CMC · California Mechanical Code

How are Category I venting systems sized (single and multiple appliances)?

For a homeowner: the California Mechanical Code requires you (or your contractor) to size natural-draft (Category I) vents from the Code’s vent-sizing tables: use the single-appliance rules in §803.1 for one appliance and the common-vent rules in §803.2 for two or more. After selecting a candidate vent from the tables, apply reductions for elbows, special liners, and altitude as the Code prescribes; when the tables don’t fit your setup, an engineer’s calculation is required. file

Ask a question free See coverage

Last reviewed: July 6, 2026

What the code requires — plain English

Sizing for Category I venting systems is done using the tabulated vent capacities and rules in the Code: use the single-appliance tables and requirements in § 803.1 for one appliance, and the multiple-appliance tables and rules in § 803.2 for common vents serving more than one appliance. The tables give the allowable vent capacities (minimum and maximum) for different vent types and available rises; the text in § 803.1 and § 803.2 prescribes how to apply reductions for offsets/elbows, how to treat zero-lateral (straight vertical) vents, corrugated liners, two-stage appliances, and other special cases. See the sizing tables and explanatory notes in the Code for the specific vent type and configuration you have (§ 803.0 → § 803.1 for single-appliance; § 803.2 for multiple-appliance) .

The single most important rule: size Category I vents from the Code tables in § 803.1 (single-appliance) or § 803.2 (multiple-appliance) and then apply the text rules in those sections (elbow reductions, zero-lateral, corrugated liner reductions, etc.).

Requirements in detail

1) What to use and where

  • Use the single-appliance tables (Table 803.1.2 series) and related text when sizing a vent serving a single Category I appliance; these tables cover Type B vents, single-wall connectors, masonry chimneys, and similar listed systems as applicable (§ 803.1) .
  • Use the multiple-appliance tables (Table 803.2(1) through Table 803.2(9)) where a common vent or common vertical serves two or more appliances and where the Code tables apply (§ 803.2) .
  • If the installation is multistory (appliances on more than one floor), the Code directs use of the multistory provisions and Table 803.2 series with the segment height method (available total height H for each segment) or engineering methods where required (§ 803.2 and related multistory provisions) .

2) Key textual adjustments and definitions (apply after reading the correct table)

  • Zero lateral (L = 0): means a straight vertical vent attached to a top-outlet draft hood or flue collar; the “zero lateral” capacity in the tables applies only to that straight vertical configuration (§ 803.1.3) .
  • Elbow/offset reductions: where lateral lengths or offsets are present the tables’ maximum capacities are reduced:
    • Each additional elbow ≤ 45° reduces the table maximum capacity by 5%.
    • Each additional elbow > 45° and ≤ 90° reduces the table maximum capacity by 10%.
    • Single-appliance venting configurations listed as “zero lateral” must not have elbows; lateral-length entries in the tables include two 90° elbows as part of their allowance (§ 803.1.2) .
  • High altitude: when sizing at altitude, use sea-level input ratings to determine the table maximum capacity, but use the appliance’s actual (altitude-derated) input to determine the minimum capacity (§ 803.1.4) .
  • Two-stage / modulating appliances: the table-determined minimum must be less than the appliance’s lowest input rate, and the table-determined maximum must be greater than the appliance’s highest input rate (§ 803.1.5) .
  • Corrugated metallic chimney liners: listed corrugated liners in masonry chimneys require reducing the table maximum capacity by 20% (multiply max capacity × 0.80); minimum capacity uses the table value. The 20% reduction includes allowance for one long-radius 90° turn at the bottom of the liner; additional offsets require further reductions per the elbow rules (§ 803.1.6) .

3) Single vs. multiple appliance special formulas (area-based rules)

  • For a single draft-hood-equipped appliance one permitted method (alternative to table use) is that the combined effective cross-sectional area of the vent connector and gas vent must be not less than the appliance draft-hood outlet area or greater than 7 times the draft-hood outlet area (§ 802.6.2.1(3)) .
  • For vents serving two draft-hood-equipped appliances, an alternative sizing rule is that the vent effective area be not less than the area of the larger draft hood outlet plus 50% of the area of the smaller draft hood outlet, or greater than 7 times the smaller draft-hood outlet area (§ 802.6.2.1(4)) .

