CMC · California Mechanical Code
What are the requirements for pressure regulators, relief devices and venting?
Regulator diaphragm vents that could discharge hazardous gas must be piped to the outdoors and sized per the manufacturer; relief valve discharge piping must be at least the same size as the valve outlet; and all fittings and pipe between the system and the relief device must be sized to prevent valve hammering or reduction of relief capacity — see **§ 1308.14**, **§ 1308.9.6**, **§ 1308.9.7** for the controlling CMC rules.
Last reviewed: July 6, 2026
What the code requires — 2-4 sentences
The California Mechanical Code requires that the atmospheric vent(s) of line and appliance pressure regulators and pressure‑limit controls be routed and sized so a hazardous discharge cannot accumulate indoors and so the vents and discharge outlets cannot be blocked. Key mandatory rules are found in § 1308.14, § 1308.9.6, and § 1308.9.7. These sections require outdoor venting where a discharge could be hazardous, discharge outlets sized at least equal to the relief outlet, and fittings/pipe/openings sized so relief capacity and valve operation are not impaired.
The single most important rule: vent the atmospheric side of regulator diaphragms to the outdoors when a gas discharge could create a hazard, use vent piping sized per the device manufacturer, and make discharge/vent outlets at least the same size as the relief device outlet.
Requirements in detail
1) Venting of regulator diaphragms — what to do and when
- Provide an independent vent pipe to the outdoors when the regulator location is such that a discharge of fuel gas would create a hazard. The vent must be sized in accordance with the device manufacturer’s instructions. See § 1308.14.
- For devices other than appliance regulators, an independent vent is not mandatory if vents are connected to a common manifold that is engineered to minimize backpressure in the event of diaphragm failure and the design is approved. See § 1308.14.
2) Where the vent/relief discharges (safety of outlet)
- The discharge stacks, vents, or outlet parts of all pressure relieving and pressure‑limiting devices must be located so gas is safely discharged to the outdoors. Design the discharge to prevent entry of water, insects, or other foreign material that could cause blockage. See § 1308.9.6.
3) Minimum sizing and sizing intent
- The discharge stack or vent line shall be at least the same size as the outlet of the pressure relieving device (do not reduce the cross‑section between the valve outlet and the outside). See § 1308.9.6.
- The fittings, pipe, and openings located between the system to be protected and the pressure‑relieving device must be sized to prevent valve hammering and to avoid impairing relief capacity (i.e., do not create restrictions that reduce the device’s rated relieving flow). See § 1308.9.7.
Decision‑relevant quick reference table
| Decision / dimension | Requirement / value | Code Reference |
|---|---|---|
| Where to vent regulator diaphragm | To outdoors when location could create a hazard; independent vent required unless approved engineered manifold for non‑appliance regulators | § 1308.14 |
| Vent sizing for regulator atmospheric side | Sized in accordance with the manufacturer’s instructions | § 1308.14 |
| Discharge outlet (relief valve) minimum size | Vent/discharge line shall be at least the same size as the relief device outlet | § 1308.9.6 |
| Protection against blockage | Discharge stacks/vents must be designed to prevent entry of water, insects, or other foreign material | § 1308.9.6 |
| Sizing between system and relieving device | Fittings/pipe/openings must be sized to prevent hammering and not impair relief capacity | § 1308.9.7 |
Exceptions & special cases
- Common manifold: For non‑appliance regulators, a common manifold is acceptable only when the manifold is designed by engineering methods to minimize backpressure in diaphragm failure scenarios and the design is approved. Appliance regulators are not covered by that allowance. See § 1308.14.
- Manufacturer listing/special products: The code defers vent sizing for regulator atmospheric sides to the device manufacturer’s instructions; if a regulator is a listed product that incorporates a vent‑limiting means (listed to the appropriate standard), other venting exceptions may apply — check the regulator listing and manufacturer instructions (see § 1308.14 and related manufacturer guidance).
If your design involves other specific circumstances (meter rooms, flood/snow risk, combination vents), consult related sections and manufacturer instructions — those provisions add specific terminal clearances, flood protection, and material requirements (see Related provisions below).
Common mistakes
- Routing a regulator atmospheric vent to an indoor space instead of outdoors when a hazardous discharge is possible (violates § 1308.14).
