CMC · California Mechanical Code
Product-conveying, flammable and abrasive exhaust systems
If your operation produces flammable vapors, dusts, corrosive fumes, or abrasive particulate, the California Mechanical Code requires capture and direct exhaust to a safe exterior location, limits flammable concentrations to 25% of a substance’s LFL unless special NFPA protections are used, forbids mixing incompatible materials in the same duct, and restricts where fire dampers and fan shutdowns can be used; see §§ 505.1–505.7 for the specific rules.
Last reviewed: July 6, 2026
What the code requires — plain English
A mechanical ventilation/exhaust system that captures and removes emissions where they create a hazard must confine and exhaust the emissions directly to a safe location or treat them; ducts conveying explosive or flammable vapors, fumes, or dusts generally must run directly to the exterior and not pass through other spaces. See § 505.1.
Systems must not carry incompatible materials together, and concentrations of flammable vapors, gases or mists in a conveying system must not exceed 25 percent of the lower flammability limit (LFL) unless protected or designed per NFPA standards. See § 505.2 and § 505.3.
The single most important rule: keep flammable / explosive emissions confined and exhausted to a safe location, and keep concentrations at or below 25% LFL unless you use NFPA 69/86/68 protections.
Requirements in detail
1) Scope and general obligation
A mechanical ventilation/exhaust system is required where emissions from product use or handling create a hazard to life or property; the system must capture, confine and exhaust the emissions or remove contaminants before recirculation. See § 505.1.
Ducts conveying explosive or flammable vapors, fumes or dusts must extend directly to the exterior and shall not enter or pass through other spaces or plenums. See § 505.1.
Exception: ducts conveying vapors/fumes with flammable constituents below 25% of the LFL may pass through other spaces (see § 505.1 exception). Bold threshold: 25% of LFL.
2) Incompatible materials
- Incompatible materials shall not be conveyed in the same system (e.g., oxidizers and combustibles). See § 505.2.
3) Flammability limits and higher concentrations
Normal limit: contaminated streams conveying flammable vapors/gases/mists must not exceed 25% of the LFL. See § 505.3.
Higher concentrations are permitted only when the exhaust system is designed and protected in accordance with NFPA 69 using strategies such as:
- combustible concentration reduction,
- oxidant concentration reduction,
- deflagration suppression, or
- deflagration pressure containment.
(See § 505.3.1.)
Ovens/furnaces (NFPA 86) and deflagration venting (NFPA 68) provide additional allowed paths for higher concentrations per § 505.3.2 and § 505.3.3.
4) Fire dampers, detection, control & shutdown
Fire dampers are permitted to be installed in exhaust systems only in the situations listed in § 505.6, including:
- where ducts pass through fire barriers,
- where a collection system at the end is protected with automatic extinguishing,
- where the duct system itself is protected with an automatic extinguishing system,
- where ducts are listed with interrupters. See § 505.6.
Fire detection and shutdown rules in § 505.7:
- Interlocks are permitted to shut down air-moving devices where needed for the effective operation of automatic extinguishing systems (see § 505.7.1).
- Shutdown by fire detection is permitted when a documented risk analysis (accepted by the AHJ) shows greater damage would occur if fans remain running (see § 505.7.2).
- Smoke control systems must be designed per NFPA 92 and use UL 864 control units where required (see § 505.7.3).
5) Duct classification and capture/airflow
Product-conveying ducts are classified Class 1–5 by what they carry (brief summaries below) per § 505.8:
- Class 1 — nonabrasives: smoke, mists, light fine dusts.
- Class 2 — moderately abrasive particulate in light concentrations (e.g., sawdust).
- Class 3 — Class 2 materials in high concentration or highly abrasive materials in low concentration.
- Class 4 — high concentrations of highly abrasive materials (e.g., sand).
- Class 5 — corrosives (acid vapors).
Minimum duct velocities and the capture airflow requirements are specified in Table 505.9 and § 505.9; systems conveying particulate matter must be designed by constant-velocity method; explosive/radioactive systems require pre‑balancing. See § 505.9 and Table reference.