Decision-relevant summary table

Decision item Key value / threshold When to apply Code Reference
Use single-appliance tables N/A — use Table 803.1.2 series Single Category I appliance § 803.1
Use multiple-appliance tables N/A — use Table 803.2 series Common vent for ≥2 appliances or multistory segments § 803.2
Zero lateral L = 0 (straight vertical only) Only applies to straight vertical with top-outlet draft hood § 803.1.3
Elbow reduction — ≤45° −5% per additional elbow Apply to table maximum capacities § 803.1.2
Elbow reduction — >45° to ≤90° −10% per additional elbow Apply to table maximum capacities § 803.1.2
Corrugated liner max reduction −20% to table max (0.80 × max) Corrugated metallic liners in masonry chimneys § 803.1.6
Alternative single-appliance area rule Vent area ≥ draft hood outlet area OR ≥ 7× draft hood outlet area Alternative method instead of tables (draft-hood appliances) § 802.6.2.1(3)
Alternative two-appliance area rule Vent area ≥ (larger draft hood area + 50% × smaller area) OR ≥ 7× smaller outlet Alternative for two draft-hood appliances § 802.6.2.1(4)
Multistory segment sizing Size common segment based on total input entering segment and available vertical height H Multistory common vents (use Table 803.2(1) or engineering method) § 803.2 / multistory rules

Exceptions & special cases

  • If the Code tables do not cover your specific combination of appliance, vent type, offsets, or multistory arrangement, the Code permits or requires engineering methods rather than tabular sizing (use engineering where tables are not applicable) (§ 803.0 / related sections) .
  • Fan-assisted (mechanical-draft) appliances or combinations of fan-assisted and draft-hood appliances are permitted to be sized by § 803.0 tables or engineering methods — check appliance listing and manufacturer instructions for applicable requirements (§ 802.6.2.1 and § 803.0 notes) .
  • Corrugated liners get the 20% maximum-capacity reduction; if additional offsets are present beyond the one long-radius turn, apply elbow/offset reductions in addition (§ 803.1.6) .

Common mistakes

  • Looking up the wrong table: using a single-appliance table for a common vent or vice versa. Always verify you are using the Table 803.1 series for single appliances and Table 803.2 series for multiple-appliance / common vent sizing (§ 803.1, § 803.2) .
  • Forgetting elbow/offset reductions: applying the table maximum capacity without reducing for additional elbows (5% or 10% reductions are required) (§ 803.1.2) .
  • Treating “zero lateral” incorrectly: adding any elbow or offset to a configuration listed as zero lateral invalidates the zero-lateral capacity — zero lateral truly means straight vertical from the appliance top outlet (§ 803.1.3) .
  • Using manufacturer nominal input instead of the proper altitude derating rules for min/max capacity checks (see § 803.1.4 for high-altitude guidance) .
  • Applying corrugated liner tables without the 20% reduction (the liner’s internal geometry reduces flow capacity and must be accounted for) (§ 803.1.6) .

Worked example — single-appliance (step-by-step)

Scenario: You have a single Category I water heater with a draft-hood outlet whose cross-sectional area is 15 in², and you plan to use a commonly available Type B double-wall gas vent. The vent will be nearly vertical (zero lateral) with no elbows.

  1. Confirm configuration matches a single-appliance, top-outlet draft-hood, straight vertical installation → zero lateral applies (§ 803.1.3) .
  2. Option A — use the single-appliance tables (Table 803.1.2(1) etc.) to find the vent size whose minimum capacity is ≤ appliance input and whose maximum capacity is ≥ appliance input, using the zero-lateral column (no elbow reductions). (You must look up the appliance input in Btu/h and compare to the table entries for the vent type and vertical rise.) The Code tables and notes in § 803.1 show how to read these values .
  3. Option B — use the alternative area rule for a single draft-hood appliance: the effective vent area (connector + vent) must be not less than the appliance draft-hood outlet area (≥ 15 in²), or be greater than 7× the draft-hood outlet area (i.e., > 105 in²) (§ 802.6.2.1(3)). If your selected vent’s effective area is ≥ 15 in² it meets the “not less than” criterion; if you choose to rely on the 7× rule you would need > 105 in² which is usually much larger than typical vents and therefore rarely the chosen route. Cite: § 802.6.2.1(3) .
  4. If the vent were to have elbows or lateral length, apply the reductions (5% or 10% per elbow) to the table maximums or follow the lateral-length column in the tables (§ 803.1.2) .

Note: I cannot pick a numeric vent diameter from the tables here because the tabulated capacities for specific vent diameters and rises are in the Code tables (Table 803.1.2(1) etc.). Consult Table 803.1.2 appropriate to the vent type and available rise to select the correct vent size and confirm min/max capacity for your appliance (§ 803.1) .

