CMC · California Mechanical Code
Chimneys and Vents
Chapter 8 of the California Mechanical Code sets the rules for safely designing, installing, inspecting and sizing chimneys and vents to carry combustion products outdoors.
Last reviewed: July 6, 2026
Overview
This hub covers Chapter 8 of the California Mechanical Code (CMC), which governs the design, installation, inspection and sizing of chimneys, gas vents, vent connectors and related venting systems for fuel‑burning appliances (§801.0–§803.2). The chapter’s purpose is to ensure combustion products are conveyed outdoors and to reduce risks from carbon monoxide and fire hazards by prescribing performance, listing, installation and inspection requirements .
Key rules you’ll find here include applicability and basic venting scope (§801.1), minimum safe performance and draft requirements for venting systems (§802.3), construction and lining requirements for masonry/metal/factory‑built chimneys (§802.5), installation and termination requirements for gas vents (§802.6), and the sizing tables and procedures for Category I venting systems (§803.0). These sections also require listed components and adherence to appliance/manufacturer instructions where indicated .
Practical consequences include mandatory inspections of installed chimneys or vents before concealment, limits and clearances for vent connectors, and prescriptive chimney height and termination rules that affect rooftop penetrations and clearances (§802.5.6, §802.10, §802.5.4). The chapter references nationally recognized standards (NFPA, UL) and requires listed materials or engineered designs where the tables do not apply .
In this section
Code references
Grounded in the retrieved California Mechanical Code — click a citation to read the verbatim passage:
CMC § 2025 High relevance — show source text
Systems that carry nonabrasive exhaust, such as smoke, moderate abrasives such as sawdust, and high abrasives such as manganese or acid vapors use product-conveying ducts. Part II provides the minimum fire safety requirements related to the design, installation, inspection, and maintenance of grease-type operations, such as cooking, for both fuel-gas and solid fuel. Cooking produces a significant amount of smoke, fumes, vapors, heat, and other pollutants. Therefore, acceptable kitchen ventilation is necessary to prevent fires from the build-up of grease and to provide the occupants protection from smoke, unpleasant odors, pollutants, and dangerous gases. There are two types of exhaust hoods (Type I and Type II) used in commercial kitchen applications.
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FORMAT OF THE UNIFORM MECHANICAL CODE
Type I hoods are intended to be installed above equipment or appliances that generate grease or smoke. Type II hoods are intended to be installed above equipment or appliances that generate steam, vapor, heat, or odors and do not generate grease or smoke.
Chapter 6 Duct Systems. Chapter 6 regulates requirements for ducts and plenums that are portions of a heating, cooling, ventilation, or exhaust system. This chapter contains material and installation requirements for metal, gypsum, factory-made, flexible, and plastic ducts. It also contains provisions for duct insulation, dampers, fire protection, and automatic shutoff for the building’s air distribution system. It is worth noting that exhaust ducts are regulated by Chapter 5, and combustion air ducts are regulated by Chapter 7.
Chapter 7 Combustion Air. Chapter 7 regulates combustion air requirements for ventilation and dilution of flue gases for appliances installed in buildings. Fuel-gas appliances not regulated by chapter include direct vent appliances and Type I clothes dryers. Makeup air requirements for Type I clothes dryers are located in Chapter 5. Chapter 7 provides acceptable methods for supplying satisfactory combustion air to ensure proper combustion. Combustion air can be supplied by using indoor combustion air or by introducing the air from the outdoors.
Combustion is the rapid oxidation of fuel to release energy. The oxygen required to release the energy from the fuel normally comes from the air. Incomplete combustion of fuel occurs when inadequate oxygen is provided to the appliance. Combustion is needed to provide ventilation cooling for the casing and internal controls. When a lack of oxygen occurs, some of the carbon is not oxidized, and carbon monoxide forms.
Chapter 8 Chimneys and Vents. Chapter 8 regulates the installation, design, and construction of venting systems for fuel-burning appliances. The provisions addressed within this chapter follow procedures an installer would use to design or evaluate a venting system. Many requirements apply to the design and construction of venting systems, chimneys, installation of gas vents, and the sizing of venting system for a Category I appliance. Sizing venting systems require rigorous engineering calculations. However, the venting sizing requirements and sizing tables in this chapter already perform the calculations for the benefit of the end user.
