dishwashers) cannot discharge over public ways (see §§ 519.1, 519.2, 519.4, 519.5) .
Last reviewed: July 6, 2026
What the code requires — 2–4 sentences
Type II hoods are required over appliances that generate steam, heat, or products of combustion where grease or smoke is not present — see § 519.1 . The hoods must be built of minimum specified metal thicknesses (for example, 0.024 inch (No. 24 gauge) steel or 0.17 oz/in² copper) with substantially tight joints and seams — see § 519.2 . Ducts and plenums serving Type II hoods must be rigid metallic materials constructed and supported per Chapter 6 and sealed where subject to positive pressure — see § 519.4 . Exhaust termination must follow the general termination rules and, for rooftop outlets, be directed away from roof surfaces within 40 inches (1016 mm); moisture‑conveying outlets (for example dishwasher exhaust) must not terminate over public ways or where condensate/vapor would be a nuisance or hazard — see § 519.5 .
The single most important point: Type II hoods are for non‑grease-producing steam/heat sources and must be metal hoods with rigid metal ducts that terminate outdoors safely (see § 519.1, § 519.2, § 519.4, § 519.5) .
Requirements in detail
Where Type II hoods are required (scope)
- Install a Type II hood above equipment and dishwashers that generate steam, heat, or products of combustion and where grease or smoke is not present — § 519.1 .
- Exceptions in § 519.1 include:
- Dishwashing machines that are connected to a Type II duct system and exhausted directly to the outdoors.
- Dishwashers with a listed self‑contained condensing system (UL 921) installed where HVAC is engineered to handle the heat/latent load; such units must have an interlock preventing opening before cycle completion — § 519.1 .
Construction of Type II hoods (materials & seams)
- Minimum materials:
- Steel hoods: not less than 0.024 inch (0.61 mm) (No. 24 gauge) — § 519.2 .
- Copper hoods: copper sheets weighing not less than 0.17 ounces per square inch (7.47 kg/m²) — § 519.2 .
- Joints/seams: must be substantially tight; solder is prohibited except for sealing a joint or seam — § 519.2 .
- (Note: Type I hood construction rules differ and are located elsewhere; only the Type II minima above are prescribed in § 519.2.)
Decision‑relevant dimensions & values
| Item | Value / Threshold | Typical use | Code Reference |
|---|---|---|---|
| Steel hood minimum thickness | 0.024 in (No. 24 gauge) | Type II hood body | § 519.2 |
| Copper hood minimum weight | 0.17 oz/in² (≈7.47 kg/m²) | Type II hood body | § 519.2 |
| Rooftop termination clearance from roof surfaces | 40 in (1016 mm) | Direction of rooftop termination | § 519.5 |
| Dishwashing hood min net airflow | 200 cfm per linear foot | Type II serving dishwashers (see § 519.3.1) | § 519.3.1 |
| Duct construction | Rigid metallic materials; bracing/support per Chapter 6 | Type II exhaust ducts/plenums | § 519.4 |
| Duct sealing (when positive pressure) | Adequately sealed | Ducts subject to positive pressure | § 519.4 |
Type II exhaust duct systems (material, supports, sealing)
- Ducts and plenums serving Type II hoods must be constructed of rigid metallic materials in accordance with Chapter 6 (see Chapter 6 for material tables, thicknesses, and installation requirements) — § 519.4 .
- Duct bracing and supports must comply with Chapter 6 requirements — § 519.4 .
- Ducts subject to positive pressure must be adequately sealed to prevent leakage — § 519.4 .
Termination requirements (discharge location & restrictions)
- The Type II exhaust system termination must be in accordance with Section 502.2.2 (general termination rules) — § 519.5 .
- Rooftop terminations associated with Type II systems shall be directed away from roof surfaces within 40 inches (1016 mm) — § 519.5 .
- Moisture‑conveying exhaust outlets (for example, dishwasher exhaust ducts) shall not terminate over a public way or where condensate or vapor could create a nuisance or hazard — § 519.5 .
Exceptions & special cases
- Dishwashing appliances have explicit exceptions: a dishwasher may be exempt from a hood if it is connected directly to a Type II duct to outdoors or if it uses a listed self‑contained condensing system (UL 921) with HVAC engineered to accept its heat load and an interlock to prevent opening during cycle — § 519.1 .
- The text of § 519.4 references Chapter 6 for detailed duct material and support requirements; designer/installer must consult Chapter 6 tables (materials/thickness/supports) for specifics — § 519.4 .
- Termination details sometimes depend on other general termination rules in Section 502.2.2; § 519.5 points you to that section for the full set of termination criteria (e.g., clearances to openings, plenums, etc.) — § 519.5 .
If you need the exact Chapter 6 thickness tables or the full language of Section 502.2.2, those are referenced by these sections but are outside the text specifically extracted here — consult Chapter 6 and Section 502.2.2 in the CMC for the detailed tables and termination figures.