Related provisions (quick list)

  • § 803.0 — Sizing of Category I venting systems (overview and table use)
  • § 803.1.2 — Elbows and lateral-length reductions (specific reduction percentages)
  • § 803.1.3 — Definition / application of zero lateral
  • § 803.1.4 — High-altitude sizing rules (sea-level vs derated input)
  • § 803.1.6 — Corrugated metallic chimney liner reductions (20% max reduction)
  • § 803.2 — Additional requirements and tables for multiple-appliance vents (including multistory segment sizing)
  • § 802.6.2.1(3)/(4) — Alternative area-based sizing rules for single and two-appliance draft-hood installations (vent area ≥ draft hood or 7×; two-appliance formula)

Code references

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

  • CMC § 802.6.2 High relevance — show source text

    installation instructions.

    802.6.2 Size of Gas Vents. Venting systems shall be sized and constructed in accordance with Section 802.6.2.1 through Section 802.6.2.3 and the appliance manufacturer’s instructions. [NFPA 54:12.7.4] 802.6.2.1 Category I Appliances. The sizing of natural draft venting systems serving one or more listed appliances equipped with a draft hood or appliances listed for use with a Type B gas vent, installed in a single story of a building, shall be in accordance with one of the following: (1) The provisions of Section 803.0. (2) Vents serving fan-assisted combustion system appliances, or combinations of fan-assisted combustion system and draft hood-equipped appliances, shall be sized in accordance with Section 803.0 or other engineering methods. (3) For sizing an individual gas vent for a single, draft hood-equipped appliance, the effective area of the vent connector and the gas vent shall be not less than the area of the appliance draft hood outlet or greater than seven times the draft hood outlet area.

    (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]

    802.6.2.2 Vent Offsets. Type B and Type L vents sized in accordance with Section 802.6.2.1(3) or Section 802.6.2.1(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 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] 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.

  • CMC § 802.1 High relevance — show source text

    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

    Table 802.10.1.4 Minimum Thickness for Steel

    Vent Connectors for Medium Heat Appliances . . . . . . . . . . . . . . 161

    Table 802.8.1 Through-the-Wall Direct Vent Termination Clearances . . . . . . . . 162

    802.11 Vent Connectors for Category II, Category III, and Category IV Appliances . . . . . . . . . . . . . . . . . . 164

    802.12 Appliances Requiring Draft Hoods . . . . . . . . . . . . . . . . . . 164

    802.13 Manually Operated Dampers . . . . 165

    802.14 Automatically Operated Vent Dampers . . . . . . . . . . . . . . . . . . . . 165

    802.15 Obstruction of Flow. . . . . . . . . . . . 165

    803.0 Sizing of Category I Venting Systems . . . . . . . . . . . . . . . . . . . . 165

    803.1 Additional Requirements to Single Appliance Vent . . . . . . . . . 165

    803.2 Additional Requirements to Multiple-Appliance Vent.. . . . . . . . 167

    Table 803.2.1 Vent Connector Maximum

    Length. . . . . . . . . . . . . . . . . . . . . . 167

    Table 803.1.2(1) Type B Double-Wall Gas Vent . . . 172

    Table 803.1.2(2) Type B Double-Wall Gas Vent . . . 175

  • CMC § 802.10.1.4 High relevance — show source text

    Table 802.10.1.4 Minimum Thickness for Steel

    Vent Connectors for Medium Heat Appliances . . . . . . . . . . . . . . 161

    Table 802.8.1 Through-the-Wall Direct Vent Termination Clearances . . . . . . . . 162

    802.11 Vent Connectors for Category II, Category III, and Category IV Appliances . . . . . . . . . . . . . . . . . . 164

    802.12 Appliances Requiring Draft Hoods . . . . . . . . . . . . . . . . . . 164

    802.13 Manually Operated Dampers . . . . 165

    802.14 Automatically Operated Vent Dampers . . . . . . . . . . . . . . . . . . . . 165

    802.15 Obstruction of Flow. . . . . . . . . . . . 165

    803.0 Sizing of Category I Venting Systems . . . . . . . . . . . . . . . . . . . . 165

    803.1 Additional Requirements to Single Appliance Vent . . . . . . . . . 165

    803.2 Additional Requirements to Multiple-Appliance Vent.. . . . . . . . 167

    Table 803.2.1 Vent Connector Maximum

    Length. . . . . . . . . . . . . . . . . . . . . . 167

    Table 803.1.2(1) Type B Double-Wall Gas Vent . . . 172

    Table 803.1.2(2) Type B Double-Wall Gas Vent . . . 175

    Table 803.1.2(3) Masonry Chimney. . . . . . . . . . . . . 177

    Table 803.1.2(4) Masonry Chimney. . . . . . . . . . . . . 179

    Table 803.1.2(5) Single-Wall Metal Pipe or Type B Asbestos-Cement Vent. . . . . . . . . 181

