CPC · California Plumbing Code

How large must a vent be (minimum vent size and sizing method)

Q: What is the very smallest vent I can run?

The vent may not be less than 1 1/4 inches in diameter and not less than one‑half the diameter of the drain it serves — whichever yields the larger required size — per § 904.1 .

Q: Can I run a long horizontal run on a 1 1/4" vent?

Not if the vent is minimum‑sized. No more than one‑third of the permitted developed length of a minimum‑sized vent may be horizontal. If you upsize the vent one pipe size for its entire length, that limit does not apply. § 904.2 .

Q: If I use Appendix C sizing, does the minimum 1 1/4" rule still apply?

Appendix C is an alternate method; however, the absolute minimum vent size stated in § 904.1 still governs unless the authority having jurisdiction explicitly approves otherwise. Consult § C401.1 and your AHJ.

Vents are sized by the total fixture units and the vent’s developed length using Table 703.2; however, an individual vent can never be smaller than 1 1/4" or less than one‑half the diameter of the drain it serves (§ 904.1).

Ask a question free See coverage

Last reviewed: July 6, 2026

What the code requires — plain English

The diameter of an individual vent is set by the vent’s developed length and the total fixture units it serves — you must pick the vent size from Table 703.2. See § 904.1 for the controlling rule.
In addition, an individual vent may never be smaller than 1 1/4 inches and never smaller than one‑half the diameter of the drain it serves. The building’s drainage must also be vented so the aggregate vent area is at least the area of the largest required building sewer per Table 703.2. See § 904.1.

The single most important rule: choose the vent diameter from Table 703.2 based on the vent’s developed length and the total fixture units served — but in no case use less than 1 1/4" or less than one‑half the drain diameter (see § 904.1).

Requirements in detail

How vent size is determined

Start by totaling the fixture units (dfu) that discharge into the vented portion.
Measure the developed (equivalent) length of the vent (vertical rise plus permitted horizontal portions counted per the tables).
Use Table 703.2 to find the maximum fixture units allowed for a given vent diameter and developed length; select the smallest diameter that accommodates your fixture unit total and length.

Absolute minimums and relational rules

Minimum individual vent diameter = 1 1/4 inches. § 904.1.
Minimum relative size: an individual vent shall be not less than one‑half the diameter of the drain to which it connects. § 904.1.
Aggregate building vent area: the drainage piping of the building shall be vented by vents whose aggregate cross‑sectional area is not less than that of the largest required building sewer as determined from Table 703.2. § 904.1.

Horizontal portion limit for minimum‑sized vents

If you use a minimum‑sized vent, no more than one‑third of its permitted developed length may be installed horizontal. If you increase the vent one full pipe size for its entire length, the one‑third horizontal limit does not apply. § 904.2.

Alternate (Appendix C) sizing method

Appendix C provides an alternate prescriptive table (Table C 401.1) for sizing vent diameters and maximum lengths. Where approved, that alternate method may be used (see § C401.1 and Table C 401.1).

Quick reference (decision‑relevant values)

Decision item	Value / Threshold	Code reference
Minimum individual vent diameter	1 1/4 in	§ 904.1
Minimum relative vent size	≥ one‑half the diameter of the drain	§ 904.1
How to size (primary method)	Lookup by fixture units + developed length in Table 703.2	Table 703.2
Alternate sizing method	Appendix C Table C 401.1 (alternate vent size/length table)	§ C401.1 / Table C 401.1
Horizontal portion limit for minimum size vents	≤ one‑third of permitted length; upsizing one pipe size removes the limitation	§ 904.2
Aggregate vent area for building	≥ cross‑sectional area of largest required building sewer (per Table 703.2)	§ 904.1

Exceptions & special cases

Two or more buildings on the same lot under one ownership may vent to a common building sewer using piping sized per Table 703.2, provided the aggregate vent area is not less than the largest common building sewer (Exception in § 904.1).
Vents downstream of obstructions: vent pipes from fixtures located upstream of pumps, ejectors, backwater valves, or other devices that obstruct free airflow between the building sewer and outside atmosphere must not be used to satisfy the aggregate cross‑sectional area requirement in § 904.1.
Appendix C (Table C 401.1) gives an alternate prescriptive sizing/length method; it may be used where accepted by the authority having jurisdiction (see § C401.1).

If a required numeric mapping (fixture‑unit → vent diameter for a specific developed length) is needed, consult Table 703.2 directly — that table is the controlling sizing table in Chapter 7.