Common mistakes
- Assuming Type II hoods can be lighter than the minimum: installing sheet metal thinner than 0.024 in (No. 24 gauge) for steel hoods violates § 519.2 .
- Using non‑rigid or non‑metallic ducts without verifying Chapter 6 allowances — § 519.4 requires rigid metallic materials unless another Chapter 6 provision allows otherwise .
- Failing to seal ducts that will operate under positive pressure (air leakage and loss of capture performance) — required by § 519.4 .
- Terminating dishwasher (moisture) exhaust over a sidewalk, alley, or other public way — prohibited by § 519.5 .
- Overlooking the exceptions in § 519.1 for condensing dishwashers or direct outdoor exhaust connections — leads to unnecessary hood installs or code non‑compliance .
Worked example — concrete scenario
Scenario: A restaurant has a commercial dishwasher that requires a Type II hood and exhaust. The hood opening length above the dishwasher is 6 feet.
Step 1 — Hood construction
- Use a steel hood at least 0.024 in thick (No. 24 gauge) per § 519.2 .
Step 2 — Airflow (dishwasher)
- For dishwashing appliances the code specifies a minimum net airflow of 200 cfm per linear foot of hood length — § 519.3.1. For a 6‑ft hood: 6 × 200 cfm/ft = 1,200 cfm required (see § 519.3.1) .
Step 3 — Duct system
- Design ducts of rigid metallic materials, sized and supported per Chapter 6; if the duct is under positive pressure, plan to seal joints to prevent leakage — § 519.4 .
Step 4 — Termination
- Terminate the exhaust in accordance with Section 502.2.2 and ensure any rooftop discharge is directed away from roof surfaces within 40 in (1016 mm). Do not terminate the dishwasher’s moisture‑laden outlet over a public way or where condensate would create a nuisance — § 519.5 .
Notes: If the dishwasher is listed with a self‑contained UL 921 condensing system and the HVAC has been engineered to accept its heat, it may be allowed without a hood per the exception in § 519.1 — confirm listing and AHJ approval .
Related provisions
- § 519.1 — Where required (Type II hood scope)
- § 519.2 — Construction of Type II hoods (material thicknesses and seams)
- § 519.3.1 — Dishwashing appliances minimum net airflow (200 cfm/ft)
- § 519.4 — Type II exhaust duct systems; references Chapter 6 for materials/supports; sealing when positive pressure applies
- § 519.5 — Termination of Type II hood exhaust systems; rooftop orientation and moisture‑conveying restrictions
- § 511.3 — Makeup air (design balance and limits for compensating hoods) — relevant when sizing exhaust and replacement air
- Chapter 6 (Ducts) — Material, thickness, supports, sealing, clearances and testing referenced by § 519.4 (see Chapter 6 tables and sections for detailed duct requirements)
Code references
Grounded in the retrieved California Mechanical Code — click a citation to read the verbatim passage:
CMC § 519.1 High relevance — show source text
519.1 Where Required. Type II hoods shall be installed above equipment and dishwashers that generate steam, heat, or products of combustion, and where grease or smoke is not present.
Exceptions:
(1) Dishwashing machines connected to a Type II duct system and exhausted directly to the outdoors.
(2) Dishwashing machines with a self-contained condensing system listed in accordance with UL 921 and installed in a space where the HVAC system has been engineered to accommodate the latent and sensible heat load emitted from such appliances as approved by the Authority Having Jurisdiction. Such equipment shall be provided with an interlocking device to prevent opening of the appliance prior to completion of its cycle. 519.2 Construction of Type II Hoods. Type II hoods constructed of steel shall be not less than 0.024 of an inch (0.61 mm) (No. 24 gauge). Hoods constructed of copper shall be of copper sheets weighing not less than 0.17 ounces per square inch (oz/in [2] ) (7.47 kg/m [2] ). Joints and seams shall be substantially tight. Solder shall not be used except for sealing a joint or seam. 519.3 Type II Hood Exhaust System Net Airflow. The net airflow for Type II hoods shall be in accordance with Section 508.5.1.5 for light-duty cooking appliances. The net airflow for Type II hoods serving dishwashing appliances shall comply with Section 519.3.1.
519.3.1 Dishwashing Appliances. The net airflow for Type II hoods used for dishwashing equipment shall be not less than 200 cubic feet per minute (0.094 m [3] /s) per linear foot (m) of hood length.
519.4 Type II Exhaust Duct Systems. Ducts and plenums serving Type II hoods shall be constructed of rigid metallic materials in accordance with Chapter 6. Duct bracing and supports shall comply with Chapter 6. Ducts subject to positive pressure shall be adequately sealed.
519.5 Termination of Type II Hood Exhaust System. The exhaust system shall terminate in accordance with Section 502.2.2. Rooftop terminations shall be directed away
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EXHAUST SYSTEMS
from roof surfaces within 40 inches (1016 mm). The discharge outlet of moisture conveying exhaust ducts, such as dishwasher exhaust ducts, shall not terminate over a public way or over an area where condensate or vapor could create a nuisance or hazard.
