CMC · California Mechanical Code
Openings, louvers, ducts & accessories
This hub orients readers to the CMC rules on required openings, louvers/grilles, duct materials/installation and access for cleaning and safety.
Last reviewed: July 6, 2026
Overview
This part of the California Mechanical Code (CMC) governs the design, materials, installation and access for openings, louvers, ducts, grease exhaust and related accessories — including access panels for cleaning and listed grease-duct construction. The code sets where openings and access panels are required, how louvers and grilles must be sized (net free area), and construction/clearance rules that affect safety and serviceability, for example in exhaust/grease systems and combustion-air installations as found in CMC §510 and §701.10
These requirements matter for fire and life-safety (access for inspection and cleaning of grease ducts), for proper combustion-air delivery and appliance interlocks, and for energy and airtightness of HVAC systems (sealed openings and closure systems). The code ties into duct material and installation rules (metal, nonmetal, flexible, plenums) and duct-sealing standards referenced elsewhere in the codes and energy requirements (UL 181 and related sealing requirements)
Key places to look in the CMC include the exhaust/grease-duct chapter (see §510 — openings, access panels, cleanouts), the combustion-air and louvers/grilles provisions (see §701.10), and the duct construction and installation chapters (see §§602–605 for materials, plenums and installation). For special topics there are louver testing and region-specific rules (for example §315 on louvers in hurricane-prone regions)
In this section
Code references
Grounded in the retrieved California Mechanical Code — click a citation to read the verbatim passage:
CMC § 506.8 High relevance — show source text
506.8 Louvers, Grilles, and Screens. The required size of openings for combustion, ventilation, and dilution air shall be based on the net free area of each opening. Where the free area through a design of louver, grille, or screen is known, it shall be used in calculating the size opening required to provide the free area specified. Where the louver and grille design and free area are not known, it shall be assumed that wood louvers have 25 percent free area, and metal louvers and grilles have 75 percent free area. Nonmotorized louvers and grilles shall be fixed in the open position. [NFPA 54:9.3.7.1]
506.8.1 Minimum Screen Mesh Size. Screens shall not be smaller than [1] ⁄ 4 of an inch (6.4 mm) mesh. [NFPA 54:9.3.7.2]
506.8.2 Motorized Louvers. Motorized louvers shall be interlocked with the appliance so they are proven in the full open position prior to main burner ignition and during main burner operation. Means shall be provided to prevent the main burner from igniting should the louver fail to open during burner startup and to shut down the main burner if the louvers close during burner operation.
[NFPA 54:9.3.7.3]
506.9 Combustion Air Ducts. Combustion air ducts shall comply with the following [NFPA 54:9.3.8]:
(1) Ducts shall be constructed of galvanized steel or a material having equivalent corrosion resistance, strength, and rigidity.
Exception: Within dwellings units, unobstructed stud and joist spaces shall not be prohibited from conveying combustion air, provided that not more than one fireblock is removed. [NFPA 54:9.3.8.1]
2025 CALIFORNIA PLUMBING CODE 87
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WATER HEATERS
(2) Ducts shall terminate in an unobstructed space, allowing free movement of combustion air to the appliances.
[NFPA 54:9.3.8.2]
(3) Ducts shall serve a single space. [NFPA 54:9.3.8.3]
(4) Ducts shall not serve both upper and lower combustion air openings where both such openings are used. The separation between ducts serving upper and lower combustion air openings shall be maintained to the source of combustion air. [NFPA 54:9.3.8.4]
(5) Ducts shall not be screened where terminating in an attic space. [NFPA 54:9.3.8.5]
(6) Combustion air intake openings located on the exterior of the building shall have the lowest side of the combustion air intake openings located at least 12 inches (305 mm) vertically from the adjoining finished ground level.