Common mistakes

Selecting a vent smaller than 1 1/4" or smaller than one‑half the drain — both are absolute prohibitions per § 904.1.
Ignoring the developed (equivalent) length: designers sometimes size by fixture units alone and forget to account for length; Table 703.2 requires both.
Installing more than one‑third of the run horizontal for a minimum‑sized vent without upsizing the entire vent by one pipe size. § 904.2 sets this limit.
Using vents that terminate upstream of pumps/ejectors/backwater valves to meet aggregate vent area — the code disallows counting those vents for that purpose. § 904.1.
Failing to follow the alternate Appendix C table only when permitted; Appendix C is optional and must be accepted by the authority having jurisdiction. § C401.1.

Worked example — applying the rule (how to check)

Scenario: a small group of fixtures discharges to a vented branch whose developed length (vertical + equivalent horizontal) measures 28 feet. The total fixture units served by that vent equal 12 dfu.

How to verify compliance:

Confirm the absolute minimum: the vent cannot be less than 1 1/4", and it must be at least one‑half the diameter of the drain it ties into (if the drain is 2", one‑half is 1" but the minimum still governs at 1 1/4"). See § 904.1.
Look up 12 dfu and 28 ft developed length in Table 703.2 to find the required vent diameter for that combination (Table 703.2 is the controlling lookup table). If Table 703.2 shows a 1 1/4" vent is allowed for 12 dfu at 28 ft, you may use 1 1/4" provided it also meets the half‑drain rule. If Table 703.2 requires 1 1/2" at that length and fixture loading, you must use 1 1/2".
If you choose minimum size (1 1/4"), ensure no more than one‑third of the permitted length is installed horizontal; otherwise either reduce horizontal length or upsize the vent one pipe size for its entire length per § 904.2.

Note: I have not asserted the specific table entry for "12 dfu at 28 ft" here because the controlling numeric mapping is exactly what Table 703.2 provides — consult that table directly and select the vent size that matches your exact dfu + developed length.

Related provisions (useful nearby CPC sections)

§ 904.1 (Size of vents; minimum diameter; aggregate area requirement).
§ 904.2 (Length and horizontal portion limit for minimum‑sized vents).
Table 703.2 (Maximum unit loading and maximum length of drainage and vent piping — primary sizing table).
§ C401.1 and Appendix C Table C 401.1 (Alternate sizing method / table in Appendix C).
§ 908.1.1 / § 908.2 (Wet‑vent sizing rules and minimum wet‑vent sizes — related vent sizing context).
§ 710.10 (Sump/receiving tank vents — minimum size reference where applicable).

Code references

Grounded in the retrieved California Plumbing Code — click a citation to read the verbatim passage:

CPC § 811.0. High relevance — show source text
defined in Section 811.0.

903.2.3 Marking. Copper or copper alloy tubing, in addition to the required incised marking, shall be marked in accordance with either ASTM B306 or ASTM B88. The colors shall be Type K, green; Type L, blue; Type M, red; and Type DWV, yellow. 903.3 Changes in Direction. Changes in the direction of vent piping shall be made by the appropriate use of approved fittings, and no such pipe shall be strained or bent. Burred ends shall be reamed to the full bore of the pipe.

904.0 Size of Vents.

904.1 Size. The size of vent piping shall be determined from its length and the total number of fixture units connected thereto, in accordance with Table 703.2. The diameter of an individual vent shall be not less than 1 [1] ⁄ 4 inches (32 mm) nor less than one-half the diameter of the drain to which it is connected. In addition, the drainage piping of each building and each connection to a public sewer or a private sewage disposal system shall be vented by means of one or more vent pipes, the aggregate cross-sectional area of which shall be not less than that of the largest required building sewer as determined from Table 703.2. Vent pipes from fixtures located upstream from pumps, ejectors, backwater valves, or other devices that obstruct the free flow of air and other gases between the building sewer and the outside atmosphere shall not be used for meeting the cross-sectional area venting requirements of this section. Exception: Where connected to a common building sewer, the drainage piping of two or more buildings located on the same lot and under one ownership shall be permitted to be vented by means of piping sized in accordance with Table 703.2, provided the aggregate cross-sectional area of vents is not less than that of the largest required common building sewer. 904.2 Length. Not more than one-third of the total permitted length, in accordance with Table 703.2, of a minimumsized vent shall be installed in a horizontal position. Where a minimum-sized vent is increased one pipe size for its entire length, the maximum length limitation shall not apply.

2025 CALIFORNIA PLUMBING CODE 189

), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.