Combustion appliances produce products of incomplete combustion, including potentially harmful carbon monoxide (CO). It is desirable to vent these products to the outdoors. Although the gas is clean-burning fuel, the products of combustion must not be allowed to collect within a building.
CMC § 2025 High relevance — show source text
Chapter 7 Combustion Air. Chapter 7 regulates combustion air requirements for ventilation and dilution of flue gases for appliances installed in buildings. Fuel-gas appliances not regulated by chapter include direct vent appliances and Type I clothes dryers. Makeup air requirements for Type I clothes dryers are located in Chapter 5. Chapter 7 provides acceptable methods for supplying satisfactory combustion air to ensure proper combustion. Combustion air can be supplied by using indoor combustion air or by introducing the air from the outdoors.
Combustion is the rapid oxidation of fuel to release energy. The oxygen required to release the energy from the fuel normally comes from the air. Incomplete combustion of fuel occurs when inadequate oxygen is provided to the appliance. Combustion is needed to provide ventilation cooling for the casing and internal controls. When a lack of oxygen occurs, some of the carbon is not oxidized, and carbon monoxide forms.
Chapter 8 Chimneys and Vents. Chapter 8 regulates the installation, design, and construction of venting systems for fuel-burning appliances. The provisions addressed within this chapter follow procedures an installer would use to design or evaluate a venting system. Many requirements apply to the design and construction of venting systems, chimneys, installation of gas vents, and the sizing of venting system for a Category I appliance. Sizing venting systems require rigorous engineering calculations. However, the venting sizing requirements and sizing tables in this chapter already perform the calculations for the benefit of the end user.
Combustion appliances produce products of incomplete combustion, including potentially harmful carbon monoxide (CO). It is desirable to vent these products to the outdoors. Although the gas is clean-burning fuel, the products of combustion must not be allowed to collect within a building.
Chapter 9 Installation of Specific Appliances. Chapter 9 regulates the minimum requirements for the design, construction and installation of specific appliances. The provisions address the minimum requirements for gas-fired appliances, oil-fired appliances, wood-fired appliances, and electric-type appliances. In addition to the requirements of this chapter, appliances are also required to comply with the general requirements of Chapter 3.
Chapter 10 Boilers and Pressure Vessels. Chapter 10 regulates the construction, installation, operation, repair, and alteration of boilers and pressure vessels. The safety provisions within this chapter address controls and limit devices for automatic boilers, methods of determining expansion tank capacities, discharge piping, relief valves, shutoff valves, gas-pressure and combustion regulators, and inspections and tests. Potable water heaters are free from the requirements of Chapter 10 as they are within the scope of the Uniform Plumbing Code (UPC).
Pressure vessels store large amounts of energy and must comply with ASME Boiler and Pressure Vessel Code (BPVC) Section VIII.1. The stored energy must be contained to prevent disastrous failures. Boilers must comply with ASME BPVC Section I, ASME BPVC Section IV, or NFPA 85. Installing a safety relief valve and expansion tank prevents pressures in the tank from exceeding the design threshold.
Chapter 11 Refrigeration. Chapter 11 regulates the design, installation, and construction requirements of refrigeration systems and the installation and construction of cooling towers. Refrigeration is a method used for achieving heat transfer to cool spaces. Refrigerants are the most common medium used to transfer the heat energy from the low-temperature level to the high-temperature level. Table
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FORMAT OF THE UNIFORM MECHANICAL CODE
CMC § 803.0. High relevance — show source text
(1) Those listed in Section 803.0.
(2) The effective areas of the vent connector and chimney flue of a venting system serving a single appliance with a draft hood shall be not less than the area
of the appliance flue collar or draft hood outlet or greater than seven times the draft hood outlet area.
(3) The effective area of the chimney flue of a venting system serving two appliances with draft hoods shall be not less than the area of the larger draft hood outlet plus 50 percent of the area of the smaller draft hood outlet or greater than seven times the smaller draft hood outlet area.
