CMC · California Mechanical Code

What are the minimum safe performance and draft requirements for venting systems?

Venting systems must carry all combustion gases safely outdoors and meet the specific draft values given by the appliance manufacturer. Mechanical draft fans must be UL‑378 listed, installed per manufacturer instructions, and any positive‑pressure portions must be sealed so flue gases cannot leak into the building; natural‑draft connectors may not tie into positive‑pressure mechanical sections.

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Last reviewed: July 6, 2026

What the code requires — 2-4 sentences

Venting systems must be designed and built to convey all flue and vent gases to the outdoors (the basic minimum safe performance) — § 802.3 . A venting system must satisfy the draft requirements specified by the appliance manufacturer — § 802.3.1 . If a mechanical draft system is used it must be listed to UL 378 and installed per the appliance and draft‑system manufacturers’ instructions, and any portions under positive pressure must be designed to prevent leakage of flue or vent gases into the building — § 802.3.3 and § 802.3.3.2 .

The single most important rule: venting systems must carry all combustion products safely outdoors and must meet the appliance maker’s draft requirements; mechanical draft parts under positive pressure must not leak into the building. § 802.3; § 802.3.1; § 802.3.3.2

Requirements in detail

Core performance requirements (plain language)

  • Convey all flue and vent gases to the outdoors — that is the baseline performance target for every venting system (§ 802.3).
  • Meet appliance draft requirements — the venting system must provide the draft (direction and magnitude as required) called out in the appliance manufacturer’s installation instructions (§ 802.3.1).
  • Mechanical draft systems (forced or induced) must be listed to UL 378 and installed exactly per the manufacturer instructions for both the appliance and the draft system (§ 802.3.3).
  • Leakage control — any portion of a forced draft system, and any portion of an induced draft system that operates under positive pressure, must be designed and installed to prevent leakage of flue or vent gases into the building (§ 802.3.3.2).

Decision‑relevant dimensions / values (quick reference table)

Requirement (decision point) When you use it What the code says (value / test) Code Reference
Minimum safe performance Every venting installation Must convey all flue/vent gases to the outdoors § 802.3
Draft level used for acceptance Commissioning / inspection Must satisfy the appliance manufacturer’s draft requirements (manufacturer’s instructions provide the numerical values and test method) § 802.3.1
Mechanical draft listing & installation When using forced/induced draft fans Mechanical draft system must be listed to UL 378 and installed per both manufacturers’ instructions § 802.3.3
Leakage control for positive pressure sections Design and installation phase Forced draft and induced-draft portions under positive pressure shall be designed/installed to prevent leakage into the building § 802.3.3.2
Prohibited connections Configuration checks Natural‑draft vent connectors shall not be connected into any portion of a mechanical draft system operating under positive pressure § 802.3.3.3
Shutoff / interlock on draft failure Safety control design Where mechanical draft is used, provide means to prevent gas flow to main burners when the draft system is not performing to satisfy safe operation § 802.3.3.4

How to interpret “satisfy the appliance draft requirements”

  • The code defers to the appliance manufacturer’s installation instructions for the required draft (magnitude and direction). Inspectors and installers must obtain the appliance’s specified draft requirement and verify field measurements meet it — this is the compliance yardstick (§ 802.3.1).
  • The CMC does not publish a single numeric draft value for all appliances — the necessary number comes from the appliance label/manufacturer. Follow those instructions for commissioning and testing. § 802.3.1

Exceptions & special cases

  • Manufacturer’s instructions control: where appliance instructions provide venting or draft criteria, those instructions govern the installation and sizing; the code requires compliance with those instructions (§ 802.3.1).
  • Mechanical draft systems must be listed to UL 378; if an installer proposes an alternative engineering method or system configuration, the installer must demonstrate equivalence and follow any listed/approved methods — the code requires listing and manufacturer installation instructions as the standard (§ 802.3.3).
  • Any mechanical draft portion that is under positive pressure during operation has stricter sealing/leakage requirements to protect building occupants — design and installation must explicitly address leakage control (§ 802.3.3.2).