[NFPA 54:9.3.8.8]
(7) Horizontal upper combustion air ducts shall not slope downward toward the source of combustion air. [NFPA 54:9.3.8.6]
CMC § 04-15 High relevance — show source text
. . . . . . . . . 063928
Table 1 Description of Items to be Furnished and Installed by Customer Item Description 1 Option 1: Meter Adapter, Cooper B-Line Cat. No. MARR20L45GRSD
(160A) Use with Customer’s Panel Rated at 160A Continuous1, 21 Option 2: Meter Adapter, Ekstrom Industries No. 722B (175A). Specify Left, Right, or Bottom Hub 2 Combination Service Meter and Breaker Panel (rating as required) 3 Pull Termination Box, 8” x 12” x 4”, Rain-Tight, Circle A-W (Cooper B-Line) No. R-9007A or Equivalent (see Note
4M on Page 2)4 Conduit, See Notes 4C and 4G on Page 1 5 Hub to Be Closed and Made Tamper Proof 6 Pull Termination Box, 12” x 26” x 6”, Rain-Tight, Circle A-W Catalog Number R-90008, or Equivalent
(see Note 4K on Page 2)1 Fifth jaw accessory, use Cooper B-Line Cat. No. 50365. 2 Reducer hub and gasket accessories for 2” conduit, use Cooper B-Line Cat. No. AW200 and 12750A.
061032 Page 2 of 4 Rev. #04: 04-15-11
UG-1: Services Greenbook Residential and Small Commercial Overhead to Underground Electric Service Conversion
Optional Removable by Customer
To Be Removed
by PG&E
Meter Socket (see Figure 2 through Figure 7 on Pages 3 through 4)
For Riser and Pull Box Detail (see Figure 2 through Figure 7 on Pages 3 through 4)
For Gas and Water Sealing Requirements,
see Documents 063927 and Document 063928
48” Min.
72” Max.
Customer Shall Dig and Backfill Trench
(see Note 4B on Page 1)
Min. Bend
Depth
To PG&E Distribution System
As required byCol2 Col3 Customer Shall Dig and Backfill Trench
(see Note 4B on Page 1)
For Riser and Pull Box Detail (see Figure 2 through Figure 7 on Pages 3 through 4)
Meter Socket (see Figure 2 through Figure 7 on Pages 3 through 4)
To Be Removed
by PG&E
For Gas and Water Sealing Requirements,
seeDocuments 063927 andDocument 063928
To PG&E Distribution System
Optional Removable by Customer
48” Min.
72” Max.
As required by
Min. Bend
DepthDocument 063927 and Document 063928
Figure 1 Typical Service Conversion
PG&E Service Termination Point
See Note 4K on Page 2
PG&E Service Termination Point (see Note 4L on Page 2)
CMC § 510.3.7 High relevance — show source text
510.3.7 Fire Protection System Devices. Openings for installation, servicing, and inspection of listed fire protection system devices and for duct cleaning shall be provided in ducts and enclosures and shall be in accordance with the requirements of Section 510.3 through Section 510.3.2 and Section 510.7.7.
Enclosure openings required to reach access panels in the ductwork shall be large enough for removal of the access panel through the enclosure opening. [NFPA 96:7.4.4.1 – 7.4.4.2]
510.4 Listed Grease Ducts. Listed grease ducts shall be installed in accordance with the terms of the listing and the manufacturer’s instructions. [NFPA 96:7.1.8]
510.4.1 Factory-Built Grease Ducts. Factory-built grease ducts in accordance with UL 1978 shall be permitted to incorporate non-welded joints in accordance with their listings.