(4) Chimney venting systems using mechanical draft shall be sized in accordance with engineering methods.
(5) Other engineering methods. [NFPA 54:12.6.3.1]
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CHIMNEYS AND VENTS
FIGURE 802.5.4
TYPICAL TERMINATION LOCATIONS FOR
CHIMNEYS AND SINGLE-WALL METAL PIPES SERVING
RESIDENTIAL-TYPE AND LOW-HEAT APPLIANCE
[NFPA 54: FIGURE A.12.6.2.1]
802.5.6 Inspection of Chimneys or Vents. This inspection shall be made after chimneys, vents, or parts thereof, authorized by the permit, have been installed and before such vent or part thereof has been covered or concealed.
802.5.7 Inspection of Chimneys. Before replacing an existing appliance or connecting a vent connector to a chimney, the chimney passageway shall be examined to ascertain that it is clear and free of obstructions and shall be cleaned if previously used for venting solid- or liquid-fuel-burning appliances or fireplaces. [NFPA 54:12.6.4.1] 802.5.7.1 Standard. Chimneys shall be lined in accordance with NFPA 211. [NFPA 54:12.6.4.2]
802.5.7.2 Cleanouts. Cleanouts shall be examined and where they do not remain tightly closed when not in use, they shall be repaired or replaced. [NFPA 54:12.6.4.3] 802.5.7.3 Existing Chimney. When inspection reveals that an existing chimney is not safe for the intended application, it shall be repaired, rebuilt, lined, relined, or replaced with a vent or chimney to conform to NFPA 211 and shall be suitable for the appliances to be attached. [NFPA 54:12.6.4.4] 802.5.8 Chimney Serving Appliances Burning Other Fuels. An appliance shall not be connected to a chimney flue serving a separate appliance designed to burn solid fuel. [NFPA 54:12.6.5.1] 802.5.8.1 Gas and Liquid Fuel-Burning Appliances. Where one chimney serves gas appliances and liquid fuel-burning appliances, the appliances shall be connected through separate openings or connected through a single opening where joined by a suitable fitting located as close as practical to the chimney. Where two or more openings are provided into one chimney flue, they shall be at different levels.
CMC § 1.11.0. High relevance — show source text
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 X X X X X X X X X Adopt Entire Chapter as
amended (amended sections
listed below)Adopt only those sections
that are listed belowChapter/Section 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.
2025 CALIFORNIA MECHANICAL CODE 151
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CHAPTER 8
CHIMNEYS AND VENTS
801.0 General.
801.1 Applicability. The requirements of this chapter shall govern the venting of fuel-burning appliances.
801.2 Venting of Gas Appliances. Low-heat and medium-heat gas appliances shall be vented in accordance with this chapter. Other gas appliances shall be vented in accordance with NFPA 211 or other applicable standards.
801.3 Appliances Fueled by Other Fuels. Appliances fueled by fuels other than gas shall be vented in accordance with NFPA 211 and the appliance manufacturer’s instructions.
802.0 Venting of Appliances.
802.1 Listing. Type B and Type B-W gas vents shall comply with UL 441, Type L gas vents shall comply with UL 641.
802.1.1 Installation. Listed chimneys and vents shall be installed in accordance with this chapter and the manufacturer’s installation instructions. [NFPA 54:12.2.1]
CMC § 1.11.0. High relevance — show source text
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.
2025 CALIFORNIA MECHANICAL CODE 151
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CHAPTER 8
CHIMNEYS AND VENTS
801.0 General.
801.1 Applicability. The requirements of this chapter shall govern the venting of fuel-burning appliances.
801.2 Venting of Gas Appliances. Low-heat and medium-heat gas appliances shall be vented in accordance with this chapter. Other gas appliances shall be vented in accordance with NFPA 211 or other applicable standards.
801.3 Appliances Fueled by Other Fuels. Appliances fueled by fuels other than gas shall be vented in accordance with NFPA 211 and the appliance manufacturer’s instructions.