519.6 Makeup Air. Makeup air shall be provided in accordance with Section 511.3.
519.7 Independent Exhaust Duct System. Single or combined Type II exhaust systems shall be independent of all other exhaust systems.
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CALIFORNIA MECHANICAL CODE – MATRIX ADOPTION TABLE
CHAPTER 6 – DUCT SYSTEMS
CMC § 519.0 High relevance — show source text
519.0 Type II Hood Exhaust System Requirements . . . . . . . . . . . . . . . . 132
519.1 Where Required . . . . . . . . . . . . . . 132
519.2 Construction of Type II Hoods . . . 132
519.3 Type II Hood Exhaust System Net Airflow . . . . . . . . . . . . . . . . . . 132
519.4 Type II Exhaust Duct Systems . . . 132
519.5 Termination of Type II Hood Exhaust System . . . . . . . . . . . . . . 132
519.6 Makeup Air . . . . . . . . . . . . . . . . . . 133
519.7 Independent Exhaust Duct System . . . . . . . . . . . . . . . . . 133
CHAPTER 6 DUCT SYSTEMS . . . . . . . . . . . . . 135
601.0 General. . . . . . . . . . . . . . . . . . . . . 137
601.1 Applicability. . . . . . . . . . . . . . . . . . 137
601.2 Sizing Requirements . . . . . . . . . . 137
602.0 Material. . . . . . . . . . . . . . . . . . . . . 137
602.1 General. . . . . . . . . . . . . . . . . . . . . 137
602.2 Combustibles Within Ducts or
Plenums . . . . . . . . . . . . . . . . . . . . 137
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602.3 Tall Wood (Mass Timber) Buildings . . . . . . . . . . . . . . . . . . . . 137
602.4 Metallic . . . . . . . . . . . . . . . . . . . . . 138
602.5 Nonmetallic Ducts. . . . . . . . . . . . . 138
602.6 Vibration Isolators. . . . . . . . . . . . . 138
602.7 Corridors. . . . . . . . . . . . . . . . . . . . 138
603.0 Installation of Ducts . . . . . . . . . . . 138
603.1 General. . . . . . . . . . . . . . . . . . . . . 138
603.2 Under-Floor or Crawl Space. . . . . 138
603.3 Metal Ducts. . . . . . . . . . . . . . . . . . 138
603.4 Flexible Air Ducts . . . . . . . . . . . . . 138
603.5 Plastic Ducts. . . . . . . . . . . . . . . . . 139
603.6 Protection of Ducts . . . . . . . . . . . . 139
603.7 Support of Ducts. . . . . . . . . . . . . . 139
603.8 Protection Against Flood Damage . 139
603.9 Joints and Seams of Ducts. . . . . . 139
Table 603.9.1 Closure Markings . . . . . . . . . . . . . 140
CMC § 513.10 High relevance — show source text
(5) Portable fire extinguishers shall be provided in accordance with Section 513.10 through Section 513.11.
[NFPA 96:16.2]
518.3.1 Integral Fire-Extinguishing System. A listed downdraft appliance ventilation system employing an integral fire-extinguishing system including detection systems that has been evaluated for grease and smoke capture, fire extinguishing, and detection shall be considered as complying with Section 518.3. [NFPA 96:16.2.1]
518.3.2 Interlocks. The downdraft appliance ventilation system shall be provided with interlocks such that the cooking fuel supply will not be activated unless the exhaust and supply air systems have been activated.
[NFPA 96:16.2.2]
518.4 Airflow Switch or Transducer. An airflow switch or transducer shall be provided after the last filter component to ensure that a minimum airflow is maintained. [NFPA 96:16.3.1]
518.4.1 Interlocks. The airflow switch or transducer shall open the interlock circuit when the airflow falls 25 percent below the system’s normal operating flow or less than 10 percent its listed minimum rating, whichever is lower. [NFPA 96:16.3.2]
518.4.2 Manual Reset. The airflow switch or transducer shall be a manual reset device or circuit. [NFPA 96:16.3.3]
518.5 Surface Materials. Any surface located directly above the cooking appliance shall be of noncombustible or limited-combustible materials. [NFPA 96:16.4]
519.0 Type II Hood Exhaust System Requirements.
519.1 Where Required. Type II hoods shall be installed above equipment and dishwashers that generate steam, heat, or products of combustion, and where grease or smoke is not present.
Exceptions:
(1) Dishwashing machines connected to a Type II duct system and exhausted directly to the outdoors.