510.5 Other Grease Ducts. Other grease ducts shall comply with the requirements of Section 510.5.1 through Section 510.5.5. [NFPA 96:7.5]
510.5.1 Materials. Ducts shall be constructed of and
supported by carbon steel not less than 0.060 of an inch (1.524 mm) (No. 16 MSG) in thickness or stainless steel not less than 0.048 of an inch (1.219 mm) (No. 18 MSG) in thickness. [NFPA 96:7.5.1.1]
510.5.2 Factory-Built Grease Ducts. Factory-built grease ducts listed in accordance with UL 1978 shall be permitted to use materials in accordance with their listing. {NFPA 96:7.5.1.2}
510.5.3 Installation. All seams, joints, penetrations, and duct-to-hood collar connections shall have a liquidtight continuous external weld. [NFPA 96:7.5.2.1]
Exceptions:
(1) Factory-built grease ducts listed in accordance with UL 1978 shall be permitted to incorporate nonwelded joint construction in accordance with their listings. {NFPA 96:7.5.2.1.1}
(2) Duct-to-hood collar connections as shown in Figure 510.5.3 shall not require a liquidtight continuous external weld. [NFPA 96:7.5.2.2]
(3) Penetrations shall be permitted to be sealed by other listed devices that are tested to be greasetight and are evaluated under the same conditions of fire severity as the hood or enclosure of listed grease extractors and whose presence does not detract from the hood’s or duct’s structural integrity. [NFPA 96:7.5.2.3]
(4) Internal welding shall be permitted, provided the joint is formed or ground smooth and is readily accessible for inspection. [NFPA 96:7.5.2.4]
For SI units: 1 inch = 25.4 mm, °C = (°F-32)/1.8
FIGURE 510.5.3
PERMITTED DUCT-TO HOOD COLLAR CONNECTION
[NFPA 96: FIGURE 7.5.2.2]
CMC § 04-15 High relevance — show source text
. . . . . . . . . . . . . . . . . . . UG-1: Services/Greenbook . . . . . . . . . . . . . . . . . . 063927 Methods and Requirements for Installing Commercial Underground Electric Services 0−600 Volts to Customer-Owned Facilities . . . . . . . . . . . . . . . . . . . . UG-1: Services/Greenbook . . . . . . . . . . . . . . . . . . 063928
Table 1 Description of Items to be Furnished and Installed by Customer Item Description 1 Option 1: Meter Adapter, Cooper B-Line Cat. No. MARR20L45GRSD
(160A) Use with Customer’s Panel Rated at 160A Continuous1, 21 Option 2: Meter Adapter, Ekstrom Industries No. 722B (175A). Specify Left, Right, or Bottom Hub 2 Combination Service Meter and Breaker Panel (rating as required) 3 Pull Termination Box, 8” x 12” x 4”, Rain-Tight, Circle A-W (Cooper B-Line) No. R-9007A or Equivalent (see Note
4M on Page 2)4 Conduit, See Notes 4C and 4G on Page 1 5 Hub to Be Closed and Made Tamper Proof 6 Pull Termination Box, 12” x 26” x 6”, Rain-Tight, Circle A-W Catalog Number R-90008, or Equivalent
(see Note 4K on Page 2)1 Fifth jaw accessory, use Cooper B-Line Cat. No. 50365. 2 Reducer hub and gasket accessories for 2” conduit, use Cooper B-Line Cat. No. AW200 and 12750A.
061032 Page 2 of 4 Rev. #04: 04-15-11
UG-1: Services Greenbook Residential and Small Commercial Overhead to Underground Electric Service Conversion
CMC § 120.4 High relevance — show source text
Nonresidential and hotel/motel buildings shall comply with the applicable requirements of Sections 120.4(a) through 120.4(g).
Exception to Section 120.4: Systems serving healthcare facilities shall comply with the applicable requirements of the California Mechanical Code.
(a) CMC compliance. All air distribution system ducts and plenums, including but not limited to building cavities, mechanical closets, air-handler boxes and support platforms used as ducts or plenums, shall meet the requirements of the CMC Sections 601.0, 602.0, 603.0, 604.0, and 605.0, and ANSI/SMACNA-006-2006 HVAC Duct Construction Standards Metal and Flexible, 3rd Edition incorporated herein by reference. Connections of metal ducts and the inner core of flexible ducts shall be mechanically fastened. Openings shall be sealed with mastic, tape, aerosol sealant or other duct-closure system that meets the applicable requirements of UL 181, UL 181A, or UL 181B. If mastic or tape is used to seal openings greater than [1] / 4 inch, the combination of mastic and either mesh or tape shall be used.
Portions of supply-air and return-air ducts conveying heated or cooled air located in one or more of the following spaces shall be insulated to a minimum installed level of R-8:
Outdoors; or
In a space between the roof and an insulated ceiling; or
In a space directly under a roof with fixed vents or openings to the outside or unconditioned spaces; or
In an unconditioned crawlspace; or
In other unconditioned spaces.
2025 CALIFORNIA ENERGY CODE 77
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NONRESIDENTIAL, HOTEL/MOTEL OCCUPANCIES, AND COVERED PROCESSES—MANDATORY REQUIREMENTS
Portions of supply-air ducts that are not in one of these spaces, including ducts buried in concrete slab, shall be insulated to a minimum installed level of R-4.2 or be enclosed in directly conditioned space.