802.0 Venting of Appliances.
802.1 Listing. Type B and Type B-W gas vents shall comply with UL 441, Type L gas vents shall comply with UL 641.
802.1.1 Installation. Listed chimneys and vents shall be installed in accordance with this chapter and the manufacturer’s installation instructions. [NFPA 54:12.2.1]
802.1.2 Prohibited Discharge. Appliance vents shall not discharge into a space enclosed by screens having openings less than [1] ⁄ 4 of an inch (6.4 mm) mesh.
802.2 Connection to Venting Systems. Except as permitted in Section 802.2.1 through Section 802.2.7, all appliances shall be connected to venting systems. [NFPA 54:12.3.1]
802.2.1 Appliances Not Required to be Vented. The following appliances shall not be required to be vented:
(1) Listed ranges.
(2) Built-in domestic cooking units listed and marked for optional venting.
(3) Listed hot plates.
(4) Listed Type 1 clothes dryers exhausted in accordance with Section 504.4.
(5) A single listed booster-type (automatic instantaneous) water heater, when designed and used solely for the sanitizing rinse requirements of a dishwashing machine, provided that the appliance is installed with the draft hood in place and unaltered, if a draft hood is required, in a commercial kitchen having a mechanical exhaust system. [Where installed in this manner, the draft hood outlet shall not be less than 36 inches (914 mm) vertically and 6 inches (152 mm) horizontally from any surface other than the appliance.]
(6) Listed refrigerators.
(7) Counter appliances.
(8) Room heaters listed for unvented use.
(9) Direct gas-fired makeup-air heaters.
(10)Other appliances listed for unvented use and not provided with flue collars.
(11)Specialized appliances of limited input such as laboratory burners or gas lights. [NFPA 54:12.3.2]
CMC § 803.1.9 High relevance — show source text
803.1.9 Draft Hood Conversion Accessories. Draft hood conversion accessories for use with masonry chimneys venting listed Category I fan-assisted appliances shall be listed and installed in accordance with the listed accessory manufacturer’s installation instructions.
[NFPA 54:13.1.10]
803.1.10 Chimney and Vent Locations. Table 803.1.2(1) through Table 803.1.2(5) shall be used only for chimneys and vents not exposed to the outdoors below the roof line. A Type B vent or listed chimney lining system passing through an unused masonry chimney flue shall not be considered to be exposed to the outdoors. Where vents extend outdoors above the roof more than 5 feet (1524 mm) higher than required by Table 802.6.1, and where vents terminate in accordance with Section 802.6.1(1)(b), the outdoor portion of the vent shall be enclosed as required by this paragraph for vents not considered to be exposed to the outdoors, or such venting system shall be engineered. A Type B vent passing through an unventilated enclosure or chase insulated to a value of not less than R8 shall not be considered to be exposed to the outdoors. Table 803.1.2(3) in combination with Table 803.1.2(6) shall be used for clay tile-lined exterior masonry chimneys, provided all of the following require ments are met:
(1) The vent connector is Type B double wall.
(2) The vent connector length is limited to 18 in./in. (18 mm/mm) of vent connector diameter.
(3) The appliance is draft hood equipped.
(4) The input rating is less than the maximum capacity given in Table 803.1.2(3).
(5) For a water heater, the outdoor design temperature shall not be less than 5°F (-15°C).
(6) For a space-heating appliance, the input rating is greater than the minimum capacity given by Table 803.1.2(6). [NFPA 54:13.1.11]
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CHIMNEYS AND VENTS
803.1.11 Residential and Low-Heat Appliances. Flue lining system for residential and low heat appliance shall be in accordance with Section 803.1.11.1 and Sec tion 803.1.11.2.
803.1.11.1 Clay Flue Lining. Clay flue lining shall be manufactured in accordance with ASTM
C315 or other approved standard.
803.1.11.2 Chimney Lining. Chimney lining shall be listed in accordance with UL 1777.
803.1.12 Corrugated Vent Connector Size. Corrugated vent connectors shall not be smaller than the listed appliance categorized vent diameter, flue collar diameter, or draft hood outlet diameter. [NFPA 54:13.1.12]
803.1.13 Upsizing. Vent connectors shall not be upsized more than two sizes greater than the listed appliance categorized vent diameter, flue collar diameter, or draft hood outlet diameter. [NFPA 54:13.1.13] **803.1.14 Multiple Vertical Vent Sizes.