VENTS

905.0 Vent Pipe Grades and Connections. 905.1 Grade. Vent and branch vent pipes shall be free from drops or sags, and each such vent shall be level or shall be so graded and connected as to drip back by gravity to the drainage pipe it serves. 905.2 Horizontal Drainage Pipe. Where vents connect to a horizontal drainage pipe, each vent pipe shall have its invert taken off above the drainage centerline of such pipe downstream of the trap being served. 905.3 Vent Pipe Rise. Unless prohibited by structural conditions, each vent shall rise vertically to a point not less than 6 inches (152 mm) above the flood-level rim of the fixture served before offsetting horizontally, and where two or more vent pipes converge, each such vent pipe shall rise to a point not less than 6 inches (152 mm) in height above the floodlevel rim of the plumbing fixture it serves before being connected to any other vent. Vents less than 6 inches (152 mm) above the flood-level rim of the fixture shall be installed with approved drainage fittings, material, and grade to the drain.
CPC § 506.4 High relevance — show source text
(3) The minimum size of outdoor opening(s) shall be the full size of outdoor opening(s) calculated in accordance with Section 506.4, multiplied by the reduction factor. The minimum dimension of air

openings shall not be less than 3 inches (76 mm).

[NFPA 54:9.3.4(3)]

506.6 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]

506.7 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 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]

506.7.1 Exhaust Fans. Where exhaust fans are installed, additional air shall be provided to replace the exhausted air. [NFPA 54:9.3.6.1]

506.7.2 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]

506.7.3 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]

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.
CPC § 509.6.1 High relevance — show source text
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 509.6.1, and where vents terminate in accordance with Section 509.6.1(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 R-8 shall not be considered to be exposed to the outdoors. Table 510.1.2(3) in combination with Table 510.1.2(6) shall be used for claytilelined exterior masonry chimneys, provided all of the following requirements are met:

(1) The vent connector is Type B double wall.

(2) The vent connector length is limited to 18 inches per inch (18 mm/mm) of vent connector diameter.

2025 CALIFORNIA PLUMBING CODE 105

), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.

WATER HEATERS

(3) The appliance is draft hood equipped.

(4) The input rating is less than the maximum capacity given in Table 510.1.2(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 510.1.2(6). [NFPA 54:13.1.11] 510.1.11 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]

510.1.12 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]

510.1.13 Multiple Vertical Vent Sizes. In a single run of vent or vent connector, more than one diameter and type shall be permitted to be used, provided that all the sizes and types are permitted by the tables. [NFPA 54:13.1.14]

510.1.14 Interpolation. Interpolation shall be permitted in calculating capacities for vent dimensions that fall between table entries. [NFPA 54:13.1.15]

510.1.15 Extrapolation. Extrapolation beyond the table entries shall not be permitted. [NFPA 54:13.1.16] 510.1.16 Sizing Vents Not Covered by Tables. Where a vent height is lower than 6 feet (1829 mm) or higher than shown in Table 510.1.2(1) through Table 510.2(9), an engineering method shall be used to calculate the vent capacity. [NFPA 54:13.1.17]
CPC § 0.79 High relevance — show source text
Any angle greater than 45 degrees (0.79 rad) from the vertical is considered horizontal. The total horizontal distance of a vent plus the horizontal vent connector serving draft hood-equipped appliances shall not be greater than 75 percent of the vertical height of the vent. [NFPA 54:12.7.4.2] 802.6.2.3 Category II, Category III, and Cate- gory IV Appliances. The sizing of gas vents for Category II, Category III, and Category IV appliances shall be in accordance with the appliance manufacturers’ instructions. The sizing of plastic pipe specified by the appliance manufacturer as a venting material for Category II, III, and IV appliances shall be in accordance with the appliance manufacturers’ instructions. [NFPA 54:12.7.4.3] 802.6.2.4 Sizing. Chimney venting systems using mechanical draft shall be sized in accordance with engineering methods. [NFPA 54:12.7.4.4] 802.6.3 Gas Vents Serving Appliances on More than One Floor. Where a common vent is installed in a multistory installation to vent Category I appliances located on more than one floor level, the venting system shall be designed and installed in accordance with engineering methods.

Crawl spaces, basements, and attics shall be considered as floor levels. [NFPA 54:12.7.5.1] 802.6.3.1 Occupiable Space. All appliances connected to the common vent shall be located in rooms separated from occupiable space. Each of these rooms shall have provisions for an adequate supply of combustion, ventilation, and dilution air that is not supplied from occupiable space. [NFPA 54:12.7.5.2] (See Figure 802.6.3.1) 802.6.3.2 Multistory Venting System. The size of the connectors and common segments of multistory venting systems for appliances listed for use with a Type B double-wall gas vent shall be in accordance with Table 803.2(1), provided all of the following apply: (1) The available total height ( H ) for each segment of a multistory venting system is the vertical distance between the level of the highest draft hood outlet or flue collar on that floor and the centerline of the next highest interconnection tee. (2) The size of the connector for a segment is determined from the appliance’s gas input rate and available connector rise and shall not be smaller

than the draft hood outlet or flue collar size.