- Reducing the vent or discharge line size downstream of the relief valve outlet (violates § 1308.9.6 requirement that the discharge line be at least the same size as the relief outlet).
- Installing undersized fittings or narrow passages between the system and the pressure‑relief device that cause valve hammering or reduce relieving capacity (violates § 1308.9.7).
- Connecting multiple regulator vents into a common manifold without engineering to address backpressure — permitted only for non‑appliance regulators with approved engineered design (§ 1308.14).
- Failing to design discharge terminals to prevent entry of water/insects/foreign material that could block the vent (see § 1308.9.6).
Worked example — concrete scenario
Scenario: A gas service has a pressure relief valve whose outlet is 1‑1/4‑inch NPS. A line pressure regulator installed indoors has an atmospheric diaphragm vent that the manufacturer specifies as a 1/2‑inch vent to outdoors.
Step 1 — Relief valve discharge sizing: the discharge vent for the pressure relief valve must be at least the same size as the relief valve outlet, so the discharge piping must be ≥ 1‑1/4" (do not step down to 1" or 3/4"). This follows § 1308.9.6.
Step 2 — Regulator atmospheric vent: because the regulator is indoors and a discharge could create a hazard, provide an independent vent pipe routed to the outdoors sized per the manufacturer (manufacturer calls for 1/2"). Comply with the manufacturer sizing and install the vent to outdoors per § 1308.14.
Step 3 — Intermediate fittings and piping: size the fittings/pipe/openings between the system and the pressure relief device so they do not cause valve hammering or limit the relief capacity (for example, avoid long, restrictive tubing or small‑bore flex connectors in the impulse/control line). This is required by § 1308.9.7.
Result: Relief discharge piping is 1‑1/4" or larger to outdoors; the regulator atmospheric vent is a separate 1/2" vent to outdoors per manufacturer instructions; all intermediate piping/fittings are full‑bore or sized to maintain rated relief capacity and avoid hammering, satisfying the cited CMC requirements.
Related provisions
- § 1308.9.1 — Separate devices (installation as integral or separate overpressure devices).
- § 1308.9.2 — Construction and installation requirements for overpressure protection devices (materials, testability).
- § 1308.9.4 — Setting limits for pressure limiting/relieving devices (limit to pressures specified in § 1308.8).
- § 1308.8 — Pressure‑limitation requirements and required limiting setpoints for connected appliances.
- § 1308.6.4 — Regulator vents reference to § 1308.14 (where regulator vents are discussed).
- Plumbing‑code cross reference: § 1208.15 (Plumbing Code) contains additional venting details such as vent material, anti‑flood fittings, and minimum clearances at termination points — consult when regulator vents penetrate plumbing/roof assemblies.
If you need a compliance checklist or a drawing note template for plan submittal that cites these sections, tell me the device types and outlet sizes and I will produce one customized to your project.
Code references
Grounded in the retrieved California Mechanical Code — click a citation to read the verbatim passage:
CMC § 1308.13.1 High relevance — show source text
1308.13.1 Special Local Conditions. Where local conditions include earthquake, tornado, unstable ground, or flood hazards, special consideration shall be given to increased strength and flexibility of piping supports and connections. [NFPA 54:5.13.2]
1308.14 Pressure Regulator and Pressure Control Venting. The venting of the atmospheric side of diaphragms in line pressure regulators, gas appliance regulators, and gas pressure limit controls shall be in accordance with all of the following:
(1) An independent vent pipe to the outdoors, sized in accordance with the device manufacturer’s instructions, shall be provided where the location of a device is such that a discharge of fuel gas will cause a hazard. For devices other than appliance regulators, vents are not required to be independent where the vents are connected to a common manifold designed in accordance with engineering methods to minimize backpressure in the event of diaphragm failure and such design is approved.
»
»
»
»
»
»
sure relieving valve or pressure limiting device inoperative, one of the following shall be accomplished:
(1) The valve shall be locked in the open position. Instruct authorized personnel in the importance of leaving the shutoff valve open and of being present while the shutoff valve is closed so that it can be locked in the open position before leaving the premises.