    Table 803.1.2(6) Exterior Masonry Chimney . . . . . . 182

    2025 CALIFORNIA MECHANICAL CODE

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

    TABLE OF CONTENTS

    Table 803.2(1) Type B Double-Wall Vent . . . . . . 183

    Table 803.2(2) Type B Double-Wall Vent . . . . . . . 187

    Table 803.2(3) Masonry Chimney. . . . . . . . . . . . . 189

    Table 803.2(4) Masonry Chimney. . . . . . . . . . . . . 191

    Table 803.2(5) Single-Wall Metal Pipe or Type B Asbestos-Cement Vent . . . 193

    Table 803.2(6) Exterior Masonry Chimney . . . . . . 193

    Table 803.2(7) Exterior Masonry Chimney . . . . . . 194

    Table 803.2(8) Exterior Masonry Chimney . . . . . . 195

    Table 803.2(9) Exterior Masonry Chimney . . . . . . 196

    CHAPTER 9 INSTALLATION OF

    SPECIFIC APPLIANCES. . . . . . . 197

  • CMC § 803.1.2 High relevance — show source text

    803.1.2 Elbows. Single-appliance venting configurations with zero (0) lateral lengths in Table 803.1.2(1), Table 803.1.2(2), and Table 803.1.2(5) shall not have elbows in the venting system. Single-appliance venting with lateral lengths include two 90 degree elbows. For each additional elbow up to and including 45 degrees, the maximum capacity listed in the venting tables shall be reduced by 5 percent. For each additional elbow greater than 45 degrees up to and including 90 degrees, the maximum capacity listed in the venting tables shall be reduced by 10 percent. Where multiple offsets occur in a vent, the total lateral length of all offsets combined shall not exceed that specified in Table 803.1.2(1) through Table 803.1.2(5). [NFPA 54:13.1.3]

    803.1.3 Zero Lateral. Zero (0) lateral ( L ) shall apply only to a straight vertical vent attached to a top outlet draft hood or flue collar. [NFPA 54:13.1.4]

    803.1.4 High-Altitude Installations. Sea level input ratings shall be used when determining maximum capacity for high-altitude installation. Actual input (derated for altitude) shall be used for determining minimum capacity for high-altitude installation. [NFPA 54:13.1.5]

    803.1.5 Two-Stage/Modulating Appliances. For appliances with more than one input rate, the minimum vent capacity (FAN Min) determined from Table 803.1.2(1) through Table 803.2(9) shall be less than the lowest appliance input rating, and the maximum vent capacity (FAN Max/NAT Max) determined from the tables shall be greater than the highest appliance rating input. [NFPA 54:13.1.6]

    803.1.6 Corrugated Chimney Liners. Listed corrugated metallic chimney liner systems in masonry chimneys shall be sized by using Table 803.1.2(1) or Table 803.1.2(2) for Type B vents, with the maximum capacity reduced by 20 percent (0.80 x maximum capacity) and the minimum capacity as shown in Table 803.1.2(1) or Table 803.1.2(2).

    Corrugated metallic liner systems installed with bends or offsets shall have their maximum capacity fur

    ther reduced in accordance with Section 803.1.2. The 20 percent reduction for corrugated metallic chimney liner systems includes an allowance for one long radius 90 degree (1.57 rad) turn at the bottom of the liner. [NFPA 54:13.1.7]

    803.1.7 Connection to Chimney Liners. Connections between chimney liners and listed double-wall connectors shall be made with listed adapters designed for such purpose. [NFPA 54:13.1.8]

  • CMC § 101.0 High relevance — show source text

    2025 CALIFORNIA MECHANICAL CODE 533

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

    534 2025 CALIFORNIA MECHANICAL CODE

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

    »

    »»

    »

    »

    APPENDIX F

    SIZING OF VENTING SYSTEMS AND OUTDOOR COMBUSTION

    AND VENTILATION OPENING DESIGN

    The provisions contained in this appendix are not mandatory unless specifically adopted by a state agency, or referenced in the adopting ordinance.

    F 101.0 General.

    F 101.1 Applicability. This appendix provides general guidelines for sizing venting systems serving appliances equipped with draft hoods, Category I appliances, and appliances listed for use with Type B vents.

    Table 803.1.2(1) is used when sizing a Type B double-wall gas vent connected directly to the appliance.

    Note: The appliance can be either Category I draft hood-equipped or fanassisted type.