(3) The size of the common vertical vent segment, and of the interconnection tee at the base of that segment, is based on the total appliance’s gas input rate entering that segment and its available total height. [NFPA 54:12.7.5.3]

158 2025 CALIFORNIA MECHANICAL CODE

), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.

CHIMNEYS AND VENTS

FIGURE 802.6.3.1

PLAN VIEW OF PRACTICAL SEPARATION METHOD

FOR MULTISTORY GAS VENTING

[NFPA 54: FIGURE A.12.7.5.2]

802.6.4 Support of Gas Vents. Gas vents shall be supported and spaced in accordance with the manufacturer’s installation instructions. [NFPA 54:12.7.6]
CPC § 907.2 Medium relevance — show source text
907.2 Yoke Vent. The yoke vent connection to the vent stack shall be placed not less than 42 inches (1067 mm) above the floor level, and the yoke vent connection to the drainage stack shall be using a wye-branch fitting placed below the lowest drainage branch connection serving that floor.

908.0 Wet Venting. 908.1 Vertical Wet Venting. Wet venting is limited to vertical drainage piping receiving the discharge from the trap arm of one and two fixture unit fixtures that also serves as a vent not exceeding four fixtures. Wet-vented fixtures shall be within the same story; provided, further, that fixtures with a continuous vent discharging into a wet vent shall be within the same story as the wet-vented fixtures. No wet vent shall exceed 6 feet (1829 mm) in developed length. 908.1.1 Size. The vertical piping between two consecutive inlet levels shall be considered a wet-vented sec tion. Each wet-vented section shall be not less than one pipe size exceeding the required minimum waste pipe size of the upper fixture or shall be one pipe size exceeding the required minimum pipe size for the sum of the fixture units served by such wet-vented section, whichever is larger, but in no case less than 2 inches (50 mm) in diameter. 908.1.2 Vent Connection. Common vent sizing shall be the sum of the fixture units served but, in no case,

190 2025 CALIFORNIA PLUMBING CODE

), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.

VENTS

smaller than the minimum vent pipe size required for a fixture served, or by Section 904.0. 908.2 Horizontal Wet Venting for a Bathroom Group. A bathroom group located on the same floor level shall be permitted to be vented by a horizontal wet vent where all of the conditions of Section 908.2.1 through Section 908.2.5 are met.

908.2.1 Vent Connection. The dry vent connection to the wet vent shall be an individual vent for the bidet, shower, or bathtub. One or two vented lavatory(s) shall be permitted to serve as a wet vent for a bathroom group. Only one wet-vented fixture drain or trap arm shall discharge upstream of the dry-vented fixture drain connection. Dry vent connections to the horizontal wet vent shall be in accordance with Section 905.2 and Section

905.3.

908.2.2 Size. The wet vent shall be sized based on the fixture unit discharge into the wet vent. The wet vent shall be not less than 2 inches (50 mm) in diameter for 4 drainage fixture units (dfu) or less, and not less than 3 inches (80 mm) in diameter for 5 dfu or more. The dry vent shall be sized in accordance with Table 702.1 and Table 703.2 based on the total fixture units discharging into the wet vent.

908.2.3 Trap Arm. The length of the trap arm shall not exceed the limits in Table 1002.2. The trap size shall be in accordance with Section 1003.3. The vent pipe opening from the horizontal wet vent, except for water closets and similar fixtures, shall not be below the weir of the trap.
CPC § 510.1.2 Medium relevance — show source text
(6) For a space-heating appliance, the input rating is greater than the minimum capacity given by Table 510.1.2(6). [NFPA 54:13.1.11] 510.1.11 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]

510.1.12 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]

510.1.13 Multiple Vertical Vent Sizes. In a single run of vent or vent connector, more than one diameter and type shall be permitted to be used, provided that all the sizes and types are permitted by the tables. [NFPA 54:13.1.14]

510.1.14 Interpolation. Interpolation shall be permitted in calculating capacities for vent dimensions that fall between table entries. [NFPA 54:13.1.15]

510.1.15 Extrapolation. Extrapolation beyond the table entries shall not be permitted. [NFPA 54:13.1.16] 510.1.16 Sizing Vents Not Covered by Tables. Where a vent height is lower than 6 feet (1829 mm) or higher than shown in Table 510.1.2(1) through Table 510.2(9), an engineering method shall be used to calculate the vent capacity. [NFPA 54:13.1.17]