(2) Dishwashing machines with a self-contained condensing system listed in accordance with UL 921 and installed in a space where the HVAC system has been engineered to accommodate the latent and sensible heat load emitted from such appliances as approved by the Authority Having Jurisdiction. Such equipment shall be provided with an interlocking device to prevent opening of the appliance prior to completion of its cycle. 519.2 Construction of Type II Hoods. Type II hoods constructed of steel shall be not less than 0.024 of an inch (0.61 mm) (No. 24 gauge). Hoods constructed of copper shall be of copper sheets weighing not less than 0.17 ounces per square inch (oz/in [2] ) (7.47 kg/m [2] ). Joints and seams shall be substantially tight. Solder shall not be used except for sealing a joint or seam. 519.3 Type II Hood Exhaust System Net Airflow. The net airflow for Type II hoods shall be in accordance with Section 508.5.1.5 for light-duty cooking appliances. The net airflow for Type II hoods serving dishwashing appliances shall comply with Section 519.3.1.
519.3.1 Dishwashing Appliances. The net airflow for Type II hoods used for dishwashing equipment shall be not less than 200 cubic feet per minute (0.094 m [3] /s) per linear foot (m) of hood length.
CMC § 516.4 High relevance — show source text
516.4 Location and Application Restrictions. . . . . . . . . . . . . . . . . . 129
516.5 Additional Fire Safety Requirements . . . . . . . . . . . . . . . . 129
516.6 Use and Maintenance. . . . . . . . . . 129
517.0 Solid-Fuel Cooking Operations. . . 129
517.1 Venting Application. . . . . . . . . . . . 129
517.2 Location of Appliances . . . . . . . . . 130
517.3 Hoods for Solid-Fuel Cooking. . . . 130
517.4 Exhaust Systems for Solid-Fuel Cooking . . . . . . . . . . . . 130
517.5 Grease Removal Devices
for Solid-Fuel Cooking . . . . . . . . . 130
517.6 Air Movement for Solid-Fuel
Cooking . . . . . . . . . . . . . . . . . . . . 130
517.7 Fire-Extinguishing Equipment for Solid-Fuel Cooking . . . . . . . . . 131
517.8 Other Safety Requirements . . . . . 131
518.0 Downdraft Appliances. . . . . . . . . . 131
518.1 General. . . . . . . . . . . . . . . . . . . . . 131
518.2 Ventilation System . . . . . . . . . . . . 132
518.3 Fire-Extinguishing Equipment. . . . 132
518.4 Airflow Switch or Transducer . . . . 132
518.5 Surface Materials . . . . . . . . . . . . . 132
519.0 Type II Hood Exhaust System Requirements . . . . . . . . . . . . . . . . 132
519.1 Where Required . . . . . . . . . . . . . . 132
519.2 Construction of Type II Hoods . . . 132
519.3 Type II Hood Exhaust System Net Airflow . . . . . . . . . . . . . . . . . . 132
519.4 Type II Exhaust Duct Systems . . . 132
519.5 Termination of Type II Hood Exhaust System . . . . . . . . . . . . . . 132
519.6 Makeup Air . . . . . . . . . . . . . . . . . . 133
519.7 Independent Exhaust Duct System . . . . . . . . . . . . . . . . . 133
CHAPTER 6 DUCT SYSTEMS . . . . . . . . . . . . . 135
601.0 General. . . . . . . . . . . . . . . . . . . . . 137
601.1 Applicability. . . . . . . . . . . . . . . . . . 137
601.2 Sizing Requirements . . . . . . . . . . 137
602.0 Material. . . . . . . . . . . . . . . . . . . . . 137
602.1 General. . . . . . . . . . . . . . . . . . . . . 137
602.2 Combustibles Within Ducts or
Plenums . . . . . . . . . . . . . . . . . . . . 137
CMC § 519.4 High relevance — show source text
519.4 Type II Exhaust Duct Systems. Ducts and plenums serving Type II hoods shall be constructed of rigid metallic materials in accordance with Chapter 6. Duct bracing and supports shall comply with Chapter 6. Ducts subject to positive pressure shall be adequately sealed.
519.5 Termination of Type II Hood Exhaust System. The exhaust system shall terminate in accordance with Section 502.2.2. Rooftop terminations shall be directed away
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EXHAUST SYSTEMS
from roof surfaces within 40 inches (1016 mm). The discharge outlet of moisture conveying exhaust ducts, such as dishwasher exhaust ducts, shall not terminate over a public way or over an area where condensate or vapor could create a nuisance or hazard.
519.6 Makeup Air. Makeup air shall be provided in accordance with Section 511.3.
519.7 Independent Exhaust Duct System. Single or combined Type II exhaust systems shall be independent of all other exhaust systems.
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CALIFORNIA MECHANICAL CODE – MATRIX ADOPTION TABLE
CHAPTER 6 – DUCT SYSTEMS
(Matrix Adoption Tables are non-regulatory, intended only as an aid to the code user. See Chapter 1 for state agency authority and building applications.)