(b) Duct and plenum materials.
- Factory-fabricated duct systems. A. All factory-fabricated duct systems shall comply with UL 181 for ducts and closure systems, including collars, connections and splices, and be labeled as complying with UL 181. UL 181 testing may be performed by UL laboratories or a laboratory approved by the Executive Director. B. All pressure-sensitive tapes, heat-activated tapes, and mastics used in the manufacture of rigid fiberglass ducts shall comply with UL 181 and UL 181A. C. All pressure-sensitive tapes and mastics used with flexible ducts shall comply with UL 181 and UL 181B. D. All ductwork and plenums with pressure class ratings shall be constructed to Seal Class A. Joints and seams of duct systems and their components shall not be sealed with cloth-back rubber adhesive duct tapes unless such tape is used in combination with mastic and drawbands.
Exception to Section 120.4(b)1D: Ductwork located in occupied space and exposed to view is not required to meet Seal Class A.
CMC § 110.3 Medium relevance — show source text
B. Ventilation. Consumer integrated HPWHs shall meet one of the ventilation requirements below. Minimum volume and opening size requirements shall be the sum of all HPWHs installed within the same space. Compressor capacity shall be determined using AHRI 540 Table 4 reference conditions for refrigeration with the “High” rating test point: i. Installed using a method provided by the manufacturer to meet or exceed the level of performance provided by the ventilation requirements of Section 110.3(c)7B2 through Section 110.3(c)B4. ii. For HPWH installation without ducts, the installation space shall have a volume not less than the greater of 100 cubic feet per kBtu per hour of compressor capacity, or the minimum volume provided by the manufacturer for this method; or
iii. For HPWH installation without ducts, the installation space shall be vented to a communicating space via permanent openings, according to the following requirements: a. Communicating space shall meet the minimum volume of Ssection 110.3(c)7B2 above, minus the volume of the HPWH installation space; and b. Permanent openings shall consist of a single layer of fixed flat slat louvers or grilles, with a total minimum Net Free Area (NFA) the larger of 125 square inches plus 25 square inches per kBtu per hour of compressor capacity, or the minimum provided by the manufacturer for this method. The permanent openings shall be fully louvered doors or two openings of equal area, one in the upper half of the enclosure and one in the bottom half of the enclosure. The top of the upper opening must be 12 inches or less from the enclosure top and the bottom of the lower vent must be 12 inches or less from the enclosure bottom; or iv. For HPWH installations with ducts, the following requirements shall be met: a. The space joined to the installation space via ducts shall meet the minimum volume of Section 110.3(c)7B2 above, minus the volume of the HPWH installation space; and b. All duct connections and building penetrations shall be sealed; and c. Exhaust air ducts and all ducts which cross pressure boundaries shall be insulated to minimum of R-6; and
d. Where only the HPWH inlet or outlet is ducted, installation space shall include permanent openings that consist of a single layer of fixed flat slat louvers or grilles in the bottom half of the room, and/or a door undercut. With a ducted inlet, the minimum NFA shall be equal to the cross-sectional area of the duct. With a ducted exhaust, the minimum NFA shall be the larger of 20 square inches or the minimum NFA provided by the manufacturer for this method; and e. Where the inlet and outlet ducts both terminate within the same pressure boundary, airflow from the termination points shall be diverted away from each other. Note: Ducting only the inlet or the exhaust across the pressure boundary could interfere with balanced ventilation systems. This should be considered when specifying HPWH location and ventilation method.
Note: Authority: Sections 25213, 25218, 25218.5, 25402 and 25402.1, Public Resources Code . Reference: Sections 25007, 25008, 25218.5, 25310, 25402, 25402.1, 25402.4, 25402.8, and 25943, Public Resources Code.