    FIGURE F 101.2(1) TYPE B DOUBLE-WALL VENT SYSTEM SERVING A SINGLE APPLIANCE WITH A TYPE B DOUBLE-WALL VENT

    [NFPA 54: FIGURE F.1(a)]

    Table 803.1.2(2) is used when sizing a single-wall metal vent connector attached to a Type B double-wall gas vent.

    Note: The appliance can be either Category I draft hood-equipped or fanassisted type.

    FIGURE F 101.2(2) TYPE B DOUBLE-WALL VENT SYSTEM SERVING A SINGLE APPLIANCE WITH A SINGLE-WALL METAL VENT CONNECTOR

    [NFPA 54: FIGURE F.1(b)]

    F 101.2 Examples Using Single Appliance Venting Tables. See Figure F 101.2(1) through Figure F 101.2(14).

    [NFPA 54:F.1]

    Table 803.1.2(3) is used when sizing a Type B double-wall gas vent connector attached to a tile-lined masonry chimney.

    Notes:

    1. A is the equivalent cross-sectional area of the tile liner.
    2. The appliance can be either Category I draft hood-equipped or fanassisted type.

    FIGURE F 101.2(3) VENT SYSTEM SERVING A SINGLE APPLIANCE WITH A MASONRY CHIMNEY AND A TYPE B DOUBLE-WALL VENT CONNECTOR

    [NFPA 54: FIGURE F.1(c)]

    Table 803.1.2(4) is used when sizing a single-wall vent connector attached to a tile-lined masonry chimney.

    Notes:

    1. A is the equivalent cross-sectional area of the tile liner.
    2. The appliance can be either Category I draft hood-equipped or fanassisted type.

    FIGURE F 101.2(4) VENT SYSTEM SERVING A SINGLE APPLIANCE USING A MASONRY CHIMNEY AND A SINGLE-WALL METAL VENT CONNECTOR

    [NFPA 54: FIGURE F.1(d)]

    «

    «

    «

    2025 CALIFORNIA MECHANICAL CODE 535

  • 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 PLUMBING CODE 423

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

    424 2025 CALIFORNIA PLUMBING CODE

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

    APPENDIX G

    SIZING OF VENTING SYSTEMS

    The provisions contained in this appendix are not mandatory unless specifically adopted by a state agency, or referenced in the adopting ordinance.

    G 101.0 General.

    G 101.1 Applicability. This appendix provides general guidelines for sizing venting systems serving appliances equipped with draft hoods, Category I appliances, and appliances listed for use with Type B vents.

    Table 510.1.2(1) is used when sizing a Type B double-wall gas vent connected directly to the appliance.

    Note: The appliance can be either Category I draft hood-equipped or fanassisted type. FIGURE G 101.2(1) TYPE B DOUBLE-WALL VENT SYSTEM SERVING A SINGLE APPLIANCE WITH A TYPE B DOUBLE-WALL VENT

    [NFPA 54: FIGURE F.1(a)]

    Table 510.1.2(2) is used when sizing a single-wall metal vent connector attached to a Type B double-wall gas vent.

    Note: The appliance can be either Category I draft hood-equipped or fanassisted type. FIGURE G 101.2(2) TYPE B DOUBLE-WALL VENT SYSTEM SERVING A SINGLE APPLIANCE WITH A SINGLE-WALL METAL VENT CONNECTOR

    [NFPA 54: FIGURE F.1(b)]

    G 101.2 Examples Using Single Appliance Venting Tables. See Figure G 101.2(1) through Figure G 101.2(14).

    [NFPA 54:F.1]

    Table 510.1.2(3) is used when sizing a Type B double-wall gas vent connector attached to a tile-lined masonry chimney.

    Notes:

    1. A is the equivalent cross-sectional area of the tile liner.
    2. The appliance can be either Category I draft hood-equipped or fan-assisted type. FIGURE G 101.2(3) VENT SYSTEM SERVING A SINGLE APPLIANCE WITH A MASONRY CHIMNEY AND A TYPE B DOUBLE-WALL VENT CONNECTOR

    [NFPA 54: FIGURE F.1(c)]

    Table 510.1.2(4) is used when sizing a single-wall vent connector attached to a tile-lined masonry chimney.