Common mistakes

  • Assuming the code supplies a single draft pressure to meet: the CMC requires meeting the appliance manufacturer’s draft requirements — do not substitute a generic value (§ 802.3.1).
  • Connecting a natural‑draft vent connector into a mechanical draft segment that runs under positive pressure — this is prohibited and creates a leakage / CO hazard (§ 802.3.3.3) .
  • Using an unlisted mechanical draft assembly or failing to follow the listing/installation instructions for UL 378‑listed components (§ 802.3.3).
  • Omitting interlocks or controls that prevent gas flow when the draft system is not operating properly — the code requires preventing burner operation when draft is inadequate for safe performance (§ 802.3.3.4) .

Worked example (how to apply the rule on a real job)

Scenario: You are installing a gas-fired appliance that has a factory-supplied draft requirement and you plan to use a mechanical induced-draft fan.

Steps to conform with the CMC:

  1. Obtain the appliance manufacturer’s installation instructions and note the required draft conditions (direction and test location) — this is the compliance requirement in § 802.3.1.
  2. Select a mechanical draft assembly that is listed to UL 378 and confirm the draft assembly’s instructions are compatible with the appliance; install both the appliance and the draft assembly per their manufacturers’ instructions (§ 802.3.3) .
  3. If the induced‑draft fan or any duct section is positive pressure during operation, ensure joints and body construction prevent leakage of flue gases into the building — design and seal accordingly per § 802.3.3.2.
  4. Provide a control/interlock that prevents the appliance main burners from receiving gas if the draft system is not performing to satisfy safe operation (per § 802.3.3.4).
  5. Commission: measure the draft as required by the appliance instructions. The code requires the venting system to satisfy the manufacturer’s draft requirement; if measured values do not meet the appliance requirement, revise the venting or controls until they do (§ 802.3.1) .

Note: the CMC sections cited above mandate the process and responsibilities; the actual numeric draft (e.g., in inches water column) is supplied by the appliance manufacturer and must be used for acceptance testing — the code itself does not prescribe a single numeric draft for all appliances (§ 802.3.1) .

Related provisions

  • § 802.3 (Minimum Safe Performance) — baseline requirement that venting systems convey all flue gases outdoors.
  • § 802.3.1 (Appliance Draft Requirements) — venting must satisfy appliance manufacturer’s draft requirements.
  • § 802.3.3 (Mechanical Draft Systems) — listing (UL 378) and installation per manufacturers’ instructions.
  • § 802.3.3.2 (Leakage) — forced draft and positive‑pressure induced‑draft portions must prevent leakage into the building.
  • § 802.3.3.3 (Vent Connectors) — natural draft vent connectors must not tie into positive pressure mechanical draft portions.
  • § 802.3.3.4 (Operation) — prevent burner gas flow when draft system is not performing to satisfy safe operation.

Code references

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

  • CMC § 802.3 High relevance — show source text

    802.3 Minimum Safe Performance. Venting systems shall be designed and constructed to convey all flue and vent gases to the outdoors. [NFPA 54:12.1]

    802.3.1 Appliance Draft Requirements. A venting system shall satisfy the draft requirements of the appliance in accordance with the manufacturer’s instructions. [NFPA 54:12.4.1]

    802.3.2 Appliance Venting Requirements. Appliances required to be vented shall be connected to a vent

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    CHIMNEYS AND VENTS

    ing system designed and installed in accordance with the provisions of Section 802.4 through Section 802.15.