CMC § 12.6.2.1 High relevance — show source text
** A chimney for residential-type or low-heat appliances shall extend at least 3 feet (914 mm) above the highest point where it passes through a roof of a building and at least 2 feet (610 mm) higher than any portion of a building within a horizontal distance of 10 feet (3048 mm). [NFPA 54:12.6.2.1] (See Figure 802.5.4) 802.5.4.1 Medium-Heat Gas Appliances. A chimney for medium-heat appliances shall extend at least 10 feet (3048 mm) higher than any portion of any building within 25 feet (7620 mm). [NFPA 54:12.6.2.2]
802.5.4.2 Chimney Height. A chimney shall extend at least 5 feet (1524 mm) above the highest connected appliance draft hood outlet or flue collar.
[NFPA 54:12.6.2.3]
802.5.4.3 Decorative Shrouds. Decorative
shrouds shall not be installed at the termination of factory-built chimneys except where such shrouds are
listed and labeled for use with the specific factorybuilt chimney system and are installed in accordance with the manufacturer’s installation instructions.
[NFPA 54:12.6.2.4]
802.5.5 Size of Chimneys. The effective area of a chimney venting system serving listed appliances with draft hoods, Category I appliances, and other appliances listed for use with Type B vents shall be in accordance with one of the following methods:
(1) Those listed in Section 803.0.
(2) The effective areas of the vent connector and chimney flue of a venting system serving a single appliance with a draft hood shall be not less than the area
of the appliance flue collar or draft hood outlet or greater than seven times the draft hood outlet area.
(3) The effective area of the chimney flue of a venting system serving two appliances with draft hoods shall be not less than the area of the larger draft hood outlet plus 50 percent of the area of the smaller draft hood outlet or greater than seven times the smaller draft hood outlet area.
(4) Chimney venting systems using mechanical draft shall be sized in accordance with engineering methods.
(5) Other engineering methods. [NFPA 54:12.6.3.1]
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CHIMNEYS AND VENTS
FIGURE 802.5.4
TYPICAL TERMINATION LOCATIONS FOR
CHIMNEYS AND SINGLE-WALL METAL PIPES SERVING
RESIDENTIAL-TYPE AND LOW-HEAT APPLIANCE
[NFPA 54: FIGURE A.12.6.2.1]
802.5.6 Inspection of Chimneys or Vents. This inspection shall be made after chimneys, vents, or parts thereof, authorized by the permit, have been installed and before such vent or part thereof has been covered or concealed.
CMC § 12.6.5.3 High relevance — show source text
The chimney flue shall be sized to properly vent the appliance.
[NFPA 54:12.6.5.3] 802.5.8.3 Combination Gas- and Oil-Burning Appliances. A single chimney flue serving a listed combination gas- and oil-burning appliance shall be sized in accordance with the appliance manufacturer’s instructions. [NFPA 54:12.6.5.4]
802.5.9 Support of Chimneys. All portions of chimneys shall be supported for the design and weight of the materials employed. Listed factory-built chimneys shall be supported and spaced in accordance with the manufacturer’s installation instructions. [NFPA 54:12.6.6]
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CHIMNEYS AND VENTS
802.5.10 Cleanouts. Where a chimney that formerly carried flue products from liquid or solid fuel-burning appliances is used with an appliance using fuel gas, an accessible cleanout shall be provided. The cleanout shall have a tight-fitting cover and be installed so its upper edge is at least 6 inches (152 mm) below the lower edge of the lowest chimney inlet opening. [NFPA 54:12.6.7] 802.5.11 Space Surrounding Lining or Vent. The remaining space surrounding a chimney liner, gas vent, special gas vent, or plastic piping installed within a masonry chimney shall not be used to vent another appli ance.