    Notes:

    1. A is the equivalent cross-sectional area of the tile liner.
    2. The appliance can be either Category I draft hood-equipped or fan-assisted type. FIGURE G 101.2(4) VENT SYSTEM SERVING A SINGLE APPLIANCE USING A MASONRY CHIMNEY AND A SINGLE-WALL METAL VENT CONNECTOR

    [NFPA 54: FIGURE F.1(d)]

  • CMC § 25.4 High relevance — show source text

    For SI Units: 1 inch = 25.4 mm, 1 foot = 304.8 mm

    509.6.1.1 Insulation Protection Shield. Where a vent passes through an insulated assembly, an approved metal shield constructed of steel having a thickness of not less than 26 gauge (0.0179 inch) (0.45 mm) (No. 26 gauge) shall be installed between the vent and insulation. The shield shall extend not less than 2 inches (51 mm) above the insulation and be secured to the structure in accordance with the

    manufacturer’s installation instructions.

    509.6.2 Size of Gas Vents. Venting systems shall be sized and constructed in accordance with Section

    2025 CALIFORNIA PLUMBING CODE 95

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

    WATER HEATERS

    509.6.2.1 through Section 509.6.2.3 and the appliance manufacturer’s instructions. [NFPA 54:12.7.4] 509.6.2.1 Category I Appliances. The sizing of natural draft venting systems serving one or more listed appliances equipped with a draft hood or appliances listed for use with a Type B gas vent, installed in a single story of a building, shall be in accordance with one of the following:

    (1) The provisions of Section 510.0.

    (2) Vents serving fan-assisted combustion system appliances, or combinations of fan-assisted combustion system and draft hood-equipped appliances, shall be sized in accordance with Section 510.0 or other engineering methods.

    (3) For sizing an individual gas vent for a single, draft hood-equipped appliance, the effective area of the vent connector and the gas vent shall be not less than the area of the appliance draft hood outlet or greater than seven times the draft hood outlet area.

    (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.

  • CMC § 1.11.0. High relevance — show source text
    Adopting Agency BSC BSC-
    CG    		 SFM 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-
    CG    		 SFM 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
    Adopt Entire Chapter as
    amended (amended sections
    listed below)
    Adopt only those sections
    that are listed below
    Chapter/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 533

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

    534 2025 CALIFORNIA MECHANICAL CODE

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

    »

    »»

    »

    »

    APPENDIX F

    SIZING OF VENTING SYSTEMS AND OUTDOOR COMBUSTION

    AND VENTILATION OPENING DESIGN

    The provisions contained in this appendix are not mandatory unless specifically adopted by a state agency, or referenced in the adopting ordinance.

    F 101.0 General.

    F 101.1 Applicability. This appendix provides general guidelines for sizing venting systems serving appliances equipped with draft hoods, Category I appliances, and appliances listed for use with Type B vents.

    Table 803.1.2(1) is used when sizing a Type B double-wall gas vent connected directly to the appliance.

    Note: The appliance can be either Category I draft hood-equipped or fanassisted type.

    FIGURE F 101.2(1) TYPE B DOUBLE-WALL VENT SYSTEM SERVING A SINGLE APPLIANCE WITH A TYPE B DOUBLE-WALL VENT

    [NFPA 54: FIGURE F.1(a)]

    Table 803.1.2(2) is used when sizing a single-wall metal vent connector attached to a Type B double-wall gas vent.

    Note: The appliance can be either Category I draft hood-equipped or fanassisted type.

  • CMC § 509.1 High relevance — show source text

    509.1 Listing . . . . . . . . . . . . . . . . . . . . . . .91

    509.2 Connection to Venting Systems . . .91

    509.3 Minimum Safe Performance . . . . . .91

    509.4 Type of Venting System to be Used . . . . . . . . . . . . . . . . . . . . .92

    Table 509.4 Type of Venting System to be Used . . . . . . . . . . . . . . . . . . .92

    509.5 Masonry, Metal, and Factory-Built Chimneys . . . . . . . . .93

    509.6 Gas Vents . . . . . . . . . . . . . . . . . . . .94

    Table 509.6.1 Roof Slope Heights . . . . . . . . . . . .95

    509.7 Single-Wall Metal Pipe . . . . . . . . . .97

    Table 509.7.3.4(1) Clearances for Connectors . . . . . . .97

    Table 509.7.3.4(2) Reduction of Clearances with Specified Forms of Protection . . . .99

    509.8 Through-the-Wall Vent Termination . . . . . . . . . . . . . . . . . .100

    509.9 Condensation Drain . . . . . . . . . . .100

    509.10 Vent Connectors for

    Category I Appliances . . . . . . . . .100

    Table 509.8.1 Through-the-Wall Direct-Vent Termination Clearances . . . . . . . .101