510.1.17 Height Entries. Where the actual height of a vent falls between entries in the height column of the applicable table in Table 510.1.2(1) through Table 510.1.2(6), 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 54:13.1.18]

510.2 Additional Requirements to Multiple-Appliance Vent. Venting Table 510.2(1) through Table 510.2(9) shall not be used where obstructions are installed in the venting system. The installation of vents serving listed appliances with vent dampers shall be in accordance with the appliance manufacturer’s instructions, or in accordance with the following:

(1) The maximum capacity of the vent connector shall be determined using the NAT Max column.

(2) The maximum capacity of the vertical vent or chimney shall be determined using the FAN + NAT column when the second appliance is a fan-assisted appliance, or the NAT + NAT column when the second appliance is equipped with a draft hood.

(3) The minimum capacity shall be determined as if the appliance were a fan-assisted appliance, as follows:

(a) The minimum capacity of the vent connector shall be determined using the FAN Min column.
CPC § 904.2 Medium relevance — show source text
904.2 Length. Not more than one-third of the total permitted length, in accordance with Table 703.2, of a minimumsized vent shall be installed in a horizontal position. Where a minimum-sized vent is increased one pipe size for its entire length, the maximum length limitation shall not apply.

2025 CALIFORNIA PLUMBING CODE 189

), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.

VENTS

905.0 Vent Pipe Grades and Connections. 905.1 Grade. Vent and branch vent pipes shall be free from drops or sags, and each such vent shall be level or shall be so graded and connected as to drip back by gravity to the drainage pipe it serves. 905.2 Horizontal Drainage Pipe. Where vents connect to a horizontal drainage pipe, each vent pipe shall have its invert taken off above the drainage centerline of such pipe downstream of the trap being served. 905.3 Vent Pipe Rise. Unless prohibited by structural conditions, each vent shall rise vertically to a point not less than 6 inches (152 mm) above the flood-level rim of the fixture served before offsetting horizontally, and where two or more vent pipes converge, each such vent pipe shall rise to a point not less than 6 inches (152 mm) in height above the floodlevel rim of the plumbing fixture it serves before being connected to any other vent. Vents less than 6 inches (152 mm) above the flood-level rim of the fixture shall be installed with approved drainage fittings, material, and grade to the drain. 905.4 Roof Termination. Vent pipes shall extend undiminished in size above the roof, or shall be reconnected with soil or waste vent of the proper size. 905.5 Location of Opening. The vent pipe opening from soil or waste pipe shall not be below the weir of the trap.

Exception: Water closets and similar fixtures. 905.6 Common Vertical Pipe. Two fixtures shall be permitted to be served by a common vertical pipe where each such fixture wastes separately into an approved double fitting having inlet openings at the same level.

906.0 Vent Termination.

906.1 Roof Termination. Each vent pipe or stack shall extend through its flashing and shall terminate vertically not less than 6 inches (152 mm) above the roof nor less than 1 foot (305 mm) from a vertical surface. ABS and PVC piping exposed to sunlight shall be protected by water based synthetic latex paints.

906.2 Clearance. Each vent shall terminate not less than 10 feet (3048 mm) from, or not less than 3 feet (914 mm) above, an openable window, door, opening, air intake, or vent shaft, or not less than 3 feet (914 mm) in every direction from a lot line, alley and street excepted. 906.2.1 [OSHPD 1,1R, 2, 3, 4 & 5] Each vent pipe shall terminate not less than twenty-five (25) feet (7620 mm) from any air intake or vent shaft. 906.3 Use of Roof. Vent pipes shall be extended separately or combined, of full required size, not less than 6 inches (152 mm) above the roof or firewall. Flagpoling of vents shall be prohibited except where the roof is used for assembly purposes or parking.
CPC § 12.8.4.6 Medium relevance — show source text
(1829 mm) of metal pipe in the opening between the draft hood outlet and the thimble, the thimble shall be a minimum of 2 inches (51 mm) larger in diameter than the metal pipe.

(2) For unlisted appliances having draft hoods, the thimble shall be a minimum of 6 inches (152 mm) larger in diameter than the metal pipe.

(3) For residential and low-heat appliances, the thimble shall be a minimum of 12 inches (305 mm) larger in diameter than the metal pipe.

Exception: In lieu of thimble protection, all combustible material in the wall shall be removed a suffi cient distance from the metal pipe to provide the

«

«

2025 CALIFORNIA MECHANICAL CODE 159

), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.