Adopting Agency BSC BSC-
CGSFM HCD Col6 Col7 DSA Col9 Col10 OSHPD Col12 Col13 Col14 Col15 Col16 Col17 BSCC DPH AGR DWR CEC CA SL SLC Adopting Agency BSC BSC-
CGSFM 1 2 1-AC AC ** SS** ** SS/CC** 1 1R 2 3 4 5 6 6 6 6 6 6 6 6 6 Adopt Entire Chapter X X X Adopt Entire Chapter as
amended (amended sections
listed below)X X X X X X X X X Adopt only those sections
that are listed belowX Chapter/Section _601.2. CMC § 5.1.5 High relevance — show source text
(2) Penetrations shall be permitted to be sealed by devices that are listed for such use and whose presence does not detract from the hood’s or duct’s structural integrity. [NFPA 96:5.1.5]
508.3.2.1 Sealed. Internal hood joints, seams, filter support frames, and appurtenances attached inside the hood shall be sealed or otherwise made greasetight. [NFPA 96:5.1.4]
508.3.3 Eyebrow-Type Hoods. Eyebrow-type hoods over gas or electric ovens shall be permitted to have a duct constructed as required in Section 510.0 from the oven flue(s) connected to the hood canopy upstream of the exhaust plenum, as shown in Figure 508.3.3. [NFPA 96:5.1.8.1]
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EXHAUST SYSTEMS
FIGURE 508.3.3
TYPICAL SECTION OF EYEBROW-TYPE HOOD
[NFPA 96: FIGURE 5.1.8.1]
508.3.3.1 Duct Connection. The duct connect ing the oven flue(s) to the hood canopy shall be connected with a continuous weld or have a duct-to-duct
connection. [See Figure 511.1.2(2) through Figure 511.1.2(4)] [NFPA 96:5.1.8.2]
508.3.4 Insulation. Insulation materials other than
electrical insulation shall have a flame spread index of 25 or less, when tested in accordance with ASTM E84 or UL 723. Adhesives or cements used in the installation of
insulating materials shall comply with the requirements of this section when tested with the specific insulating material. [NFPA 96:5.1.9, 5.1.10]
508.3.5 Exhaust Hood Assemblies with Integrated Supply-Air Plenums. The construction and size of exhaust hood assemblies with integrated supply air plenums shall comply with the requirements of Section 508.2.3 through Section 508.3.4 and Section 508.5. [NFPA 96:5.3.1]
508.3.5.1 Outer Shell. The construction of the outer
shell or the inner exhaust shell shall comply with Section 508.2.3 through Section 508.3.4. [NFPA 96:5.3.2]
508.3.5.2 Inner Shell. Where the outer shell is
welded, the inner shell shall be of greasetight construction. [NFPA 96:5.3.3]
508.3.5.3 Fire Dampers. A fire-actuated damper shall be installed in the supply air plenum at each point where a supply air duct inlet or a supply air outlet penetrates the continuously welded shell of the assembly. [NFPA 96:5.3.4.1]
508.3.5.3.1 Listing. The fire damper shall be listed for such use or be part of a listed exhaust hood with or without exhaust damper. [NFPA 96:5.3.4.2]
CMC § 505.3. High relevance — show source text
The design of the hood or enclosure shall be such that air currents created by the exhaust systems will capture the contaminants and transport them directly to the exhaust duct. The volume of air shall be sufficient to dilute explosive or flammable vapors, fumes, or dusts in accordance with Section 505.3. Hoods of steel shall have a base metal thickness not less than 0.027 of an inch (0.686 mm) (No. 22 gauge) for Class 1 and Class 5 metal duct systems; 0.033 of an inch (0.838 mm) (No. 20 gauge) for hoods serving a Class 2 duct system; 0.044 of an inch (1.118 mm) (No. 18 gauge) for hoods serving a Class 3 duct system; and 0.068 of an inch (1.727 mm) (No. 14 gauge) for hoods serving a Class 4 duct system.
Approved nonmetallic hoods and duct systems shall be permitted to be used for Class 5 corrosive systems where the corrosive mixture is nonflammable. Metal hoods used with Class 5 duct systems shall be protected with an approved corrosion-resistant material. Edges of hoods shall be rounded. The minimum clearance between hoods and combustible construction shall be the clearance required by the duct system.
505.12 Pharmacies – Compounding Area of Par- enteral Solutions. [CA – Board of Pharmacy] The pharmacy shall have a designated area for the preparation of sterile products for dispensing which shall be ventilated in a manner not interfering with laminar air flow.
Note: For additional pharmacy building standard require- ments, see Chapter 12, California Building Code.
505.12.1 P harmacies – Laminar Flow Biological Safety Cabinet. [CA – Board of Pharmacy] In all pharmacies preparing parenteral cytotoxic agents, all com- pounding shall be conducted within a certified Class II Type A or Class II Type B vertical laminar airflow hood with bag in – bag out design. The pharmacy must ensure that con-
taminated air plenums that are under positive air pressure are leak tight. Note: For additional pharmacy building standard requirements, see Chapter 12, California Building Code.
506.0 Product-Conveying Ducts.
506.1 Materials. Materials used in product-conveying duct systems shall be suitable for the intended use and shall be of rigid sheet metal.