SECTION 110.4—MANDATORY REQUIREMENTS FOR POOL AND SPA SYSTEMS AND EQUIPMENT
CMC § 9.3.7.1 Medium relevance — show source text
Where the free area through a design of louver, grille, or screen is known, it shall be used in calculating the size opening required to provide the free area specified. Where the louver and grille design and free area are not known, it shall be assumed that wood louvers have 25 percent free area and metal louvers and grilles have 75 percent free area. Nonmotorized louvers and grilles shall be fixed in the open position. [NFPA 54:9.3.7.1]
701.10.1 Minimum Screen Mesh Size. Screens shall not be smaller than [1] ⁄ 4 of an inch (6.4 mm) mesh. [NFPA 54:9.3.7.2]
701.10.2 Motorized Louvers. Motorized louvers shall be interlocked with the appliance so they are proven in the full open position prior to main burner ignition and during main burner operation. Means shall be provided to prevent the main burner from igniting should the louver fail to open during burner startup and to shut down the main burner if the louvers close during burner operation.
[NFPA 54:9.3.7.3]
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COMBUSTION AIR
701.11 Combustion Air Ducts. Combustion air ducts shall comply with the following [NFPA 54:9.3.8]:
(1) Ducts shall be constructed of galvanized steel or a material having equivalent corrosion resistance, strength, and rigidity.
Exception: Within dwellings units, unobstructed stud and joist spaces shall not be prohibited from conveying combustion air, provided that not more than one fireblock is removed. [NFPA 54:9.3.8.1]
(2) Ducts shall terminate in an unobstructed space, allowing free movement of combustion air to the appliances.
[NFPA 54:9.3.8.2]
(3) Ducts shall serve a single space. [NFPA 54:9.3.8.3]
(4) Ducts shall not serve both upper and lower combustion air openings where both such openings are used. The separation between ducts serving upper and lower combustion air openings shall be maintained to the source of combustion air. [NFPA 54:9.3.8.4]
(5) Ducts shall not be screened where terminating in an attic space. [NFPA 54:9.3.8.5]
(6) Horizontal upper combustion air ducts shall not slope downward toward the source of combustion air. [NFPA 54:9.3.8.6]
(7) The remaining space surrounding a chimney liner, gas vent, special gas vent, or plastic piping installed within a masonry, metal, or factory built chimney shall not be used to supply combustion air.
Exception: Direct vent appliances designed for installation in a solid fuel-burning fireplace where installed in accordance with the manufacturer’s installation instructions. [NFPA 54:9.3.8.7]
(8) Combustion air intake openings located on the exterior of the building shall have the lowest side of the combustion air intake openings located at least 12 inches (305 mm) vertically from the adjoining finished ground level.
CMC § 1.0 Medium relevance — show source text
D. The outdoor design temperatures for cooling shall be no greater than the 1.0 percent Cooling Dry Bulb and Mean Coincident Wet Bulb values.
214 2025 CALIFORNIA ENERGY CODE
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MULTIFAMILY BUILDINGS—MANDATORY REQUIREMENTS
- Outdoor condensing units . A. Clearances. Installed air conditioner and heat pump outdoor condensing units shall have a clearance of at least 5 feet (1.5 meters) from the outlet of any dryer vent. B. Liquid line drier. Installed air conditioner and heat pump systems shall be equipped with liquid line filter driers if required, as specified by manufacturer’s instructions.
- Central forced-air heating furnaces . A. Temperature rise . Central forced-air heating furnace installations shall be configured to operate in conformance with the furnace manufacturer's maximum inlet-to-outlet temperature rise specifications.
- Air-distribution and ventilation system ducts, plenums and fans. A. CMC compliance.