Decision-relevant dimensions/values (where to look in the code)
| Decision or value | Typical action you take | Code Reference |
|---|---|---|
| Maximum normal flammable concentration in duct | Keep ≤ 25% of LFL unless NFPA protections used | § 505.3 |
| Allowed duct routing through other spaces when flammable content is low | ≤ 25% LFL allows passing through other spaces (exception) | § 505.1 (exception) |
| Where fire dampers may be used | Only in the specific situations listed (fire barriers, protected collection systems, protected duct system, listed interrupters) | § 505.6 |
| When fans may be shut down by fire systems | Interlock allowed for extinguisher effectiveness or after AHJ‑approved risk analysis | § 505.7.1–505.7.2 |
| Duct classification (Class 1–5) — choose materials/construction | Select duct materials and construction per the material class; corrosives are Class 5 | § 505.8 |
| Minimum duct design velocities and capture airflow | See Table 505.9 and § 505.9 for the required velocities and constant‑velocity design | § 505.9 / Table 505.9 |
Note: numeric velocities and the full ranges are given in Table 505.9 in the code. The table values are referenced in the code text but were not reproduced in the preview I accessed; consult Table 505.9 in the CMC for exact velocities.
Exceptions & special cases
Passing through other spaces: Ducts carrying flammable vapors/fumes with constituents less than 25% of the LFL are permitted to pass through other spaces (exception to § 505.1).
Higher concentrations: If concentration exceeds 25% LFL, higher concentrations are allowed only when the system is designed/protected per NFPA 69, or other specific NFPA provisions (NFPA 86 for ovens/furnaces, NFPA 68 for venting) as listed in § 505.3.1–505.3.3.
Fire dampers: § 505.6 lists only certain circumstances where fire dampers are permitted in exhaust systems; otherwise, avoid adding dampers in product-conveying ducts unless one of the listed conditions applies.
Shutdown interlocks: Shutdown by detection/alarm is permitted but must follow either the automatic extinguishing system interlock rule or a risk analysis accepted by the Authority Having Jurisdiction per § 505.7.1–505.7.2.
Common mistakes
Treating “less than 25% LFL” as a guideline rather than a hard threshold for routing through other spaces — the CMC uses 25% of LFL as the dividing point for allowable exceptions (see § 505.3 and § 505.1 exception).
Manifold-ing hot-work or spark‑generating operations into the same exhaust that handles flammable material — the code forbids manifold-ing operations that generate sparks/hot materials into ducts conveying flammable/combustible materials (see § 505.5).
Installing fire dampers or other closures in product-conveying ducts without confirming the installation falls into one of the permitted conditions in § 505.6.
Recirculating contaminated air back into occupied spaces without pollutant removal — air with flammable/explosive vapors, toxic gases, or radioactive material must not be recirculated (see § 505.3).
Selecting duct materials that are incompatible with Class 5 corrosive streams or using aluminum/galvanized where prohibited by temperature/material restrictions — check § 506.1 and the material rules for product ducts (see Related provisions).
Worked example — vapour concentration check and routing decision
Scenario: A small solvent‑handling hood exhaust measures an in‑duct solvent vapor concentration of 1.1% by volume; the solvent’s Safety Data Sheet lists its LFL = 4.4% by volume (SDS source, not the code).
Step 1 — compute percent of LFL:
- 1.1% ÷ 4.4% = 0.25 → 25% of LFL
Step 2 — apply the CMC rule:
- Because the in‑duct concentration is exactly 25% of the LFL, it meets the normal allowable limit in § 505.3 (the code prohibits concentrations that exceed 25% of LFL unless NFPA protections are applied). If concentrations exceeded 25%, you would need NFPA 69 protections or other permitted NFPA measures per § 505.3.1.
Step 3 — routing decision:
- At 25% LFL or less, the exception in § 505.1 may permit the duct to pass through other spaces (subject to AHJ approval and other code requirements). If you plan to route the duct through other spaces, document the measured concentration and confirm AHJ acceptance. See § 505.1 (exception) and § 505.3.
Caveats:
- LFL values come from the chemical’s SDS/MSDS; the code references the LFL but does not publish material-specific LFLs. Follow the SDS and confirm test methods/locations for in‑duct sampling. See § 505.3.
Related provisions (select)
- § 505.1 — General product-conveying system requirements and routing to exterior (see the general capture/confinement rule and exception).
- § 505.2 — Incompatible materials shall not be conveyed in the same system.
- § 505.3 — Flammability limit: 25% of LFL and provisions for higher concentrations (NFPA 69/86/68 references).
- § 505.6 — Fire damper conditions permitted in exhaust systems.
- § 505.7 — Fire detection, smoke control and alarm interlocks; NFPA 92/UL 864 references.
- § 505.8 — Product‑conveying duct classifications (Class 1–5).
- § 505.9 / Table 505.9 — Minimum duct velocities and constant‑velocity design requirement.
- § 506.1 — Materials and construction requirements for product‑conveying ducts (see metal vs nonmetallic rules).
- § 502.2.3 — Termination distances for product‑conveying / flammable exhausts (e.g., 30 ft distances specified for Class 4 / flammable ducts) — relevant for where the duct must discharge outdoors.