(2) Duplicate relief valves shall be installed, each having adequate capacity to protect the system, and arrange the isolating valves or three-way valve so that only one relief valve can be rendered inoperative at a time.
[NFPA 54:5.8.7] 1308.9.6 Discharge of Vents. The discharge stacks, vents, or outlet parts of all pressure relieving and pressure limiting devices shall be located so that gas is safely discharged to the outdoors. Discharge stacks or vents shall be designed to prevent the entry of water, insects, or other foreign material that could cause blockage. The discharge stack or vent line shall be at least the same size as the outlet of the pressure relieving device. [NFPA 54:5.8.8.1, 5.8.8.2] 1308.9.7 Size of Fittings, Pipe, and Openings. The fittings, pipe, and openings located between the system to be protected and the pressure relieving device shall be sized to prevent hammering of the valve and to prevent impairment of relief capacity. [NFPA 54:5.8.9] 1308.10 Backpressure Protection. Protective devices shall be installed as close to the equipment as practical where the design of equipment connected is such that air, oxygen, or standby gases could be forced into the gas supply system. Gas and air combustion mixers incorporating double diaphragm “zero” or “atmosphere” governors or regulators shall require no further protection unless connected directly to compressed air or oxygen at pressures of 5 psi (34 kPa) or more. [NFPA 54:5.9.1.1, 5.9.1.2]
1308.10.1 Protective Devices. Protective devices shall include but not be limited to the following:
(1) Check valves.
(2) Three-way valves (of the type that completely closes one side before starting to open the other side).
(3) Reverse flow indicators controlling positive shutoff valves.
CMC § 6.32 High relevance — show source text
32 with
ANSI/ASME B16.44 labeled 2G or
labeled 5G| |
Valve up to 2 psi|CSA/ANSI LC 4/CSA 6.32 with
ANSI/ASME B16.33 marked 125 G| |
Valve up to 5 psi|ASME B16.44 labeled 5G| |
Valve up to 5 psi|ASME B16.33| |
Valve up to 5 psi|CSA/ANSI LC 4/CSA 6.32 with
ANSI/ASME B16.44 marked 5G| |
Valve up to 5 psi|CSA/ANSI LC 4/CSA 6.32 with
ANSI/ASME B16.33 marked 125 G| |
Valve up to 125 psi|ASME B16.33 marked 125 G| |
Valve up to 125 psi|CSA/ANSI LC 4/CSA 6.32 with
ASME B16.33 marked 125 G|For SI Units: 1 pound-force per square inch = 6.8947 kPa
1308.13 Expansion and Flexibility. Piping systems shall be designed to prevent failure from thermal expansion or contraction. [NFPA 54:5.13.1]
1308.13.1 Special Local Conditions. Where local conditions include earthquake, tornado, unstable ground, or flood hazards, special consideration shall be given to increased strength and flexibility of piping supports and connections. [NFPA 54:5.13.2]
1308.14 Pressure Regulator and Pressure Control Venting. The venting of the atmospheric side of diaphragms in line pressure regulators, gas appliance regulators, and gas pressure limit controls shall be in accordance with all of the following:
(1) An independent vent pipe to the outdoors, sized in accordance with the device manufacturer’s instructions, shall be provided where the location of a device is such that a discharge of fuel gas will cause a hazard. For devices other than appliance regulators, vents are not required to be independent where the vents are connected to a common manifold designed in accordance with engineering methods to minimize backpressure in the event of diaphragm failure and such design is approved.
»
»
»
»
»
»
sure relieving valve or pressure limiting device inoperative, one of the following shall be accomplished:
(1) The valve shall be locked in the open position. Instruct authorized personnel in the importance of leaving the shutoff valve open and of being present while the shutoff valve is closed so that it can be locked in the open position before leaving the premises.
(2) Duplicate relief valves shall be installed, each having adequate capacity to protect the system, and arrange the isolating valves or three-way valve so that only one relief valve can be rendered inoperative at a time.
[NFPA 54:5.8.7] **1308.9.6 Discharge of Vents.
CMC § 1308.8 High relevance — show source text
have a flow capacity such that the pressure in the protected system is maintained at or below the limits specified in Section 1308.8 under the following conditions:
(1) The line pressure regulator for which the relief valve is providing overpressure protection has failed wide
open.