Exception: The insertion of another liner or vent within the chimney as provided in this code and the liner or vent manufacturer’s instructions. [NFPA 54:12.6.8.1] 802.5.11.1 Combustion Air. The remaining space surrounding a chimney liner, gas vent, special gas vent, or plastic piping installed within a masonry, metal or factory-built chimney flue 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 54:12.6.8.2] 802.6 Gas Vents. The installation of gas vents shall meet the following requirements:
(1) Gas vents shall be installed in accordance with the manufacturer’s installation instructions.
(2) A Type B-W gas vent shall have a listed capacity not less than that of the listed vented wall furnace to which it is
connected.
(3) Gas vents installed within masonry chimneys shall be installed in accordance with the manufacturer’s installation instructions. Gas vents installed within masonry chimneys shall be identified with a permanent label installed at the point where the vent enters the chimney. The label shall contain the following language: “This gas vent is for appliances that burn gas. Do not connect to solid or liquid fuel–burning appliances or incinerators.”
(4) Screws, rivets, and other fasteners shall not penetrate the inner wall of double-wall gas vents, except at the transition from the appliance draft hood outlet, flue collar, or single-wall metal connector to a double-wall vent.
[NFPA 54:12.7.2] 802.6.1 Gas Vent Termination. The termination of gas vents shall comply with the following requirements:
(1) A gas vent shall terminate in accordance with one of the following:
CMC § 803.2.23 High relevance — show source text
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CHIMNEYS AND VENTS
803.2.23 Multiple Connector and Vent Sizes. Where Table 803.1.2(1) through Table 803.2(9) permits more than one diameter of pipe to be used for a connector or vent, all the permitted sizes shall be permitted to be used. [NFPA 54:13.2.26]
803.2.24 Interpolation. Interpolation shall be permitted in calculating capacities for vent dimensions that fall between table entries. [NFPA 54:13.2.27]
803.2.25 Extrapolation. Extrapolation beyond the table entries shall not be permitted. [NFPA 54:13.2.28]
803.2.26 Sizing Vents Not Covered by Tables. For vent heights lower than 6 feet (1829 mm) and higher than shown in the tables, engineering methods shall be used to calculate vent capacities. [NFPA 54:13.2.29]
803.2.27 Height Entries. Where the actual height of a vent falls between entries in the height column of the applicable table in Table 803.2(1) through Table 803.2(9), either of the following shall be used:
(1) Interpolation.
(2) The lower appliance input rating shown in the table entries, for FAN MAX and NAT MAX column values; and the higher appliance input rating for the FAN MIN column values. [NFPA 54:13.2.30]
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CHIMNEYS AND VENTS
99% Winter Design Temperatures for the Contiguous United States
This map is a necessarily generalized guide to temperatures in the contiguous United States. Temperatures shown for areas such
as mountainous regions and large urban centers are not necessarily accurate. The climate data used to develop this map are from
the ASHRAE Handbook – Fundamentals (Climate Conditions for the United States).
For 99% winter design temperatures in Alaska, consult the ASHRAE Handbook — Fundamentals.
99% winter design temperatures for Hawaii are greater than 37°F
For SI units: °C = (°F-32)/1.8
FIGURE 803.1.2(6) RANGE OF WINTER DESIGN TEMPERATURES USED IN ANALYZING EXTERIOR MASONRY CHIMNEYS IN THE UNITED STATES
[NFPA 54: FIGURE F.2.4]
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|TABLE 803.1.2(1) TYPE B DOUBLE-WALL GAS VENT [NFPA 54: TABLE 13.1(a)]*|Col2|Col3|Col4|Col5|54: TABLE 13.