    [NFPA 54:12.4.2]

    802.3.3 Mechanical Draft Systems. Mechanical draft systems shall be listed in accordance with UL 378 and installed in accordance with both the appliance and the mechanical draft system manufacturer’s installation instructions. [NFPA 54:12.4.3.1]

    802.3.3.1 Venting. Appliances requiring venting shall be permitted to be vented by means of mechanical draft systems of either forced or induced draft design. [NFPA 54:12.4.3.2]

    802.3.3.2 Leakage. Forced draft systems and all portions of induced draft systems under positive pressure during operation shall be designed and installed so as to prevent leakage of flue or vent gases into a building. [NFPA 54:12.4.3.3]

    802.3.3.3 Vent Connectors. Vent connectors

    serving appliances vented by natural draft shall not be connected into any portion of mechanical draft systems operating under positive pressure. [NFPA 54:12.4.3.4]

    802.3.3.4 Operation. Where a mechanical draft system is employed, provision shall be made to prevent the flow of gas to the main burners when the draft system is not performing so as to satisfy the operating requirements of the appliance for safe performance. [NFPA 54:12.4.3.5]

    802.3.4 Ventilating Hoods and Exhaust Systems. Where automatically operated appliances, other than food service appliances, are vented through a ventilating hood or exhaust system equipped with a damper or with a power means of exhaust, provisions shall be made to allow the flow of gas to the main burners only when the damper is open to a position to properly vent the appliance and when the power means of exhaust is in operation. [NFPA 54:12.4.4.1]

    802.3.5 Circulating Air Ducts, Above-Ceiling Air- Handling Spaces, and Furnace Plenums. Venting systems shall not extend into or pass through any fabricated air duct or furnace plenum. [NFPA 54:12.4.5.1]

    802.3.6 Above-Ceiling or Nonducted Air Han- dling System. Where a venting system passes through an above-ceiling air space or other nonducted portion of an air-handling system, it shall conform to one of the following requirements:

  • CMC § 802.2.3 High relevance — show source text

    802.2.3 Adjacent Room or Space. Where the calculation includes the volume of an adjacent room or space, the room or space in which the appliances are installed shall be directly connected to the adjacent room or space by a doorway, archway, or other opening of comparable size that cannot be closed. [NFPA 54:12.3.2.2]

    802.2.4 Ventilating Hoods. The use of ventilating hoods and exhaust systems to vent appliances shall be limited to industrial appliances and appliances installed in commercial applications. [NFPA 54:12.3.3]

    802.2.5 Well-Ventilated Spaces. The flue gases from industrial-type appliances shall not be required to be vented to the outdoors where such gases are discharged into a large and well-ventilated industrial space. [NFPA 54:12.3.4]

    802.2.6 Direct Vent Appliances. Listed direct vent appliances shall be installed in accordance with the manufacturer’s installation instructions. [NFPA 54:12.3.5.1]

    802.2.6.1 Through-the-Wall Vent Terminations. Through-the-wall vent terminations for listed direct vent appliances shall be in accordance with Section 802.8.1. [NFPA 54:12.3.5.2]

    802.2.7 Appliances with Integral Vents. Appliances incorporating integral venting means shall be installed in accordance with Section 802.8.1. [NFPA 54:12.3.6]

    802.2.8 Incinerators, Commercial–Industrial. Incinerators shall be vented in accordance with NFPA 82.

    [NFPA 54:12.3.7]

    802.3 Minimum Safe Performance. Venting systems shall be designed and constructed to convey all flue and vent gases to the outdoors. [NFPA 54:12.1]

    802.3.1 Appliance Draft Requirements. A venting system shall satisfy the draft requirements of the appliance in accordance with the manufacturer’s instructions. [NFPA 54:12.4.1]

    802.3.2 Appliance Venting Requirements. Appliances required to be vented shall be connected to a vent

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    ing system designed and installed in accordance with the provisions of Section 802.4 through Section 802.15.

    [NFPA 54:12.4.2]

    802.3.3 Mechanical Draft Systems. Mechanical draft systems shall be listed in accordance with UL 378 and installed in accordance with both the appliance and the mechanical draft system manufacturer’s installation instructions. [NFPA 54:12.4.3.1]

    802.3.3.1 Venting. Appliances requiring venting shall be permitted to be vented by means of mechanical draft systems of either forced or induced draft design. [NFPA 54:12.4.3.2]

    802.3.3.2 Leakage. Forced draft systems and all portions of induced draft systems under positive pressure during operation shall be designed and installed so as to prevent leakage of flue or vent gases into a building. [NFPA 54:12.4.3.3]