Code references
Grounded in the retrieved California Mechanical Code — click a citation to read the verbatim passage:
CMC § 504.5 High relevance — show source text
shaft for lint removal.
504.5 Heat Recovery Ventilation (HRV) and Energy Recovery Ventilation (ERV) Systems. Heat recovery ventilators (HRV) and energy recovery ventilators (ERV) shall be installed in accordance with their listings and comply with the appliance manufacturer’s installation instructions. Non-ducted heat recovery ventilators shall comply with UL 1815. Ducted heat recovery ventilators shall comply with UL 1812. Heat recovery ventilator and energy recovery ventilator ducts shall comply with Chapter 6.
505.0 Product-Conveying Systems.
505.1 General. A mechanical ventilation or exhaust system shall be installed to control, capture, and remove emissions generated from product use or handling where required in accordance with the building code or fire code and where such emissions result in a hazard to life or property. The design of the system shall be such that the emissions are confined to the area in which they are generated by air currents, hoods, or enclosures and shall be exhausted by a duct system to a safe location or treated by removing contaminants. Ducts conveying explosives or flammable vapors, fumes, or dusts shall extend directly to the exterior of the building without entering other spaces and shall not extend into or through ducts and plenums.
Exception: Ducts conveying vapor or fumes having flammable constituents less than 25 percent of their Lower Flammability Limit (LFL) shall be permitted to pass through other
spaces.
505.1.1 Mechanical Ventilation. A mechanical ven tilation system shall be interlocked to operate with the
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equipment used to produce vapors, fumes, or dusts that are flammable or hazardous.
505.2 Incompatible Materials. Incompatible materials shall not be conveyed in the same system. [NFPA 91:4.2.2] 505.3 Flammability Limit. Unless the circumstances stipulated in Section 505.3.1, Section 505.3.2, or Section 505.3.3 exist, in systems conveying flammable vapors, gases, or mists, the concentration shall not exceed 25 percent of the lower flammability limit (LFL). [NFPA 91:4.2.3] 505.3.1 Higher Concentrations. Higher concentrations shall be permitted if the exhaust system is designed and protected in accordance with NFPA 69, using one or more of the following techniques:
(1) Combustible concentration reduction
(2) Oxidant concentration reduction
(3) Deflagration suppression
(4) Deflagration pressure containment [NFPA 91:4.2.3.1]
Contaminated air shall not be recirculated to occupied areas unless contaminants have been removed. Air contaminated with explosive or flammable vapors, fumes, or dusts; flammable or toxic gases; or radioactive material shall not be recirculated.
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 § 505.1.1 High relevance — show source text
spaces.
505.1.1 Mechanical Ventilation. A mechanical ven tilation system shall be interlocked to operate with the
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equipment used to produce vapors, fumes, or dusts that are flammable or hazardous.
505.2 Incompatible Materials. Incompatible materials shall not be conveyed in the same system. [NFPA 91:4.2.2] 505.3 Flammability Limit. Unless the circumstances stipulated in Section 505.3.1, Section 505.3.2, or Section 505.3.3 exist, in systems conveying flammable vapors, gases, or mists, the concentration shall not exceed 25 percent of the lower flammability limit (LFL). [NFPA 91:4.2.3] 505.3.1 Higher Concentrations. Higher concentrations shall be permitted if the exhaust system is designed and protected in accordance with NFPA 69, using one or more of the following techniques:
(1) Combustible concentration reduction
(2) Oxidant concentration reduction
(3) Deflagration suppression
(4) Deflagration pressure containment [NFPA 91:4.2.3.1]
Contaminated air shall not be recirculated to occupied areas unless contaminants have been removed. Air contaminated with explosive or flammable vapors, fumes, or dusts; flammable or toxic gases; or radioactive material shall not be recirculated.