Exceptions:
(1) Asbestos-cement, concrete, clay, or ceramic materials shall be permitted to be used where it is shown that these materials will be equivalent to metal ducts installed in accordance with this chapter.
(2) Ducts serving a Class 5 system shall be permitted to be constructed of approved nonmetallic material where the corrosive characteristics of the material being conveyed make a metal system unsuitable and where the mixture being conveyed is nonflammable.
Approved nonmetallic material shall be either a listed product having a flame-spread index not exceeding 25 and a smoke-developed rating of 50 or less on both inside and outside surfaces without evidence of continued progressive combustion, or shall have a flame-spread index not exceeding 25 and shall be installed with an automatic fire-sprinkler protection system inside the duct.
CMC § 511.3 High relevance — show source text
E NET = E HOOD – MA ID (Equation 511.3)
Where:
E NET = net hood exhaust, CFM (L/s)
E HOOD = total hood exhaust, CFM (L/s)
MA ID = makeup air, internal discharge, CFM (L/s)
For SI units: 1 cubic foot per minute = 0.0283 m [3] /min, 1 cubic foot per
minute = 0.4719 L/s
511.3.1 Air Balance. Design plans for a facility with a commercial kitchen ventilation system shall include a schedule or diagram indicating the design outdoor air balance. The design outdoor air balance shall indicate the exhaust and replacement air for the facility and the net exfiltration where applicable. The total replacement airflow rate shall equal the total exhaust airflow rate and the net exfiltration.
511.4 Common Duct (Manifold) Systems. Master kitchen exhaust ducts that serve multiple tenants shall include provision to bleed air from outdoors or from adjacent spaces into the master exhaust duct where required to maintain the necessary minimum air velocity in the master exhaust duct.
[NFPA 96:8.4.1]
511.4.1 Connections. Bleed air ducts shall connect to the top or side of the master exhaust duct. [NFPA 96:8.4.2] 511.4.2 Fire Damper. The bleed-air duct shall have a fire damper at least 12 inches (305 mm) from the exhaust duct connection. [NFPA 96:8.4.3]
511.4.3 Construction and Clearance. The bleed-air
duct shall have the same construction and clearance requirements as the main exhaust duct from the connection to the exhaust duct to at least 12 inches (305 mm) on both sides of the fire damper. [NFPA 96:8.4.4] 511.4.4 Adjustment. Each bleed air duct shall have a means of adjusting (e.g., by using volume dampers) the bleed air quantity. [NFPA 96:8.4.5] 511.4.5 Adjustment Location. Means to adjust the bleed air quantity shall be installed between the fire damper and the source of bleed air. [NFPA 96:8.4.6]
511.4.6 Bleed Air Duct. A bleed air duct shall not be used for the exhaust of grease-laden vapors and shall be so labeled. [NFPA 96:8.4.7]
511.4.7 Disconnect. Unused tenant exhaust connec tions to the master exhaust duct that are not used as bleed
air connections shall be disconnected and sealed at the main duct. [NFPA 96:8.4.8] 511.5 Solid-Fuel Air Movement Requirements. Where solid-fuel cooking equipment is used, exhaust and replacement air also shall be in accordance with Section 517.0.
512.0 Auxiliary Equipment.
512.1 Dampers. Dampers shall not be installed in exhaust ducts or exhaust duct systems. [NFPA 96:9.1.1]
CMC § 505.8 Medium relevance — show source text
505.8 Product-Conveying Duct Classifications. . . . . . . . . . . . . . . . 102
505.9 Minimum Velocities and
Circulation. . . . . . . . . . . . . . . . . . . 102
505.10 Makeup Air . . . . . . . . . . . . . . . . . . 102
505.11 Hoods and Enclosures . . . . . . . . . 103
505.12 Pharmacies – Compounding Area of Parenteral Solutions . . . . 103
506.0 Product-Conveying Ducts. . . . . . . 103
506.1 Materials . . . . . . . . . . . . . . . . . . . . 103
Table 505.9 Range of Minimum Duct Design Velocities. . . . . . . . . . . . . . . . . . . . 103
Table 506.2(1) Minimum Sheet Metal Thickness for Round Ducts . . . . . . . . . . . . . . 104
506.2 Construction . . . . . . . . . . . . . . . . . 105
Table 506.2(2) Minimum Sheet Metal Thickness for Rectangular Ducts. . . . . . . . . . 106
506.3 Penetrations . . . . . . . . . . . . . . . . . 106