CMC § 608.9 High relevance — show source text
608.9 Floor Registers . . . . . . . . . . . . . . . 143
608.10 Exterior Wall and Interior Stud
Partitions. . . . . . . . . . . . . . . . . . . . 143
608.11 Wall Register . . . . . . . . . . . . . . . . 143
608.12 Distance from Combustible . . . . . 143
608.13 Vapor Barrier . . . . . . . . . . . . . . . . 143
608.14 Prohibited . . . . . . . . . . . . . . . . . . . 143
609.0 Automatic Shutoffs . . . . . . . . . . . . 143
609.1 Air-Moving Systems and Smoke Detectors . . . . . . . . . . . . . . . . . . . 143
609.2 Air-Moving Systems and Smoke Detectors in Group I-2 Occupancies . . . . . . . . . . . . . . . . 144
CHAPTER 7 COMBUSTION AIR . . . . . . . . . . . 145
701.0 General. . . . . . . . . . . . . . . . . . . . . 147
701.1 Applicability. . . . . . . . . . . . . . . . . . 147
701.2 Pressure Difference . . . . . . . . . . . 147
701.3 Makeup Air . . . . . . . . . . . . . . . . . . 147
701.4 Indoor Combustion Air . . . . . . . . . 147
701.5 Indoor Opening Size and Location . . . . . . . . . . . . . . . . . . . . 147
701.6 Outdoor Combustion Air . . . . . . . . 147
701.7 Combination Indoor and Outdoor
Combustion Air . . . . . . . . . . . . . . . 148
701.8 Engineered Installations . . . . . . . . 149
701.9 Mechanical Combustion Air
Supply. . . . . . . . . . . . . . . . . . . . . . 149
701.10 Louvers, Grilles, and Screens . . . 149
701.11 Combustion Air Ducts. . . . . . . . . . 150
701.12 Dampers Prohibited . . . . . . . . . . . 150
702.0 Extra Device or Attachment . . . . . 150
702.1 General. . . . . . . . . . . . . . . . . . . . . 150
CHAPTER 8 CHIMNEYS AND VENTS. . . . . . . 151
801.0 General. . . . . . . . . . . . . . . . . . . . . 153
801.1 Applicability. . . . . . . . . . . . . . . . . . 153
801.2 Venting of Gas Appliances . . . . . . 153
801.3 Appliances Fueled by Other Fuels. 153
802.0 Venting of Appliances. . . . . . . . . . 153
xxxiv
CMC § 701.4 High relevance — show source text
701.4 Indoor Combustion Air . . . . . . . . . 147
701.5 Indoor Opening Size and Location . . . . . . . . . . . . . . . . . . . . 147
701.6 Outdoor Combustion Air . . . . . . . . 147
701.7 Combination Indoor and Outdoor
Combustion Air . . . . . . . . . . . . . . . 148
701.8 Engineered Installations . . . . . . . . 149
701.9 Mechanical Combustion Air
Supply. . . . . . . . . . . . . . . . . . . . . . 149
701.10 Louvers, Grilles, and Screens . . . 149
701.11 Combustion Air Ducts. . . . . . . . . . 150
701.12 Dampers Prohibited . . . . . . . . . . . 150
702.0 Extra Device or Attachment . . . . . 150
702.1 General. . . . . . . . . . . . . . . . . . . . . 150
CHAPTER 8 CHIMNEYS AND VENTS. . . . . . . 151
801.0 General. . . . . . . . . . . . . . . . . . . . . 153
801.1 Applicability. . . . . . . . . . . . . . . . . . 153
801.2 Venting of Gas Appliances . . . . . . 153
801.3 Appliances Fueled by Other Fuels. 153
802.0 Venting of Appliances. . . . . . . . . . 153
xxxiv
802.1 Listing . . . . . . . . . . . . . . . . . . . . . . 153
802.2 Connection to Venting Systems . . 153
802.3 Minimum Safe Performance . . . . . 153
802.4 Type of Venting System to be Used . . . . . . . . . . . . . . . . . . . . . . . 154
802.5 Masonry, Metal, and Factory-Built Chimneys . . . . . . . . . . . . . . . . . . . 154
Table 802.4 Type of Venting System to be Used . . . . . . . . . . . . . . . . . . . . . . . 155
802.6 Gas Vents. . . . . . . . . . . . . . . . . . . 157
Table 802.6.1 Roof Slope Heights. . . . . . . . . . . . 158
802.7 Single-Wall Metal Pipe . . . . . . . . . 159
Table 802.7.3.3 Clearance for Connectors. . . . . . . 160
802.8 Through-the-Wall Vent Termination . . . . . . . . . . . . . . . . . . 160
802.9 Condensation Drain . . . . . . . . . . . 160
802.10 Vent Connectors for Category I Appliances . . . . . . . . . . . . . . . . . . 160
Table 802.10.1.3 Minimum Thickness for Galvanized
Steel Vent Connectors for Low Heat Appliances . . . . . . . . . . . . . . 161
CMC § 12.4.5.2 High relevance — show source text
(2) The vent system shall be installed such that no fittings or joints between sections are installed in the above-ceiling space.