i. All air-distribution system ducts and plenums, including, but not limited to, mechanical closets and air-handler boxes, shall meet the requirements of the CMC Sections 601.0, 602.0, 603.0, 604.0 and 605.0 and ANSI/SMACNA006-2006 HVAC Duct Construction Standards Metal and Flexible 3rd Edition, incorporated herein by reference. ii. Portions of supply-air and return-air ducts and plenums of a space heating or cooling system shall be insulated in accordance with either Subsection a or b below:
a. Ducts shall have a minimum installed level of R-6.0, or
Exception to Section 160.3(b)5Aiia : Portions of the duct system located in conditioned space below the ceiling separating the occupiable space from the attic are not required to be insulated if all of the following conditions are met: i. The noninsulated portion of the duct system is located entirely inside the building’s thermal envelope as confirmed by visual inspection. ii. At all locations where noninsulated portions of the duct system penetrate into unconditioned space, the penetration shall be draft stopped compliant with CFC Sections 703.1 and 704.1 and air-sealed to the construction materials that are penetrated, using materials compliant with CMC Section E502.4.2 to prevent air infiltration into the cavity. All connections in unconditioned space are insulated to a minimum of R-6.0 as confirmed by visual inspection. b. Ducts do not require insulation when the duct system is located entirely in conditioned space. For buildings with three or fewer habitable stories, duct systems located entirely in conditioned space shall be confirmed through field verification and diagnostic testing in accordance with the requirements of Reference Residential Appendix RA3.1.4.3.8. iii. Connections of metal ducts and the inner core of flexible ducts shall be mechanically fastened. iv. Openings shall be sealed with mastic, tape or other duct-closure system that meets the applicable requirements of UL 181, UL 181A or UL 181B or aerosol sealant that meets the requirements of UL 723. If mastic or tape is used to seal openings greater than [1] / 4 inch, the combination of mastic and either mesh or tape shall be used. v. Building cavities, support platforms for air handlers, and plenums designed or constructed with materials other than sealed sheet metal, duct board or flexible duct shall not be used for conveying conditioned air.
CMC § 9.3.4 Medium relevance — show source text
[NFPA 54:9.3.4(3)] 701.8 Engineered Installations. Engineered combustion air installations shall provide an adequate supply of combustion, ventilation, and dilution air determined using engineering methods. [NFPA 54:9.3.5] 701.9 Mechanical Combustion Air Supply. Where all combustion air is provided by a mechanical air supply system, the combustion air shall be supplied from outdoors at the minimum rate of not less than 0.35 cubic feet per minute per 1000 Btu/h [0.034 (m [3] /min)/kW] for all appliances located within the space. [NFPA 54:9.3.6]
701.9.1 Exhaust Fans. Where exhaust fans are installed, additional air shall be provided to replace the exhausted air. [NFPA 54:9.3.6.1]
701.9.2 Interlock. Each of the appliances served shall be interlocked to the mechanical air supply system to prevent main burner operation where the mechanical air supply system is not in operation. [NFPA 54:9.3.6.2] 701.9.3 Specified Combustion Air. Where combustion air is provided by the building’s mechanical ventilation system, the system shall provide the specified combustion air rate in addition to the required ventilation air. [NFPA 54:9.3.6.3] 701.10 Louvers, Grilles, and Screens. The required size of openings for combustion, ventilation, and dilution air shall be based on the net free area of each opening. Where the free area through a design of louver, grille, or screen is known, it shall be used in calculating the size opening required to provide the free area specified. Where the louver and grille design and free area are not known, it shall be assumed that wood louvers have 25 percent free area and metal louvers and grilles have 75 percent free area. Nonmotorized louvers and grilles shall be fixed in the open position. [NFPA 54:9.3.7.1]
701.10.1 Minimum Screen Mesh Size. Screens shall not be smaller than [1] ⁄ 4 of an inch (6.4 mm) mesh. [NFPA 54:9.3.7.2]
701.10.2 Motorized Louvers. Motorized louvers shall be interlocked with the appliance so they are proven in the full open position prior to main burner ignition and during main burner operation. Means shall be provided to prevent the main burner from igniting should the louver fail to open during burner startup and to shut down the main burner if the louvers close during burner operation.
[NFPA 54:9.3.7.3]
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COMBUSTION AIR
701.11 Combustion Air Ducts. Combustion air ducts shall comply with the following [NFPA 54:9.3.8]:
(1) Ducts shall be constructed of galvanized steel or a material having equivalent corrosion resistance, strength, and rigidity.
CMC § 510.1.2 Medium relevance — show source text
510.1.2 Interconnection. Duct systems shall not be interconnected with any other building ventilation or exhaust system. [NFPA 96:7.1.3]
510.1.3 Duct Installation. All ducts shall be installed with a minimum slope of ¼ inch per linear foot (20.8 mm/m) on horizontal runs up to 75 feet (22 860 mm) and a minimum slope of 1 inch per linear foot (83.2 mm/m) on horizontal runs greater than 75 feet (22 860 mm). Factory-built grease ducts shall be permitted to be installed at a lesser slope in accordance with the listing and the manufacturer’s instructions. All horizontal ducts shall be provided with access in accordance with Section 510.3.3.