505.3.2 Ovens and Furnaces. Higher concentrations shall be permitted for ovens and furnaces designed and protected in accordance with NFPA 86. [NFPA 91:4.2.3.2] 505.3.3 Deflagration. Higher concentrations shall be permitted where deflagration venting is provided in accordance with NFPA 68. [NFPA 91:4.2.3.3]
505.4 Air-Moving Devices. Air-moving devices shall be sized to establish the velocity required to capture, control, and convey materials through the exhaust system. [NFPA 91:4.2.5] 505.5 Generating Flames, Sparks, or Hot Materials. Operations generating flames, sparks, or hot material such as from grinding wheels and welding shall not be manifolded into any exhaust system that air conveys flammable or combustible materials. [NFPA 91:4.2.6]
505.6 Fire Dampers. Fire dampers shall be permitted to be installed in exhaust systems in accordance with the following:
(1) Where ducts pass through fire barriers
(2) Where a collection system installed on the end of the system is protected with an automatic extinguishing system
(3) Where the duct system is protected with an automatic extinguishing system
(4) Where ducts have been listed with interrupters
CMC § 0.097 High relevance — show source text
)|0.097 (12 ga.)|0.097 (12 ga.)|0.097 (12 ga.)| |16 to 20|24|0.055 (16 ga.)|0.055 (16 ga.)|0.055 (16 ga.)|0.055 (16 ga.)|0.068 (14 ga.)|0.068 (14 ga.)|0.097 (12 ga.)|0.097 (12 ga.)|0.097 (12 ga.)|
For SI units: 1 inch = 25.4 mm, 1 inch water column = 0.249 kPa
Aluminum ducts shall not be used in systems conveying flammable vapors, fumes, or explosive dusts, nor in Class 2, 3, or 4 systems. Galvanized steel and aluminum ducts shall not be used where the temperature of the material being conveyed exceeds 400°F (204°C). Metal ducts used in Class 5 systems that are not resistant to the corrosiveness of the product shall be protected with an approved corrosion-resistant material. 506.2 Construction. Ducts used for conveying products shall be airtight construction as approved by the Authority Having Jurisdiction, and shall not have openings other than those required for operation and maintenance of the system. Ducts constructed of steel shall comply with Table 506.2(1) or Table 506.2(2). Exceptions: (1) Class 1 product-conveying ducts that operate at less than 4 inches water column (0.9 kPa) negative pressure and con
vey noncorrosive, nonflammable and nonexplosive materials at temperatures not exceeding 250°F (121°C) shall be permitted to be constructed in accordance with SMACNA HVAC Duct Construction Standards–Metal and Flexible.
(2) Ducts used in central vacuuming systems within a dwelling unit shall be constructed of materials in accordance with ASTM F2158 or the applicable standards referenced in Chapter 18. Penetrations of fire-resistive walls or floor-ceiling or roof-ceiling assemblies shall be in accordance with the building code. Copper or ferrous pipes or conduit extending from within the separation between a garage and dwelling unit to the central vacuum unit shall be permitted to be used. The use of rectangular ducts conveying particulates shall be subject to approval of the Authority Having Jurisdiction. The design of rectangular ducts shall consider the adhesiveness and buildup of products being conveyed within the duct.
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Col1 Col2 TABLE 506.2(2) MINIMUM SHEET METAL THICKNESS FOR RECTANGULAR DUCTS Col4 Col5 Col6 Col7 Col8 Col9 Col10 Col11 Col12 NEGATIVE
PRESSURE
(inches
water
column)
**REINF.CMC § 505.9 High relevance — show source text
Class 4 - Ducts conveying Class 3 materials in high concentrations and highly abrasive material in high concentrations, such as alumina, bauxite, iron silicate, sand, and slag.
Class 5 - Ducts conveying corrosives, such as acid vapors.
505.9 Minimum Velocities and Circulation. The velocity and circulation of air in work areas shall be such that contaminant’s are captured by an airstream at the area where the emissions are generated and conveyed into a product-conveying duct system. Mixtures within work areas where contaminants are generated shall be diluted to be accordance with Section 505.3 with air that does not contain other contaminants. The velocity of air within the duct shall be not less than set forth in Table 505.9.
Systems conveying particulate matter shall be designed by employing the constant velocity method. Systems conveying explosive or radioactive materials shall be pre-balanced through duct sizing. Other systems shall be permitted to be designed with balancing devices such as dampers. Dampers provided to balance airflow shall be provided with securely fixed minimum-position blocking devices to prevent restricting flow below the required volume or velocity. 505.10 Makeup Air. Makeup air shall be provided to replenish air exhausted by the ventilation system. Makeup air
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intakes shall be located so as to avoid recirculation of con taminated air within enclosures.
505.11 Hoods and Enclosures. Hoods and enclosures shall be used where contaminants originate in a concentrated area. 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.
CMC § 502.2.4 High relevance — show source text
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502.2.4 Commercial Kitchen Ducts. Commercial
kitchens exhaust ducts shall terminate in accordance with
Section 510.9 for Type I exhaust systems or Section 519.5 for Type II exhaust systems.
Part I – Environmental Air Ducts and Product-Conveying Systems.