  • CMC § 509.2.6.1 High relevance — show source text

    509.2.6.1 Through-the-Wall Vent Termination. Through-the-wall vent terminations for listed directvent appliances shall be in accordance with Section 509.8.1. [NFPA 54:12.3.5.2]

    509.2.7 Appliances with Integral Vents. Appliances incorporating integral venting means shall be installed in accordance with Section 509.8. [NFPA 54:12.3.6] 509.3 Minimum Safe Performance. Venting systems shall be designed and constructed to convey all flue and vent gases to the outdoors. [NFPA 54:12.1] 509.3.1 Appliance Draft Requirements. A venting system shall satisfy the draft requirements of the appliance in accordance with the manufacturer’s instructions.

    [NFPA 54:12.4.1]

    509.3.2 Appliance Venting Requirements. Appliances required to be vented shall be connected to a venting system designed and installed in accordance with the provisions of Section 509.4 through Section 509.15.

    [NFPA 54:12.4.2] 509.3.3 Mechanical Draft Systems. Mechanical draft systems shall be listed in accordance with UL 378 and installed in accordance with both the appliance and the mechanical draft system manufacturer’s installation instructions. [NFPA 54:12.4.3.1]

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    509.3.3.1 Venting. Appliances requiring venting shall be permitted to be vented by means of mechanical draft systems of either forced or induced draft design. [NFPA 54:12.4.3.2]

    509.3.3.2 Leakage. Forced draft systems and all portions of induced draft systems under positive pressure during operation shall be designed and installed so as to prevent leakage of flue or vent gases into a building. [NFPA 54:12.4.3.3]

    509.3.3.3 Vent Connectors. Vent connectors serving appliances vented by natural draft shall not be connected into any portion of mechanical draft systems operating under positive pressure. [NFPA 54:12.4.3.4]

    509.3.3.4 Operation. Where a mechanical draft system is employed, provision shall be made to prevent the flow of gas to the main burners when the draft system is not performing so as to satisfy the operating requirements of the appliance for safe performance. [NFPA 54:12.4.3.5]

    509.3.4 Ventilating Hoods and Exhaust Systems. Where automatically operated appliances, other than food service appliances, are vented through a ventilating hood or exhaust system equipped with a damper or with a power means of exhaust, provisions shall be made to allow the flow of gas to the main burners only when the damper is open to a position to properly vent the appliance and when the power means of exhaust is in operation. [NFPA 54:12.4.4.1]

  • 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 § 509.2.1 High relevance — show source text

    ** Where any or all of the appliances in Section 509.2.1(1) and Section 509.2.1(2) are installed so the aggregate input rating exceeds 20 (Btu/h)/ft [3] (207 W/m [3] ) of room or space in which it is installed, one or more shall be provided with venting systems or other approved means for conveying the vent gases to the outdoors so that the aggregate input rating of the remaining unvented appliances does not exceed 20 (Btu/h)/ft [3] (207 W/m [3] ). [NFPA 54:12.3.2.1] 509.2.3 Adjacent Room or Space. Where the calculation includes the volume of an adjacent room or space, the room or space in which the appliances are installed shall be directly connected to the adjacent room or space by a doorway, archway, or other opening of comparable size that cannot be closed. [NFPA 54:12.3.2.2]

    509.2.4 Ventilating Hoods. The use of ventilating hoods and exhaust systems to vent appliances shall be limited to industrial appliances and appliances installed in commercial applications. [NFPA 54:12.3.3] 509.2.5 Well-Ventilated Spaces. The flue gases from industrial-type appliances shall not be required to be vented to the outdoors where such gases are discharged into a large and well-ventilated industrial space. [NFPA 54:12.3.4]

    509.2.6 Direct-Vent Appliances. Listed direct vent appliances shall be installed in accordance with the manufacturer’s installation instructions. [NFPA 54:12.3.5.1]

    509.2.6.1 Through-the-Wall Vent Termination. Through-the-wall vent terminations for listed directvent appliances shall be in accordance with Section 509.8.1. [NFPA 54:12.3.5.2]

    509.2.7 Appliances with Integral Vents. Appliances incorporating integral venting means shall be installed in accordance with Section 509.8. [NFPA 54:12.3.6] 509.3 Minimum Safe Performance. Venting systems shall be designed and constructed to convey all flue and vent gases to the outdoors. [NFPA 54:12.1] 509.3.1 Appliance Draft Requirements. A venting system shall satisfy the draft requirements of the appliance in accordance with the manufacturer’s instructions.