Drains shall be provided at low points in horizontal ducts. Where provided, drains shall be continuously welded to the exhaust duct or in accordance with the terms of the listing and the manufacturer’s installation manual.
All ducts shall be installed without forming dips or traps. In manifold (common duct) systems, the lowest end of the main duct shall be connected flush on the bottom with the branch duct. [NFPA 96:7.1.5 – 7.1.5.5]
510.1.4 Accessibility. Openings required for accessibility shall comply with Section 510.3 through Section 510.3.2. [NFPA 96:7.1.6]
510.1.5 Sign. A sign stating the following shall be placed on all access panels:
ACCESS PANEL – DO NOT OBSTRUCT [NFPA 96:7.1.7]
510.1.6 Bracing and Supports. Duct bracing and supports shall be of noncombustible material, securely attached to the structure and designed to carry gravity and lateral loads within the stress limitations of the building code. Bolts, screws, rivets, and other mechanical fasteners shall not penetrate duct walls.
510.1.7 Type I Exhaust Duct Systems. Listed grease ducts shall be installed in accordance with the terms of their listing and the manufacturer’s instructions. [NFPA 96:7.1.8]
510.1.8 Independent Grease Duct System. Single or combined Type I exhaust systems shall be independent of other exhaust systems.
510.2 Clearance. Clearance between ducts and combustible materials shall be provided in accordance with the requirements of Section 507.4 through Section 507.4.3.3. [NFPA 96:7.2] 510.3 Openings. Openings shall be provided at the sides or at the top of the duct, whichever is more accessible, and at changes of direction. Openings shall be protected by approved access constructed and installed in accordance with the requirements of Section 510.3.7. [NFPA 96:7.3.1, 7.3.2]
Exception: Openings shall not be required in portions of the duct that are accessible from the duct entry or discharge.
[NFPA 96:7.3.3]
510.3.1 Access Panel. For hoods with dampers in the exhaust or supply collar, an access panel for cleaning and inspection shall be provided in the duct or the hood within 18 inches (457 mm) of the damper. [NFPA 96:7.3.4]
Exception: Dampers that are accessible from under the hood.
CMC § 8.5 Medium relevance — show source text
21_–0.27|75|0.75 inch|0.75 inch|1.0 inch|1.0 inch|1.0 inch| |Residential
40–60|0.21–0.27|75|R-6|R-5|R-7|R-6|R-5| |Nonresidential
40–60|0.21–0.27|75|0.5 inch|0.5 inch|1.0 inch|1.0 inch|1.0 inch| |Nonresidential
40–60|0.21–0.27|75|R-3|R-3|R-7|R-6|R-5| |Below 40|0.20–0.26|50|1.0 inch|1.5 inches|1.5 inches|1.5 inches|1.5 inches| |Below 40|0.20–0.26|50|R 8.5|R 14|R 12|R 10|R 9|Footnote to Table 120.3-A-1 and Table 120.3-A-2: These thicknesses are based on energy efficiency considerations only. Issues such as water vapor permeability or surface condensation sometimes require vapor retarders or additional insulation.
SECTION 120.4—REQUIREMENTS FOR AIR DISTRIBUTION SYSTEM DUCTS AND PLENUMS
Nonresidential and hotel/motel buildings shall comply with the applicable requirements of Sections 120.4(a) through 120.4(g).
Exception to Section 120.4: Systems serving healthcare facilities shall comply with the applicable requirements of the California Mechanical Code.
(a) CMC compliance. All air distribution system ducts and plenums, including but not limited to building cavities, mechanical closets, air-handler boxes and support platforms used as ducts or plenums, shall meet the requirements of the CMC Sections 601.0, 602.0, 603.0, 604.0, and 605.0, and ANSI/SMACNA-006-2006 HVAC Duct Construction Standards Metal and Flexible, 3rd Edition incorporated herein by reference. Connections of metal ducts and the inner core of flexible ducts shall be mechanically fastened. Openings shall be sealed with mastic, tape, aerosol sealant or other duct-closure system that meets the applicable requirements of UL 181, UL 181A, or UL 181B. If mastic or tape is used to seal openings greater than [1] / 4 inch, the combination of mastic and either mesh or tape shall be used.