    Table 509.10.1.3 Minimum Thickness for

    Galvanized Steel Vent

    Connectors for Low-Heat

    Appliances . . . . . . . . . . . . . . . . . .102

    Table 509.10.1.4 Minimum Thickness for Steel

    Vent Connectors for

    Medium-Heat Appliances . . . . . . .102

    509.11 Vent Connectors for

    Category II, Category III, and Category IV Appliances . . . . . . . .104

    509.12 Appliances Requiring Draft Hoods . . . . . . . . . . . . . . . . . . . . . .104

    509.13 Manually Operated Dampers . . . .104

    509.14 Automatically Operated Vent Dampers . . . . . . . . . . . . . . . .104

    509.15 Obstructions . . . . . . . . . . . . . . . . .104

    510.0 Sizing of Category I Venting Systems . . . . . . . . . . . . . . . . . . . .104

    510.1 Additional Requirements to Single Appliance Vent . . . . . . . . .104

    510.2 Additional Requirements to Multiple-Appliance Vent . . . . . . . . .106

    Table 510.2.1 Vent Connector Maximum

    Length . . . . . . . . . . . . . . . . . . . . . .106

    Table 510.1.2(1) Type B Double-Wall Gas Vent . . .110

    Table 510.1.2(2) Type B Double-Wall Gas Vent . . .113

    Table 510.1.2(3) Masonry Chimney . . . . . . . . . . . . .115

  • CMC § 140.4 High relevance — show source text

    w.g.) of the device/component by the watts/cfm in Table 140.4-B.
    5.
    This allowance can only be taken for healthcare facilities.|1.
    See FAN SYSTEM, MULTI-ZONE VARIABLE AIR VOLUME (VAV) in definitions for “Multi-zone” to be classified as a multi-zone VAV system.
    2.
    Filter pressure loss can only be counted once per fan system.
    3.
    Enthalpy recovery ratio (ERR) calculated per ANSI/ASHRAE 84-2020.
    4.
    Power allowance requires further calculation, multiplying the actual pressure drop (in. w.g.) of the device/component by the watts/cfm in Table 140.4-B.
    5.
    This allowance can only be taken for healthcare facilities.|1.
    See FAN SYSTEM, MULTI-ZONE VARIABLE AIR VOLUME (VAV) in definitions for “Multi-zone” to be classified as a multi-zone VAV system.
    2.
    Filter pressure loss can only be counted once per fan system.
    3.
    Enthalpy recovery ratio (ERR) calculated per ANSI/ASHRAE 84-2020.
    4.
    Power allowance requires further calculation, multiplying the actual pressure drop (in. w.g.) of the device/component by the watts/cfm in Table 140.4-B.
    5.
    This allowance can only be taken for healthcare facilities.|1.
    See FAN SYSTEM, MULTI-ZONE VARIABLE AIR VOLUME (VAV) in definitions for “Multi-zone” to be classified as a multi-zone VAV system.
    2.
    Filter pressure loss can only be counted once per fan system.
    3.
    Enthalpy recovery ratio (ERR) calculated per ANSI/ASHRAE 84-2020.
    4.
    Power allowance requires further calculation, multiplying the actual pressure drop (in. w.g.) of the device/component by the watts/cfm in Table 140.4-B.
    5.
    This allowance can only be taken for healthcare facilities.|1.
    See FAN SYSTEM, MULTI-ZONE VARIABLE AIR VOLUME (VAV) in definitions for “Multi-zone” to be classified as a multi-zone VAV system.
    2.
    Filter pressure loss can only be counted once per fan system.
    3.
    Enthalpy recovery ratio (ERR) calculated per ANSI/ASHRAE 84-2020.
    4.
    Power allowance requires further calculation, multiplying the actual pressure drop (in. w.g.) of the device/component by the watts/cfm in Table 140.4-B.
    5.
    This allowance can only be taken for healthcare facilities.|1.
    See FAN SYSTEM, MULTI-ZONE VARIABLE AIR VOLUME (VAV) in definitions for “Multi-zone” to be classified as a multi-zone VAV system.
    2.
    Filter pressure loss can only be counted once per fan system.
    3.
    Enthalpy recovery ratio (ERR) calculated per ANSI/ASHRAE 84-2020.
    4.
    Power allowance requires further calculation, multiplying the actual pressure drop (in. w.g.) of the device/component by the watts/cfm in Table 140.4-B.
    5.
    This allowance can only be taken for healthcare facilities.|

  • CMC § 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]