506.4 Condensate . . . . . . . . . . . . . . . . . 107
506.5 Fittings . . . . . . . . . . . . . . . . . . . . . 107
506.6 Explosion Venting. . . . . . . . . . . . . 107
506.7 Supports . . . . . . . . . . . . . . . . . . . . 107
506.8 Fire Protection . . . . . . . . . . . . . . . 107
506.9 Protection from Physical Damage . . . . . . . . . . . . . . . . . . . . 107
xxxi
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TABLE OF CONTENTS
506.10 Duct Clearances . . . . . . . . . . . . . . 107
Table 506.10.4 Basic Minimum Clearances to
Unprotected Surfaces. . . . . . . . . . 107
506.11 Clearance Reduction Methods . . . 108
Table 506.11 Reduction of Duct Clearance with
Specified Forms of Protection . . . 108
Part II Commercial Hoods and
Kitchen Ventilation . . . . . . . . . . . 109
507.0 General Requirements . . . . . . . . . 109
507.1 Exhaust Systems . . . . . . . . . . . . . 109
507.2 Type I Hood Exhaust System Requirements . . . . . . . . . . . . . . . . 109
507.3 Listed Devices . . . . . . . . . . . . . . . 109
507.4 Clearance . . . . . . . . . . . . . . . . . . . 109
507.5 Drawings. . . . . . . . . . . . . . . . . . . . 110
CMC § 903.2.7 Medium relevance — show source text
SUSTAINABLE PRACTICES . . . . . . . . . . . . . . .Appendix E SWITCHES, ELECTRICAL SUPPLY LINE . . . . . . .903.2.7
– T –
TANKLESS WATER HEATERS . . . . . . . . . . . . . . . . .1203.3
TERMINATION
Chimneys . . . . . . . . . . . . . . . . . . . . .802.5.4, 802.5.4.2
Environmental air ducts . . . . . . . . . . . . . . . . . . .502.2.1
Exhaust duct systems,
commercial kitchen . . . . . . . . . . . . . . . . .510.9, 519.5
Exhaust
ducts, clothes dryer . . . . . . . . . . . . . . . . . . . . . .504.4 Gas vents . . . . . . . . . . . . . . . . . . . . . . . .802.6, 802.6.1,
802.7.2, 802.8, 802.8.1
Product conveying ducts . . . . . . . . . . . . . . . . . .502.2.3 Type I hood exhaust system . . . . . . . . . . . . . . . . .510.9 Type II hood exhaust system . . . . . . . . . . . . . . . . .519.5
TESTING
Of boilers . . . . . . . . . . . . . . . . . . . . . . . . .1012.1, 1013.0
Of ducts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .602.2
Of exhaust systems . . . . . . . . . . . . . . . . . . . . . . . .514.0 Of fuel-gas piping . . . . . . . . . . . . . . . . . . .105.3, 1313.0
Of hood assemblies . . . . . . . . . . . . . . . . . . . . . . .508.2
Of insulating materials . . . . . . . . . . . . . .508.3.4, 602.2
Of louvers . . . . . . . . . . . . . . . . . . . . . . .315.1.1, 315.1.2
Of recirculating systems . . . . . . . . . . .516.2.2, 516.2.5,
516.6.4
Of refrigerating equipment . . . . . . . . . . . .105.3, 1114.3,
1116.0
Of safety interlocks . . . . . . . . . . . . . . . . . . . . . . .516.6.4
Required by the Authority
Having Jurisdiction . . . . . . . . . . . . . . . . . . . . .302.2.1
TESTING AGENCY
Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .222.0
CMC § 25.4 Medium relevance — show source text
For SI units: 1 inch = 25.4 mm, °C = (°F-32)/1.8
FIGURE 511.1.2(1) TYPICAL SECTION OF DUCT-TO-FAN
CONNECTION-BUTT JOINT METHOD
[NFPA 96: FIGURE 8.1.3.2(a)]
For SI units: 1 inch = 25.4 mm, °C = (°F-32)/1.8
FIGURE 511.1.2(2) TYPICAL SECTION OF DUCT-TO-FAN
CONNECTION-OVERLAPPING METHOD
[NFPA 96: FIGURE 8.1.3.2(b)]
and exhaust fans that do not meet this velocity. [NFPA 96:8.2.1.2]
511.2.2 Exhaust-Air Volumes. Exhaust air volumes for hoods shall be of sufficient level to provide for capture and removal of grease-laden cooking vapors. Test data, performance tests acceptable to the Authority Having Jurisdiction, or both shall be displayed, provided on request, or both. [NFPA 96:8.2.2.1, 8.2.2.2] Lower exhaust air volumes shall be permitted during no-load and partial load cooking conditions, provided they are sufficient to capture and remove flue gases and cooking effluent from cooking equipment.
For SI units: 1 inch = 25.4 mm, °C = (°F-32)/1.8)
FIGURE 511.1.2(3) TYPICAL SECTION OF DUCT-TO-FAN
CONNECTION–SEALANT METHOD
[NFPA 96: FIGURE 8.1.3.2(c)]
FIGURE 511.1.2(4) TYPICAL SECTION OF DUCT-TO-FAN CONNECTION-DIRECT
TO FAN INLET CONE METHOD
[NFPA 96: FIGURE 8.1.3.2(d)]
2025 CALIFORNIA MECHANICAL CODE 121
), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.