503.0 Motors, Fans, and Filters.
503.1 General. Motors and fans shall be sized to provide the required air movement. Motors in areas that contain flammable vapors or dusts shall be of a type approved for such environments. A manually operated remote control installed at an approved location shall be provided to shut off fans or blowers in flammable vapor or dust systems. Equipment used in operations that generate explosive or flammable vapors, fumes, or dusts shall be interlocked with the ventilation system so that the equipment cannot be operated unless the ventilation fans are in operation. Motors for fans used to convey flammable vapors or dusts shall be located outside the duct or shall be protected with approved shields and dustproofing. Where belts are used, they shall not enter the duct unless the belt and pulley within the duct are enclosed. Motors and fans shall be accessible for servicing and maintenance.
503.2 Fans. Parts of fans in contact with explosive or flammable vapors, fumes, or dusts shall be of nonferrous or nonsparking materials, or their casing shall be lined or constructed of such material. Where the size and hardness of
materials passing through a fan are capable of producing a spark, both the fan, and the casing shall be of nonsparking materials. Where fans are required to be spark-resistant, their bearings shall not be within the airstream, and parts of the fan shall be grounded. Fans in systems handling materials that are likely to clog the blades, and fans in buffing or woodworking exhaust systems, shall be of the radial-blade or tube-axial type.
Equipment used to exhaust explosive or flammable vapors, fumes, or dusts shall bear an identification plate stating the ventilation rate for which the system was designed.
Fans located in systems conveying corrosives shall be of materials that are resistant to the corrosive or shall be coated
with corrosion-resistant materials.
504.0 Environmental Air Ducts.
504.1 General. Where not specified in this chapter, exhaust ducts shall be constructed and installed in accordance with
Chapter 6 and shall be airtight as approved by the Authority Having Jurisdiction. Environmental air ducts that have an alternate function as a part of an approved smoke control sys
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501.0 General.
501.1 Applicability. This chapter includes requirements for environmental air ducts, product-conveying systems, and commercial hoods and kitchen ventilation. Part I addresses environmental air ducts and product conveying systems. Part II addresses commercial hoods and kitchen ventilation.
502.0 Termination.
502.1 Exhaust Opening Protection. Exhaust openings terminating to the outdoors shall be covered with a corrosionresistant screen having not less than [1] ⁄ 4 of an inch (6.4 mm) openings, and shall have not more than [1] ⁄ 2 of an inch (12.7 mm) openings.
CMC § 0.0929 High relevance — show source text
(1) 60 CFM/person for cultivation, production, and processing facilities or factories.
(2) 15 CFM/person for centers and dispensaries.
Ra = outdoor airflow rate required per unit area, 1 CFM/ft [2]
For SI Units: 1 square foot = 0.0929 m [2], 1 cubic foot per minute =
0.0283 m [3] /min, 1 cubic foot per minute = 0.4719 L/s, 1 cubic foot per
minute per square foot = 5.08 [(L/s)/m [2] ]
Sample Calculation: Determine the outdoor airflow required in the breathing zone ( Vbz ) of an indoor space used for horticultural cultivation with an occupiable floor area of 800 square feet and a maximum of 5 people expected to occupy the zone.
Sample Solution: Vbz = Rp - Pz + Ra - Az Vbz = (60 CFM/person • 5 people) + (1 CFM • 800 SF) Vbz = 1100 CFM. The ventilation system shall be capable of providing not less than 1100 CFM of outdoor air.
I 801.1.2 Ventilation System Requirements. When activated by the gas detection system required by Section I 601.1, the mechanical purge ventilation system shall remain on until manually reset. The purge ventilation system ducting shall terminate outdoors in an approved location. The ventilation system shall be designed to operate at a negative pressure of 0.01 inches water column (0.002 kPa) in relation to the exhausted indoor space.
I 801.1.3 Ventilation for Indoor Cultivation and Storage Spaces. Indoor spaces used for horticultural cultivation and processing and storage shall be provided with ventilation in accordance with Section 402.2. Where
mechanical ventilation is provided, the systems shall be operational when the indoor space(s) are occupied. Air in indoor cultivation and storage spaces shall be classified as Class 2 Air in accordance with Section 403.9.2.
I 801.1.4 Exhaust Ventilation Rate. The exhaust and ventilation system required in this section shall not create a lesser standard of installation than prescribed by the
minimum safety standards adopted by the Authority Having Jurisdiction. Exhaust airflow shall be provided at not less than 0.2 CFM/ft [2] [1.02 (L/s)/m [2] ] of floor area. Exhaust air shall not be used as makeup air, recirculated air, or transfer air. Makeup air shall be provided for more than 150 CFM (70.8 L/s) exhaust. I 801.1.5 Exhaust System Requirements. An exhaust system shall be installed in accordance with Section 505.0 and the following requirements:
(1) Exhaust outlet location(s) in accordance with Section 502.2.3 for product conveying ducts as classified in Section 505.8.