    [NFPA 54:12.4.1]

    509.3.2 Appliance Venting Requirements. Appliances required to be vented shall be connected to a venting system designed and installed in accordance with the provisions of Section 509.4 through Section 509.15.

    [NFPA 54:12.4.2] 509.3.3 Mechanical Draft Systems. Mechanical draft systems shall be listed in accordance with UL 378 and installed in accordance with both the appliance and the mechanical draft system manufacturer’s installation instructions. [NFPA 54:12.4.3.1]

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  • CMC § 106.0 High relevance — show source text

    C 106.0 Pilot Operation.

    C 106.1 General. Igniter or pilot flames shall be effective to ignite the oil or liquid fuel at the main burner or burners and shall be adequately protected from drafts. Pilot flames shall not become extinguished during the pilot cycle where the main burner or burners are turned on or off in a normal manner either manually or by automatic controls.

    C 107.0 Burner Operation.

    C 107.1 General. In making tests to determine compliance with the requirements of this section, the following care shall be exercised to prevent the accumulation of unburned liquid fuel in the appliance that will result in an explosion or fire:

    (1) The flames from the burner shall ignite freely the liquid fuel where operating at the lowest firing position.

    (2) Burner flames shall not flash back where the liquid fuel is turned on or off by an automatic control mechanism.

    (3) Main burner flames shall ignite freely from the pilot where the pilot flame is reduced to a minimum point that will actuate the pilot safety device.

    (4) Where ignition is made in a normal manner, the flame will not flash outside the appliance.

    (5) Burners shall not expel liquid fuel through air openings where operating at prevailing pressure.

    (6) Burners shall have a liquid-fuel air mixture to ensure smooth ignition of the main burner.

    C 108.0 Method of Test.

    C 108.1 General Test Methods.

    (1) The flue gas, venting, safety and operating controls of the appliance shall be checked by the installer to ensure their proper and safe operation.

    (2) Method of test – atmospheric, induced draft or fan assisted types.

    (a) The appliance shall be allowed to operate until the stack temperature becomes stabilized, after which a sample of the undiluted flue products shall be taken from the appliance flue outlet. The sample taken shall be analyzed for carbon monoxide, carbon dioxide, and oxygen. Stack temperature shall be noted.

    Appliance designs incorporating induced-draft assemblies require a flue gas sample to be taken ahead of the draft regulator or induced draft fan.

    (3) Performance standards for atmospheric type shall be provided in accordance with the following:

    (a) Shall be not less than 75 percent efficiency as determined by flue gas analysis method at the appliance flue outlet.

    (b) Carbon monoxide concentration in flue gas shall not exceed 0.04 percent.

    (c) Stack temperature shall not exceed 700°F (371°C) plus ambient.

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    APPENDIX C

    (d) Carbon dioxide concentration shall be between 8 percent and 13 percent.

    (e) Oxygen concentration shall be between 4 percent and 10 percent.

    (f) Smoke test shall not exceed number 2 for light oils or number 4 for oils heavier than number 4.

    (g) Draft shall comply with the burner manufacturer’s instructions.

    (4) Performance standards for induced-draft or fan-assisted types shall comply with the following:

    (a) Shall be not less than 75 percent efficiency, as determined by flue gas analysis method at appliance flue outlet.

    (b) Carbon monoxide concentration in flue gas not exceeding 0.04 percent.

    (c) Stack temperature shall not exceed 700°F (371°C) plus ambient.

  • 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 § 0.04 High relevance — show source text

    (g) Draft shall comply with the burner manufacturer’s instructions.