EXHAUST SYSTEMS
511.2.2.1 Performance Test. A performance test shall be conducted upon completion and before final approval of the installation of a ventilation system serving commercial cooking appliances. The test shall verify the rate of exhaust airflow in accordance with Section 508.5.1.2 through Section 508.5.1.5. The permit holder shall furnish the necessary test equipment and devices required to perform the tests.
[ASHRAE 154:4.7.1]
511.2.2.2 Capture and Containment Test. The permit holder shall verify the capture and containment performance of Type I hoods. A field test shall be conducted with all appliances under the hood at operating temperatures, all the hoods operating at design airflows, and with all sources of replacement air operating at design airflows for the restaurant. Capture and containment shall be verified visually by observing smoke or steam produced by actual cooking operation or by simulating cooking using devices such as smoke candles or smoke puffers. Smoke bombs shall not be used. [ASHRAE 154:4.7.2]
CMC § 517.3.1 Medium relevance — show source text
517.3.1 Separation. Except as permitted in Section 517.3.1.1, exhaust systems serving solid-fuel cooking equipment in buildings, including gas or electrically operated equipment, shall be separate from all other exhaust systems. [NFPA 96:15.3.3]
Exception: Cooking equipment not requiring automatic fire-extinguishing equipment (per Section 513.0) shall be permitted to be installed under a common hood with solid-fuel cooking equipment that is served by a duct system separate from all other exhaust systems. [NFPA 96:15.3.5]
517.3.1.1 Equipment with Solid Fuel for Fla- voring. Gas-operated equipment utilizing solid fuel for flavoring that meets all the following conditions shall not be required to have a separate exhaust sys tem:
(1) The solid fuel holder (smoker box) shall be listed with the gas-operated equipment.
(2) The solid fuel holder shall be located underneath the gas burners.
(3) Spark arresters conforming with Section 517.1.6 shall be provided.
(4) The maximum quantity of solid fuel consumed shall not exceed 1 pound (0.45 kg) per hour per 100 000 Btu/h (29 kW) of gas burner capacity.
(5) The gas-operated equipment shall be protected by a fire suppression system listed for the equipment, including the solid fuel holder.
(6) Gas-operated equipment with integral solid fuel holder(s) intended for flavoring, such as radiant charbroiler(s), shall comply simultaneously with the requirements of UL 300 that address the gas radiant charbroiler(s) and mesquite wood charbroiler(s).
(7) A fire suppression system nozzle(s) shall be installed to protect the solid fuel holder.
(8) The fire suppression system shall be designed and installed to protect the entire cooking operation.
(9) Each solid fuel holder shall be limited to a size of 150 cubic inches (2.5 L), with no dimension to exceed 20 inches (508 mm).
(10) A maximum of one solid fuel holder for each 100 000 Btu/h (29 kW), or portion thereof, of burner capacity shall be permitted.
(11) Solid fuel shall be immersed in water for a continuous period of at least 24 hours immediately prior to being placed in the cooking equipment.
(12) The inspection frequency shall be the same as for solid fuel cooking operations in Table 514.3.
[NFPA 96:15.3.4] 517.4 Exhaust Systems for Solid-Fuel Cooking. Where a hood is not required, in buildings where the duct system is three stories or less in height, a duct complying with Section 510.0 shall be provided. [NFPA 96:15.4] 517.4.1 Hood. If a hood is used in buildings where the duct system is three stories or less in height, the duct system shall comply with Section 510.0. [NFPA 96:15.4.1] 517.4.2 Building Exceeding Four Stories. A listed or approved grease duct system that is four stories in height or greater shall be provided for solid-fuel cooking exhaust systems. [NFPA 96:15.4.2]
517.4.3 Prohibited. Wall terminations of solid-fuel exhaust systems shall be prohibited. [NFPA 96:15.4.4]
Frequently asked questions
Can a Type II hood be made from lighter sheet metal than No. 24 gauge?
No. For steel Type II hoods the code requires a minimum thickness of 0.024 in (No. 24 gauge); copper has its own minimum (0.17 oz/in²). See § 519.2 .
Do Type II ducts have to be metallic?
Yes — § 519.4 requires ducts and plenums serving Type II hoods to be constructed of rigid metallic materials in accordance with Chapter 6, unless another Chapter 6 provision allows an approved alternative .
Where can a dishwasher exhaust terminate?
Dishwasher (moisture‑conveying) exhaust must not terminate over a public way or where condensate/vapor would create a nuisance or hazard, and rooftop terminations must be directed away from roof surfaces within 40 in (1016 mm) — § 519.5 .
If my dishwasher has a listed condensing unit, do I still need a hood?
If the dishwasher has a listed self‑contained condensing system (UL 921) and the HVAC is engineered to accept the appliance’s latent and sensible heat load (and the unit has the required interlock), the hood requirement may be waived per the exception in § 519.1 — approval by the Authority Having Jurisdiction is required .
Do ducts under positive pressure need special treatment?
Yes. Ducts subject to positive pressure must be adequately sealed to prevent leakage and maintain system performance — § 519.4 .
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