(3) The venting system shall be installed in a conduit or enclosure with joints between the interior of the enclosure and the ceiling space sealed. [NFPA 54:12.4.5.2] 802.4 Type of Venting System to be Used. The type of venting system to be used shall be in accordance with Table 802.4. [NFPA 54:12.5.1]
802.4.1 Plastic Piping. Where plastic piping is used to vent an appliance, the appliance shall be listed for use with such venting materials and the appliance manufacturer’s installation instructions shall identify the specific plastic piping material. The plastic pipe venting materials shall be labeled in accordance with the product standards specified by the appliance manufacturer or shall be listed and labeled in accordance with UL 1738. [NFPA 54:12.5.2]
802.4.2 Plastic Vent Joints. Plastic pipe and fittings used to vent appliances shall be installed in accordance with the appliance manufacturer’s installation instructions. Plastic pipe venting materials listed and labeled in accordance with UL 1738 shall be installed in accordance
with the vent manufacturer’s installation instructions. Where primer is required, it shall be of a contrasting color.
[NFPA 54:12.5.3]
802.4.3 Special Gas Vents. Special gas vents shall be listed and labeled in accordance with UL 1738 and installed in accordance with the special gas vent manufacturer’s installation instructions. [NFPA 54:12.5.4]
802.5 Masonry, Metal, and Factory-Built Chimneys. Chimneys shall be installed in accordance with Section 802.5.1 through Section 802.5.3.
802.5.1 Factory-Built Chimneys. Factory-built chimneys shall be listed in accordance with UL 103, UL 959, or UL 2561. Factory-built chimneys used to vent appliances that operate at positive vent pressure shall be listed for such application. [NFPA 54:12.6.1.1]
802.5.1.1 Decorative Shrouds. Decorative shrouds addressed in Section 802.5.4.3 shall comply with UL 103 for factory-built residential chimneys, UL 127 for factory-built fireplaces, or UL 1482 for solid-fuel room heaters.
802.5.2 Metal Chimneys. Metal chimneys shall be built and installed in accordance with NFPA 211. [NFPA 54:12.6.1.2]
802.5.3 Masonry Chimneys. Masonry chimneys shall be built and installed in accordance with NFPA 211 and lined with one of the following:
(1) Approved clay flue lining.
(2) A chimney lining system listed and labeled in accordance with UL 1777.
(3) Other approved material that resists corrosion, erosion, softening, or cracking from vent gases at temperatures up to 1800°F (982°C).
Exception: Masonry chimney flues lined with a chimney lining system specifically listed for use with listed appliances with draft hoods, Category I appliances, and
154 2025 CALIFORNIA MECHANICAL CODE
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CHIMNEYS AND VENTS
Frequently asked questions
Do all gas appliances have to be vented?
Not always—Chapter 8 requires venting for most fuel‑burning appliances but lists specific exceptions (for example, some listed ranges, refrigerators, certain unvented room heaters and listed direct‑vent appliances). See the list of appliances not required to be vented in §802.2.1 .
How high must a chimney extend above the roof or nearby structures?
Chimney termination height is prescriptive: for residential/low‑heat appliances a chimney must extend at least 3 ft above the highest roof penetration and at least 2 ft higher than any portion of a building within 10 ft horizontally; medium‑heat appliances have different (greater) height requirements—see §802.5.4 and its subsections for the exact criteria .
Can plastic piping or corrugated connectors be used?
Plastic piping may be used only where the appliance is listed for plastic venting and the piping/fittings are listed (UL 1738) or identified by the appliance manufacturer; corrugated connectors have sizing limits and must not be smaller than the appliance flue/collar per §802.4.1 and §803.1.12 .
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