(2) Exhaust air shall terminate directly to the outdoors and shall not be directed onto a public way.
(3) The exhaust inlet shall be not less than 12 inches (305 mm) above the finished floor.
(4) Chemical fume hoods shall be required for flammable solvent processing, shall be listed, and shall be installed in accordance with the manufacturer’s
installation instructions.
(5) Exhaust ducts shall be independent of all other exhaust systems.
(6) The exhaust rate shall be in accordance with the appliable occupancy category as shown in Table 403.7.
CMC § 7.3 High relevance — show source text
7.3|||X|||||||||||||||||||||| |505.12||||||||||||||||||||||X||| |505.12.1||||||||||||||||||||||X||| |507.7||||||||||X||X|X|X|X|||||||||| |508.4||||||||||X|X|X||X|X|||||||||| |509.2.3|||X|||||||||||||||||||||| |509.2.3.4||||||||||||||||||X||||||| |513.2.2|||X|||||||||||||||||||||| |513.2.2 Exception 2|||X|||||X||||||||||||||||| |513.3.5|||X|||||||||||||||||||||| |513.10|||X|||||||||||||||||||||| |513.10.1|||X||||||||||||||||||||||
This state agency does not adopt sections identified with the following symbol: The Office of the State Fire Marshal’s adoption of this chapter or individual sections is applicable to structures regulated by other state agencies pursuant to Section 1.11.0.
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502.2.4 Commercial Kitchen Ducts. Commercial
kitchens exhaust ducts shall terminate in accordance with
Section 510.9 for Type I exhaust systems or Section 519.5 for Type II exhaust systems.
Part I – Environmental Air Ducts and Product-Conveying Systems.
503.0 Motors, Fans, and Filters.
503.1 General. Motors and fans shall be sized to provide the required air movement. Motors in areas that contain flammable vapors or dusts shall be of a type approved for such environments. A manually operated remote control installed at an approved location shall be provided to shut off fans or blowers in flammable vapor or dust systems. Equipment used in operations that generate explosive or flammable vapors, fumes, or dusts shall be interlocked with the ventilation system so that the equipment cannot be operated unless the ventilation fans are in operation. Motors for fans used to convey flammable vapors or dusts shall be located outside the duct or shall be protected with approved shields and dustproofing. Where belts are used, they shall not enter the duct unless the belt and pulley within the duct are enclosed. Motors and fans shall be accessible for servicing and maintenance.
503.2 Fans. Parts of fans in contact with explosive or flammable vapors, fumes, or dusts shall be of nonferrous or nonsparking materials, or their casing shall be lined or constructed of such material. Where the size and hardness of
CMC § 502.2 High relevance — show source text
Exception: Clothes dryers.
502.2 Termination of Exhaust Ducts. Exhaust ducts shall terminate in accordance with Section 502.2.1 through Section 502.2.4. Classes of air shall be as defined in Section
203.0 and classified in Section 403.9.
502.2.1 Environmental, Class 1, and Class 2 Air Ducts. Environmental, Class 1, and Class 2 air duct exhaust shall terminate not less than 3 feet (914 mm) from a property line, 10 feet (3048 mm) above a public way, 3 feet (914 mm) from openings into the building and the minimum separation distance from ventilation system outdoor air intakes determined in accordance with Section 402.4.1. The discharge of dryer exhaust ducts shall not terminate over a public way or over an area where condensate or vapor could create a nuisance or hazard.
502.2.2 Class 3 Air Ducts. Class 3 air duct exhaust shall terminate not less than 10 feet (3048 mm) from a property line, 3 feet (914 mm) from exterior walls or roofs that are in the direction of the exhaust discharge, 10 feet (3048 mm) from openings into the building, 10 feet (3048 mm) above adjoining grade, and the minimum separation distance from ventilation system outdoor air intakes determined in accordance with Section 402.4.1.
502.2.3 Product Conveying, Flammable, and Class 4 Air Ducts. Ducts conveying Class 4 air or explosive or flammable vapors, fumes, or dusts shall terminate not less than 30 feet (9144 mm) from a property line, 10 feet (3048 mm) from openings into the building, 6 feet (1829 mm) from exterior walls or roofs that are in the direction of the exhaust discharge, 30 feet (9144 mm) from combustible walls or openings into the building that are in the direction of the exhaust discharge, 10 feet (3048 mm) above adjoining grade, and the minimum separation distance from ventilation system outdoor air intakes determined in accordance with Section 402.4.1.