  • CMC § 110.2 High relevance — show source text

    Compliance with the cooling-only perfor-
    mance is required as defined in Notes b and c of Table 110.2-I.
    h. Water-to-water heat pumps with a capacity less than 135,000 Btu/h are included in Table 110.2-B, Heat Pumps—Minimum Efficiency Requirements.
    i. Source leaving liquid temperature.
    1. The cooling evaporator liquid flow rate used for the heating rating for a reverse cycle air-to-water heat pump shall be the flow rate determined during the full-load cool-
    ing rating.
    2. The cooling evaporator liquid flow rate for the simultaneous cooling and heating and heat recovery liquid cooled chilling packages rating shall be the liquid flow rates
    from the cooling operation full-load rating.
    3. For heating-only fluid-to-fluid chiller packages, the evaporator flow rate obtained with an entering liquid temperature of 54°F and a leaving liquid temperature of 44°F
    shall be used.
    j. NA means the requirements are not applicable.|a. The size category is the full-load net refrigeration cooling mode capacity, which is the capacity of the evaporator available for cooling of the thermal load external to the chill-
    ing package.
    b. For air source heat pumps, compliance with both the 47°F and 17°F heating source outdoor air temperature (OAT) rating efficiency is required for heating.
    c. Heating full-load rating conditions are at standard rating conditions defined in AHRI 550/590 (I-P), Table 4, which includes the impact of defrost for air source heating ratings.
    d. For units that operate in both cooling and heating, compliance with both the cooling and heating efficiency is required.
    e. For heat recovery heating chilling package applications where there is simultaneous cooling and heating, compliance with the heating performance heat recover COPHR is
    only required at one of the four heating AHRI 550/590 (I-P) standard ratings conditions of Low, Medium, Hot-Water 1 or Hot-Water 2. Compliance with the cooling-only perfor-
    mance is required as defined in Notes b and c of Table 110.2-I.
    f. For liquid source heat recovery chilling packages that have capabilities for heat rejection to a heat recovery condenser and a tower condenser, the COPHR applies to operation
    at full load with 100 percent heat recovery (no tower rejection). Units that only have capabilities for partial heat recovery shall meet the requirements of Table 110.2-D, Water
    Chilling Packages—Minimum Efficiency Requirements.
    g. For heat recovery heating chilling package applications where there is simultaneous cooling and heating, compliance with the heating performance heat recover COPHR is
    only required at one of the four heating AHRI 550/590 (I-P) standard ratings conditions of Low, Medium, Hot-Water 1 or Hot-Water 2. Compliance with the cooling-only perfor-
    mance is required as defined in Notes b and c of Table 110.2-I.
    h. Water-to-water heat pumps with a capacity less than 135,000 Btu/h are included in Table 110.2-B, Heat Pumps—Minimum Efficiency Requirements.
    i. Source leaving liquid temperature.
    1. The cooling evaporator liquid flow rate used for the heating rating for a reverse cycle air-to-water heat pump shall be the flow rate determined during the full-load cool-
    ing rating.
    2. The cooling evaporator liquid flow rate for the simultaneous cooling and heating and heat recovery liquid cooled chilling packages rating shall be the liquid flow rates
    from the cooling operation full-load rating.
    3.

Frequently asked questions

How do I know whether to use the 803.1 tables or an area-based rule?

If you’re sizing a single draft-hood appliance you may either use the Code single-appliance tables in § 803.1 or the alternative area rules found in § 802.6.2.1(3). For multiple appliances or multistory common vents use § 803.2 tables; if tables don’t cover your case, use engineering methods file.

What is “zero lateral” and why does it matter?

Zero lateral means a straight vertical vent attached to a top-outlet draft hood or flue collar. The Code’s zero-lateral table entries give higher allowable capacities and must not be used if any elbows or offsets are present (§ 803.1.3) .

How do elbows affect the tabulated vent capacities?

Each additional elbow ≤ 45° reduces the table maximum capacity by 5%; each additional elbow >45° and ≤90° reduces the table maximum by 10%. Also, some lateral-length table entries already include allowance for two 90° elbows — read the table footnotes (§ 803.1.2) .

My installation uses a corrugated chimney liner — what changes?

Reduce the table maximum capacity by 20% (multiply max × 0.80). The minimum capacity remains the table minimum. The 20% reduction includes one long-radius 90° turn allowance; additional offsets require elbow reductions too (§ 803.1.6) .

If appliances are on different floors, can I use the same Table 803.2 entries?

Yes — multistory common vents are sized by considering each vertical segment’s available total height H and the total input entering that segment; use Table 803.2 series for Type B vents or use engineering methods when required (§ 803.2 and multistory rules) .

More in California Mechanical Code

Ask about the CMC

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

Start Free Trial

Related in the CMC

What tables and limits govern vent connector lengths and vent diameters?

What are the vent connector construction thickness and damper rules for Category I appliances?

When can corrugated or upsized vent connectors be used?

Sizing Category I Venting Systems & Vent Connectors

California Mechanical Code