CHIMNEYS AND VENTS

specified clearance from such metal pipe to combustible material. Any material used to close up such opening shall be noncombustible. [NFPA 54:12.8.4.6] 802.7.4 Size of Single-Wall Metal Pipe. Single-wall metal piping shall comply with the following require ments:

(1) A venting system of a single-wall metal pipe shall be sized in accordance with one of the following methods and the appliance manufacturer’s instructions:

(a) For a draft hood-equipped appliance, in accordance with Section 803.0.

(b) For a venting system for a single appliance with a draft hood, the areas of the connector and the pipe each shall not be less than the area of the appliance flue collar or draft hood outlet, whichever is smaller. The vent area shall not be greater than seven times the draft hood outlet area.

(c) Engineering methods.

(2) Where a single-wall metal pipe is used and has a shape other than round, it shall have an equivalent effective area equal to the effective area of the round pipe for which it is substituted and the minimum internal dimension of the pipe shall be 2 inches (51 mm).

(3) The vent cap or a roof assembly shall have a venting capacity not less than that of the pipe to which it is attached. [NFPA 54:12.8.5]

802.7.5 Support of Single-Wall Metal Pipe. All portions of single-wall metal pipe shall be supported for the design and weight of the material employed. [NFPA 54:12.8.6]

802.7.6 Marking. Single-wall metal pipe shall comply with the marking provisions of Section 802.6.5. [NFPA 54:12.8.7]

802.8 Through-the-Wall Vent Termination. Throughthe-wall vent termination shall be in accordance with Section

802.8.1 through Section 802.8.3.

802.8.1 Clearance for Through-the-Wall Vent Ter- mination. The clearance for through-the-wall direct vent and non-direct vent terminals shall be in accordance with

Table 802.8.1 and Figure 802.8.1.

Exception: The clearances in Table 802.8.1 shall not apply to the combustion air intake of a direct vent appliance. [NFPA 54:12.9.1]

CPC § 802.10.1.4 Medium relevance — show source text

TABLE 802.10.1.4 MINIMUM THICKNESS FOR STEEL VENT CONNECTORS FOR MEDIUM-HEAT APPLIANCES [NFPA 54: TABLE 12.11.2.5]	Col2	Col3
VENT CONNECTOR SIZE	VENT CONNECTOR SIZE	VENT CONNECTOR SIZE
DIAMETER (inches)	AREA (square inches)	MINIMUM THICKNESS (inches)
Up to 14	Up to 154	0.053
Over 14 to 16	154 to 201	0.067
Over 16 to 18	201 to 254	0.093
Over 18	Larger than 254	0.123

For SI units: 1 inch = 25.4 mm, 1 square inch = 0.000645 m [2]

802.10.2 Size of Vent Connector. A vent connector for an appliance with a single draft hood or for a Category I fan-assisted combustion system appliance shall be sized and installed in accordance with Section 803.0 or engineering methods. [NFPA 54:12.11.3.1]

For SI units: 1 inch = 25.4 mm, 1 square inch = 0.000645 m [2]

2025 CALIFORNIA MECHANICAL CODE 161

CHIMNEYS AND VENTS

TABLE 802.8.1

THROUGH-THE-WALL DIRECT VENT TERMINATION CLEARANCES

[NFPA 54: TABLE 12.9.1]

FIGURE CLEARANCE	CLEARANCE LOCATION	MINIMUM CLEARANCES FOR DIRECT VENT TERMINALS	MINIMUM CLEARANCES FOR NON-DIRECT VENT TERMINALS
A	Clearance above finished grade level, veranda, porch, deck, or balcony	12 inches	12 inches
B	Clearance to window or door that is openable	6 inches for Appliances≤ 10 000 Btu/hr 9 inches for Appliances > 10 000 Btu/hrs≤ 50 000 Btu/hr 12 inches for Appliances > 50 000 Btu/hrs≤ 150 000 Btu/hr Appliances > 150 000 Btu/hr, in accordance with the appliance manufacturer’s instructions and not less than the clearances specified for non-direct vent terminals in row B	4 feet below or to side of opening or 1 foot above opening
C	Clearance to non-openable win- dow	None unless otherwise specified by the appliance manufacturer	None unless otherwise specified by the appliance manufacturer
D	Vertical clearance to ventilated soffit located above the terminal within a horizontal distance of 2 feet from the center line of the terminal	None unless otherwise specified by the appliance manufacturer	None unless otherwise specified by the appliance manufacturer
E	Clearance to unventilated soffit	None unless otherwise specified by the appliance manufacturer	None unless otherwise specified by the appliance manufacturer
F	Clearance to outside corner

CPC § 12.7.4.1 Medium relevance — show source text
(4) For sizing a gas vent connected to two appliances with draft hoods, the effective area of the vent 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.