Exception: Type I Hood exhaust termination shall be in accordance with Section 510.9.
2025 CALIFORNIA MECHANICAL CODE 99
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EXHAUST SYSTEMS
tem in accordance with Section 505.7.3 do not require design as Class 1 product-conveying ducts.
Exceptions:
(1) Ductless range hoods where installed in accordance with the manufacturer’s installation instructions.
(2) Condensing clothes dryers where installed in accordance with the manufacturer’s installation instructions.
504.1.1 Backdraft Protection. Exhaust ducts shall terminate outside the building and shall be equipped with backdraft dampers or with motorized dampers that automatically shut where the systems or spaces served are not in use. [OSHPD 1, 1R, 2, 4 & 5] Exception: Back- draft dampers are not required when the exhaust fan must operate continuously.
Exception: Where the exhaust duct does not discharge into a common exhaust plenum and one of the following:
(1) The exhaust fan runs continuously.
CMC § 503.1 High relevance — show source text
503.1 General. . . . . . . . . . . . . . . . . . . . . . 99
503.2 Fans . . . . . . . . . . . . . . . . . . . . . . . . 99
504.0 Environmental Air Ducts . . . . . . . . . 99
504.1 General. . . . . . . . . . . . . . . . . . . . . . 99
504.2 Independent Exhaust Systems. . . 100
504.3 Domestic Cooking Exhaust Equipment . . . . . . . . . . . . . . . . . . 100
504.4 Clothes Dryers . . . . . . . . . . . . . . . 100
504.5 Heat Recovery Ventilation (HRV) and Energy Recovery Ventilation (ERV) Systems . . . . . . 101
505.0 Product-Conveying Systems . . . . 101
505.1 General. . . . . . . . . . . . . . . . . . . . . 101
505.2 Incompatible Materials . . . . . . . . . 102
505.3 Flammability Limit. . . . . . . . . . . . . 102
505.4 Air-Moving Devices. . . . . . . . . . . . 102
505.5 Generating Flames, Sparks, or Hot Materials . . . . . . . . . . . . . . 102
505.6 Fire Dampers . . . . . . . . . . . . . . . . 102
505.7 Fire Detection, Smoke Control and Alarm Systems . . . . . . . . . . . . 102
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
CMC § 4.2.14 High relevance — show source text
shall not be interlocked to shut down air-moving devices.
[NFPA 91:4.2.14] 505.7.1 Automatic Extinguishing System. Where shutdown is necessary for the effective operation of an automatic extinguishing system, it shall be permitted to interlock fire detection and alarm systems to shut down air-moving devices. [NFPA 91:4.2.14.1]
505.7.2 Shut Down Permitted. Where a documented risk analysis acceptable to the Authority Having Jurisdiction shows that the risk of damage from fire and the products of combustion would be higher with air-moving devices operating, it shall be permitted to interlock fire detection and alarm systems to shut down air-moving devices. [NFPA 91:4.2.14.2]
505.7.3 Smoke Control Systems. Smoke control systems shall be designed in accordance with NFPA 92 and installed where required by the California Building and the California Fire Code . Smoke control systems shall be equipped with a control unit that complies with UL 864.
505.8 Product-Conveying Duct Classifications. Product-conveying ducts shall be classified according to their use, as follows:
Class 1 - Ducts conveying nonabrasives, such as smoke, spray, mists, fogs, noncorrosive fumes and gases, light fine dusts, or powders.
Class 2 - Ducts conveying moderately abrasive particulate in light concentrations, such as sawdust and grain dust, and buffing and polishing dust.
Class 3 - Ducts conveying Class 2 materials in high concentrations and highly abrasive materials in low concentrations, such as manganese, steel chips, and coke.
Class 4 - Ducts conveying Class 3 materials in high concentrations and highly abrasive material in high concentrations, such as alumina, bauxite, iron silicate, sand, and slag.
Class 5 - Ducts conveying corrosives, such as acid vapors.
505.9 Minimum Velocities and Circulation. The velocity and circulation of air in work areas shall be such that contaminant’s are captured by an airstream at the area where the emissions are generated and conveyed into a product-conveying duct system. Mixtures within work areas where contaminants are generated shall be diluted to be accordance with Section 505.3 with air that does not contain other contaminants. The velocity of air within the duct shall be not less than set forth in Table 505.9.