    Induced-draft and fan-assisted types of appliances require a sample be taken after the induceddraft fan that will cause oxygen figures in excess of the limits stated. In such cases, safe liquid fuel combustion ratios shall be maintained and be consistent with approvals and listings of the appliance.

    (5) Method of test – power type.

    (a) The appliance shall be allowed to operate until the stack temperature becomes stabilized; after that, a sample of the undiluted flue products shall be taken from the appliance flue outlet. The sample shall be analyzed for carbon monoxide, carbon dioxide, and oxygen. Stack temperature shall be noted.

    (6) Performance standards for power type.

    (a) Shall be not less than 80 percent efficiency as determined by flue gas analysis method at the appliance flue outlet.

    (b) Carbon monoxide concentration in the flue gas shall not exceed 0.04 percent.

    (c) Stack temperature shall not exceed 700°F (371°C) plus ambient.

    (d) Carbon dioxide concentration shall be between 8 percent and 13 percent.

    (e) Oxygen concentration shall be between 4 percent and 10 percent.

    (f) Smoke test shall not exceed number 2 for light oils or number 4 for oils heavier than number 4.

    (g) Draft shall comply with the burner manufacturer’s instructions.

    (7) After completion of the test of newly installed oil or liquid fuel burner equipment as provided in this section, the installer shall file with the Authority Having Jurisdiction complete records of the test on a form approved by the Authority Having Jurisdiction. The tag stating the date of the test and the name of the installer shall be attached to the appliance at the main valve.

    (8) Listing and approval.

    (a) The concentration of oxygen in the undiluted flue products of oil or liquid fuel burners shall in no case be less than 3 percent nor exceed 10 percent and shall be in accordance with performance standards and shall be consistent with the listing and approval of the equipment.

    (b) The allowable limit of carbon monoxide shall not exceed 0.04 percent.

    (c) The flue gas temperature of an oil appliance, as taken on the appliance side of the draft regulator, shall not exceed applicable performance standards and shall be consistent with the listing and approvals of the equipment.

    (9) The oxygen figures shall not apply where there is an approved oxygen trim system on the burner that is designed for that use, including a low oxygen interlock where approved by the Authority Having Jurisdiction.

    (10)Supervision shall be as follows:

    (a) Supervised startup shall be required to verify the safe operation of an oil or liquid fuel burner and to provide documentation that operation is consistent with this code, listing, and approval. Supervised startup shall be required for liquid fuel burners in Section C 109.1(2) through Section C 109.1(5). Supervised startup requires that the liquid-fuel burner shall be tested in the presence of the mechanical official in a manner set forth by the Authority Having Jurisdiction before the installation is approved. Testing shall include safety and operating controls, input, flue gas analysis, and venting. Flue gas shall be tested at high, medium, and low fires. Provisions shall be made in the system to allow a firing test in warm weather.

  • CMC § 802.3.3.3 High relevance — show source text

    802.3.3.3 Vent Connectors. Vent connectors

    serving appliances vented by natural draft shall not be connected into any portion of mechanical draft systems operating under positive pressure. [NFPA 54:12.4.3.4]

    802.3.3.4 Operation. Where a mechanical draft system is employed, provision shall be made to prevent the flow of gas to the main burners when the draft system is not performing so as to satisfy the operating requirements of the appliance for safe performance. [NFPA 54:12.4.3.5]

    802.3.4 Ventilating Hoods and Exhaust Systems. Where automatically operated appliances, other than food service appliances, are vented through a ventilating hood or exhaust system equipped with a damper or with a power means of exhaust, provisions shall be made to allow the flow of gas to the main burners only when the damper is open to a position to properly vent the appliance and when the power means of exhaust is in operation. [NFPA 54:12.4.4.1]

    802.3.5 Circulating Air Ducts, Above-Ceiling Air- Handling Spaces, and Furnace Plenums. Venting systems shall not extend into or pass through any fabricated air duct or furnace plenum. [NFPA 54:12.4.5.1]

    802.3.6 Above-Ceiling or Nonducted Air Han- dling System. Where a venting system passes through an above-ceiling air space or other nonducted portion of an air-handling system, it shall conform to one of the following requirements:

    (1) The venting system shall be a listed special gas vent, other system serving a Category III or Category IV appliance, or other positive pressure vent, with joints sealed in accordance with the appliance or vent manufacturer’s instructions.