(5) Engineering methods. [NFPA 54:12.7.4.1] 509.6.2.2 Vent Offsets. Type B and Type L vents sized in accordance with Section 509.6.2.1(3) or Section 509.6.2.1(4) shall extend in a generally vertical direction with offsets not exceeding 45 degrees (0.79 rad), except that a vent system having not more than one 60 degree (1.05 rad) offset shall be permitted. Any angle greater than 45 degrees (0.79 rad) from the vertical is considered horizontal. The total horizontal distance of a vent plus the horizontal vent connector serving draft hood-equipped appliances shall not be greater than 75 percent of the vertical height of the vent. [NFPA 54:12.7.4.2] 509.6.2.3 Category II, Category III, and Cate- gory IV Appliances. The sizing of gas vents for Category II, Category III, and Category IV appliances shall be in accordance with the appliance manufacturers’ instructions. The sizing of plastic pipe specified by the appliance manufacturer as a venting material for Category II, III, and IV appliances shall be in accordance with the appliance manufacturers’ instructions. [NFPA 54:12.7.4.3] 509.6.2.4 Sizing. Chimney venting systems using mechanical draft shall be sized in accordance with engineering methods. [NFPA 54:12.7.4.4] 509.6.3 Gas Vents Serving Appliances on More than One Floor. Where a common vent is installed in a multistory installation to vent Category I appliances located on more than one floor level, the venting system shall be designed and installed in accordance with engineering methods. Crawl spaces, basements, and attics shall be considered as floor levels. [NFPA 54:12.7.5.1]

509.6.3.1 Occupiable Space. All appliances connected to the common vent shall be located in rooms separated from occupiable space. Each of these rooms shall have provisions for an adequate supply of combustion, ventilation, and dilution air that is not supplied from occupiable space. [NFPA 54:12.7.5.2] (See Figure 509.6.3.1)

FIGURE 509.6.3.1

PLAN VIEW OF PRACTICAL SEPARATION

METHOD FOR MULTISTORY GAS VENTING

[NFPA 54: FIGURE A.12.7.5.2]

509.6.3.2 Multistory Venting System. The size of the connectors and common segments of multistory venting systems for appliances listed for use with a Type B double-wall gas vent shall be in accordance with Table 510.2(1), provided all of the following apply:

(1) The available total height ( H ) for each segment of a multistory venting system is the vertical distance between the level of the highest draft hood outlet or flue collar on that floor and the centerline of the next highest interconnection

tee.

CPC § 301.3. Medium relevance — show source text

form plumbing design work. Details are necessary to ensure compliance with the requirements of this section, together with a full description of the complete installation including quality, grade of materials, equipment, construction, and methods of assembly and installation. Components, materials, and equipment shall comply with standards and specifications listed in Chapter 17 of this code or approved by the Authority Having Jurisdiction and other national consensus standards applicable to plumbing systems and materials. Where such standards and specifications are not available, alternate materials and equipment shall be approved in accordance with Section 301.3.

390 2025 CALIFORNIA PLUMBING CODE

APPENDIX C

Col1	Col2	TABLE C 401.1 SIZE AND LENGTH OF VENTS	Col4	Col5	Col6	Col7	Col8	Col9	Col10	Col11
SIZE OF SOIL OR WASTE STACK (inches)	FIXTURE UNITS CONNECTED	DIAMETER OF VENT REQUIRED (inches)	DIAMETER OF VENT REQUIRED (inches)	DIAMETER OF VENT REQUIRED (inches)	DIAMETER OF VENT REQUIRED (inches)	DIAMETER OF VENT REQUIRED (inches)	DIAMETER OF VENT REQUIRED (inches)	DIAMETER OF VENT REQUIRED (inches)	DIAMETER OF VENT REQUIRED (inches)	DIAMETER OF VENT REQUIRED (inches)
SIZE OF SOIL OR WASTE STACK (inches)	FIXTURE UNITS CONNECTED	11⁄4	11⁄2	2	21⁄2	3	4	5	6	8
SIZE OF SOIL OR WASTE STACK (inches)	FIXTURE UNITS CONNECTED	MAXIMUM LENGTH OF VENT (feet)	MAXIMUM LENGTH OF VENT (feet)	MAXIMUM LENGTH OF VENT (feet)	MAXIMUM LENGTH OF VENT (feet)	MAXIMUM LENGTH OF VENT (feet)	MAXIMUM LENGTH OF VENT (feet)	MAXIMUM LENGTH OF VENT (feet)	MAXIMUM LENGTH OF VENT (feet)	MAXIMUM LENGTH OF VENT (feet)
11⁄2	8	50	150	––	––	––	––	––	––	––
2	12	30	75	200	––	––	––	––	––	––
2	20	26	50	150	––	––	––	––	––	––
21⁄2	42	––	30	1