Systems conveying particulate matter shall be designed by employing the constant velocity method. Systems conveying explosive or radioactive materials shall be pre-balanced through duct sizing. Other systems shall be permitted to be designed with balancing devices such as dampers. Dampers provided to balance airflow shall be provided with securely fixed minimum-position blocking devices to prevent restricting flow below the required volume or velocity. 505.10 Makeup Air. Makeup air shall be provided to replenish air exhausted by the ventilation system. Makeup air
102 2025 CALIFORNIA MECHANICAL CODE
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intakes shall be located so as to avoid recirculation of con taminated air within enclosures.
505.11 Hoods and Enclosures. Hoods and enclosures shall be used where contaminants originate in a concentrated area.
CMC § 402.1 High relevance — show source text
Table 4-B Filter Efficiencies for Central
Ventilation and Air-Conditioning Systems In General Acute Care Hospitals, Outpatient Facilities,
Licensed Clinics and Acute
Psychiatric Hospitals . . . . . . . . . . . 91
Table 4-C Filter Efficiencies for Central
Ventilation and Air-Conditioning Systems In Skilled Nursing Facilities, Intermediate Care
Facilities and Correctional
Treatment Centers . . . . . . . . . . . . . 91
Table 402.1 Minimum Ventilation Rates in
Breathing Zone . . . . . . . . . . . . . . . . 92
Table 403.2.2 Zone Air Distribution Effectiveness. . 95
Table 403.7 Minimum Exhaust Rates. . . . . . . . . 96
CHAPTER 5 EXHAUST SYSTEMS . . . . . . . . . . 97
501.0 General. . . . . . . . . . . . . . . . . . . . . . 99
501.1 Applicability. . . . . . . . . . . . . . . . . . . 99
502.0 Termination. . . . . . . . . . . . . . . . . . . 99
502.1 Exhaust Opening Protection. . . . . . 99
502.2 Termination of Exhaust Ducts . . . . 99
2025 CALIFORNIA MECHANICAL CODE
Part I Environmental Air Ducts and
Product-Conveying Systems . . . . 99
503.0 Motors, Fans, and Filters . . . . . . . . 99
503.1 General. . . . . . . . . . . . . . . . . . . . . . 99
503.2 Fans . . . . . . . . . . . . . . . . . . . . . . . . 99
504.0 Environmental Air Ducts . . . . . . . . . 99
504.1 General. . . . . . . . . . . . . . . . . . . . . . 99
504.2 Independent Exhaust Systems. . . 100
504.3 Domestic Cooking Exhaust Equipment . . . . . . . . . . . . . . . . . . 100
504.4 Clothes Dryers . . . . . . . . . . . . . . . 100
504.5 Heat Recovery Ventilation (HRV) and Energy Recovery Ventilation (ERV) Systems . . . . . . 101
505.0 Product-Conveying Systems . . . . 101
505.1 General. . . . . . . . . . . . . . . . . . . . . 101
505.2 Incompatible Materials . . . . . . . . . 102
505.3 Flammability Limit. . . . . . . . . . . . . 102
505.4 Air-Moving Devices. . . . . . . . . . . . 102
505.5 Generating Flames, Sparks, or Hot Materials . . . . . . . . . . . . . . 102
505.6 Fire Dampers . . . . . . . . . . . . . . . . 102
505.7 Fire Detection, Smoke Control and Alarm Systems . . . . . . . . . . . . 102
Frequently asked questions
What does “25% of the LFL” mean in practice?
It means the concentration of a flammable vapor, gas, or mist in the duct must not exceed one-quarter of that substance’s published Lower Flammability Limit (LFL). The LFL is a material property (SDS) and the CMC’s limit and exception reference is § 505.3 and the § 505.1 exception.
Can I run an exhaust duct with flammable vapors through an unoccupied storage room?
Only if the in‑duct concentration is ≤ 25% of the LFL (the code’s exception to § 505.1) and the Authority Having Jurisdiction approves the routing; otherwise ducts conveying flammable vapors must go directly to the exterior.
Are fire dampers required in these ducts?
Fire dampers are not generally required; § 505.6 lists limited conditions where fire dampers are permitted in exhaust systems (e.g., when passing fire barriers or when the system/collection is protected by automatic extinguishing). Consult § 505.6 and the AHJ.
Can contaminated air be recirculated into the building?
No. Air contaminated with explosive or flammable vapors, fumes, dusts, flammable or toxic gases, or radioactive material must not be recirculated into occupied spaces — see § 505.3.
If my measured concentration is above 25% of LFL, what options are there?
You must either redesign to dilute below 25% LFL, or implement protective measures per NFPA 69 (combustible concentration reduction, oxidant reduction, deflagration suppression, or pressure containment), or use NFPA 86 / NFPA 68 as applicable — see § 505.3.1–505.3.3.
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