Portions of supply-air and return-air ducts conveying heated or cooled air located in one or more of the following spaces shall be insulated to a minimum installed level of R-8:
Outdoors; or
In a space between the roof and an insulated ceiling; or
In a space directly under a roof with fixed vents or openings to the outside or unconditioned spaces; or
In an unconditioned crawlspace; or
In other unconditioned spaces.
2025 CALIFORNIA ENERGY CODE 77
CMC § 160.3 Medium relevance — show source text
iii. Pipe insulation buried below grade must be installed in a waterproof and noncrushable casing or sleeve.
D. Insulation thickness.
i. For insulation with a conductivity in the range shown in Table 160.3-D for the applicable fluid temperature range, the insulation shall have the applicable minimum thickness or R- value shown in Table 160.3-D. ii. For insulation with a conductivity outside the range shown in Table 160.3-D for the applicable fluid temperature range, the insulation shall have a minimum R- value shown in Table 160.3-D or thickness as calculated with Equation 160.3-A:
(Equation 160.3-A)
T = PR 1 + ------ PRt
K
[-] k [-] = PR 1 + ------ t – 1
where:
T = Minimum insulation thickness for material with conductivity K, inches. PR = Pipe actual outside radius, inches.
218 2025 CALIFORNIA ENERGY CODE
on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.
MULTIFAMILY BUILDINGS—MANDATORY REQUIREMENTS
t = Insulation thickness from 160.3-D, inches.
K = Conductivity of alternate material at the mean rating temperature indicated in Table 160.3-D for the applicable fluid temperature range, in Btu-inch per hour per square foot per °F. k = The lower value of the conductivity range listed in Table 160.3-D for the applicable fluid temperature range, Btu-inch per hour per square foot per °F. Exception 1 to Section 160.3(c)1: Factory-installed piping within space-conditioning equipment certified under Section 110.1 or 110.2.
Exception 2 to Section 160.3(c)1: Piping that conveys fluids with a design operating temperature range between 60°F and 105°F.
Exception 3 to Section 160.3(c)1: Where the heat gain or heat loss to or from piping without insulation will not increase building source energy use. Exception 4 to Section 160.3(c)1: Piping that penetrates framing members shall not be required to have pipe insulation for the distance of the framing penetration. Metal piping that penetrates metal framing shall use grommets, plugs, wrapping or other insulating material to ensure that no contact is made with the metal framing. 2. Requirements for air distribution system, ducts and plenum. Multifamily common areas shall comply with the applicable requirements of Sections 160.3(c)2A through 160.3(c)2F. A. CMC compliance. All air distribution system ducts and plenums, including, but not limited to, building cavities, mechanical closets, air-handler boxes and support platforms used as ducts or plenums shall meet the requirements of CMC Sections 601.0, 602.0, 603.0, 604.0 and 605.0, and ANSI/SMACNA-006-2006 HVAC Duct Construction Standards Metal and Flexible 3rd Edition, incorporated herein by reference. Connections of metal ducts and the inner core of flexible ducts shall be mechanically fastened. Openings shall be sealed with mastic, tape, aerosol sealant or other duct-closure system that meets the applicable requirements of UL 181, UL 181A or UL 181B.
Frequently asked questions
How are louvers sized for combustion or ventilation openings?
Louvers and grilles are sized by net free area (NFA); where a louver’s rated free area isn’t known the code prescribes assumed NFAs (e.g., wood 25%, metal 75%) and requires nonmotorized louvers to be fixed open per CMC §701.10
When does a duct need an access opening or panel?
Openings for cleaning, inspection and servicing must be provided at accessible locations (sides or top, at changes of direction, near dampers where applicable) — see the access and opening requirements in §510.3 and related sub-sections
Must HVAC duct openings be sealed to UL 181 or equivalent?
Duct openings and closure systems used to meet energy and airtightness rules are required to meet applicable UL 181 classifications (and the Energy Code references these sealing requirements for ducts and closure systems)
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