CPC § 0.06 Medium relevance — show source text

INTERMITTENT FLOW ONLY*

GPM	FIXTURE UNITS
Up to 71⁄2	Equals 1 Fixture Unit
Greater than 71⁄2 to 15	Equals 2 Fixture Units
Greater than 15 to 30	Equals 4 Fixture Units
Greater than 30 to 50	Equals 6 Fixture Units

For SI units: 1 gallon per minute = 0.06 L/s

Discharge capacity exceeding 50 gallons per minute (3.15 L/s) shall be determined by the Authority Having Jurisdiction.

702.3 Continuous Flow. For a continuous flow into a drainage system, such as from a pump, sump ejector, air conditioning equipment, or similar device, 2 fixture units shall be equal to each gallon per minute (gpm) (L/s) of flow.

703.0 Size of Drainage Piping. 703.1 Minimum Size. The minimum sizes of vertical, horizontal, or both drainage piping shall be determined from the total of fixture units connected thereto, and additionally, in the case of vertical drainage pipes, in accordance with their length.

703.2 Maximum Number of Fixture Units. Table 703.2

shows the maximum number of fixture units allowed on a vertical or horizontal drainage pipe, building drain, or building sewer of a given size; the maximum number of fixture units allowed on a branch interval of a given size; and the maximum length (in feet and meters) of a vertical drainage pipe of a given size. 703.3 Sizing per Appendix C. For alternate method of sizing drainage piping, see Appendix C.

704.0 Fixture Connections (Drainage). 704.1 Inlet Fittings. Drainage piping shall be provided with approved inlet fittings for fixture connections, correctly located according to the size and type of fixture proposed to be connected.

704.2 Single Vertical Drainage Pipe. Two fixtures set back-to-back, or side-by-side, within the distance allowed between a trap and its vent, shall be permitted to be served by a single vertical drainage pipe provided that each fixture wastes separately into an approved double-fixture fitting having inlet openings at the same level. 704.3 Commercial Sinks. Pot sinks, scullery sinks, dishwashing sinks, silverware sinks, and other similar fixtures shall be connected directly to the drainage system. A floor drain shall be provided adjacent to the fixture and shall be connected on the sewer side of the sink. No other drainage

168 2025 CALIFORNIA PLUMBING CODE

SANITARY DRAINAGE

TABLE 703.2

MAXIMUM UNIT LOADING AND MAXIMUM LENGTH OF DRAINAGE AND VENT PIPING

SIZE OF PIPE (inches)	11⁄4	11⁄2	2	3	4	5	6	8	10	12
Maximum Units Drainage Piping1 Vertical Horizontal	1 1	22,

Frequently asked questions

What is the very smallest vent I can run?

The vent may not be less than 1 1/4 inches in diameter and not less than one‑half the diameter of the drain it serves — whichever yields the larger required size — per § 904.1.

Do I always use Table 703.2 to size vents?

Yes — the size of vent piping is determined from total fixture units and developed length using Table 703.2. Appendix C provides an alternate table (Table C 401.1) that may be used where accepted. § 904.1 and § C401.1.

Can I run a long horizontal run on a 1 1/4" vent?

Not if the vent is minimum‑sized. No more than one‑third of the permitted developed length of a minimum‑sized vent may be horizontal. If you upsize the vent one pipe size for its entire length, that limit does not apply. § 904.2.

Can vents behind a pump count toward the building’s aggregate vent area?

No. Vents from fixtures located upstream of pumps, ejectors, backwater valves, or similar devices that obstruct the free flow of air cannot be used to meet the aggregate vent area requirement in § 904.1.

If I use Appendix C sizing, does the minimum 1 1/4" rule still apply?

Appendix C is an alternate method; however, the absolute minimum vent size stated in § 904.1 still governs unless the authority having jurisdiction explicitly approves otherwise. Consult § C401.1 and your AHJ.

Ask about the CPC

Get cited, plain-English answers on the California Plumbing Code for your project — any code section, any scenario.

Start Free Trial

Related in the CPC

How far can a minimum-sized vent run horizontally?

Sizing vent stacks, branch vents and when to increase vent size

Vent sizing, minimums and permitted lengths

California Plumbing Code