    (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]

  • 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 § 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 § 12.14.1 Medium relevance — show source text

    placing the appliance in service. Balancing baffles shall be listed in accordance with UL 378. [NFPA 54:12.14.1 – 12.14.3]

    802.14 Automatically Operated Vent Dampers. An automatically operated vent damper shall be listed. [NFPA 54:12.15]

    802.14.1 Listing. Automatically operated vent dampers for oil-fired appliances shall comply with UL 17. The automatic damper control shall comply with UL 378.

    802.15 Obstruction of Flow. Devices that retard the flow of vent gases shall not be installed in a vent connector, chimney, or vent. The following shall not be considered as obstructions:

    (1) Draft regulators and safety controls specifically listed for installation in venting systems and installed in accordance with the manufacturer’s installation instructions.

    (2) Approved draft regulators and safety controls designed and installed in accordance with engineering methods.

    (3) Listed heat reclaimers and automatically operated vent dampers installed in accordance with the manufacturers’ installation instructions.

    (4) Vent dampers serving listed appliances installed in accordance with Section 803.1 or Section 803.2 or engineering methods.

    (5) Approved economizers, heat reclaimers, and recuperators installed in venting systems of appliances not required to be equipped with draft hoods, provided the appliance manufacturer’s instructions cover the installation of such a device in the venting system and performance in accordance with Section 802.3 and Section

    802.3.1 is obtained. [NFPA 54:12.16]

    803.0 Sizing of Category I Venting Systems.

    803.1 Additional Requirements to Single Appliance Vent. Venting Table 803.1.2(1) through Table 803.1.2(6) 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 system shall be determined using the “NAT Max” column.

    (2) The minimum capacity shall be determined as though the appliance were a fan-assisted appliance, using the “FAN Min” column to determine the minimum capacity of the vent system. Where the corresponding “FAN Min” is “NA,” the vent configuration shall not be permitted and an alternative venting configuration shall be utilized.

    [NFPA 54:13.1.1]

    803.1.1 Vent Downsizing. Where the vent size determined from the tables is smaller than the appliance draft hood outlet or flue collar, the use of the smaller size shall be permitted, provided that the installation complies with all of the following requirements:

    «

    «

    «

    2025 CALIFORNIA MECHANICAL CODE 165

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

    CHIMNEYS AND VENTS

    (1) The total vent height ( H ) is at least 10 feet (3048 mm).

    (2) Vents for appliance draft hood outlets or flue collars 12 inches (305mm) in diameter or smaller are not reduced more than one table size.

    (3) Vents for appliance draft hood outlets or flue collars larger than 12 inches (305 mm) in diameter are not reduced more than two table sizes.

Frequently asked questions

What numeric draft value should I design to?

The CMC defers to the appliance manufacturer. The venting system must satisfy the appliance maker’s draft requirements; the code does not give a single numeric draft for all appliances (§ 802.3.1) .

Do mechanical draft fans have to be listed?

Yes — mechanical draft systems must be listed to UL 378 and installed per the appliance and draft‑system manufacturers’ instructions (§ 802.3.3) .

What does “prevent leakage” mean for positive‑pressure sections?

Any forced draft or induced draft portion that operates under positive pressure must be designed and installed so flue/vent gases do not leak into the building; this typically means sealed joints, approved closures, and materials/install methods compatible with the positive‑pressure service (§ 802.3.3.2) .

Can I connect a natural‑draft appliance into a mechanical draft stack?

No. Vent connectors serving natural‑draft appliances must not be connected into any portion of a mechanical draft system that operates under positive pressure (§ 802.3.3.3) .

What if the draft system fails during operation?

Where a mechanical draft system is employed, provisions must be made to prevent flow of gas to the main burners when the draft system is not performing so as to satisfy safe operation (§ 802.3.3.4) .

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