(2) The gas pressure at the inlet of the line pressure regulator for which the relief valve is providing overpressure protection is not less than the regulator’s normal operating inlet pressure. [NFPA 54:5.8.2.5]
1308.9 Overpressure Protection Devices. Overpressure protection devices shall be one of the following:
(1) Pressure relief valve.
(2) Monitor regulator.
(3) Series regulator installed upstream from the line regulator and set to continuously limit the pressure on the inlet of the line regulator to the maximum values specified by Section 1308.8 or less.
(4) Automatic shutoff device installed in series with the line pressure regulator and set to shut off when the pressure on the downstream piping system reaches the maximum values specified by Section 1308.8 or less. This device shall be designed so that it will remain closed until manually reset. [NFPA 54:5.8.3.1]
1308.9.1 Separate Devices. The devices in Section 1308.9 shall be installed either as an integral part of the service or line pressure regulator or as separate units. Where separate overpressure protection devices are installed, they shall comply with Section 1308.9.2 through Section 1308.9.7. [NFPA 54:5.8.3.2]
1308.9.2 Construction and Installation. All overpressure protection devices shall meet the following requirements:
(1) Be constructed of materials so that the operation of the device is not impaired by corrosion of external parts by the atmosphere or of internal parts by the gas.
(2) Be designed and installed so they can be operated to determine whether the valve is free. The devices shall also be designed and installed so they can be tested to determine the pressure at which they operate and be examined for leakage when in the closed position. [NFPA 54:5.8.4]
1308.9.3 External Control Piping. External control piping shall be designed and installed so that damage to the control piping of one device does not render both the regulator and the overpressure protective device inoperative. [NFPA 54:5.8.5]
1308.9.4 Setting. Each pressure limiting or pressure relieving device shall be set so that the gas pressure supplied to the connected appliance(s) does not exceed the limits specified in Section 1308.8 and Section 1308.8.1.
[NFPA 54:5.8.6]
1308.9.5 Unauthorized Operation. Where unauthorized operation of any shutoff valve could render a pres
«
«
«
«
«
«
2025 CALIFORNIA MECHANICAL CODE 281
), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.
FUEL GAS PIPING
TABLE 1308.12
MANUAL GAS VALVE STANDARDS
[NFPA 54: TABLE 5.11]
CMC § 1308.6.1 High relevance — show source text
FUEL GAS PIPING
»
»
»
»»
»
»
»
»
»
1308.6.1 Listing. Line pressure regulators shall be listed in accordance with CSA/ANSI Z21.80/CSA 6.22
where the outlet pressure is set to 2 psi (14 kPa) or less.
[NFPA 54:5.7.2]
1308.6.2 Location. The gas pressure regulator shall be accessible for servicing. [NFPA 54:5.7.3]
1308.6.3 Regulator Protection. Pressure regulators shall be protected against physical damage. [NFPA 54:5.7.4]
1308.6.4 Regulator Vents. Regulator vents shall be in accordance with Section 1308.14. [NFPA 54:5.7.5]
1308.6.5 Identification. Line pressure regulators at multiple regulator installations shall be marked by a metal tag or other permanent means designating the building or the part of the building being supplied.
[NFPA 54:5.7.6]
1308.7 Overpressure Protection. Where the serving gas supplier delivers gas at a pressure greater than 2 psi for piping systems serving appliances designed to operate at a gas pressure of 14 inches water column or less, overpressure protection devices shall be installed. Piping systems serving equipment designed to operate at inlet pressures greater than 14 inches water column (3.5 kPa) shall be equipped with overpressure protection devices as required by the appliance manufacturer’s installation instructions. [NFPA 54:5.8.1]
1308.8 Pressure Limitation Requirements. Where piping systems serving appliances designed to operate with a gas supply pressure of 14 inches water column (3.5 kPa) or less are required to be equipped with overpressure protection by Section 1308.7, each overpressure protection device shall be adjusted to limit the gas pressure to each connected appliance to 2 psi (14 kPa) or less upon a failure of the line pressure regulator. [NFPA 54:5.8.2.1]
1308.8.1 Overpressure Protection Required. Where piping systems serving appliances designed to operate with a gas supply pressure greater than 14 inches water column (3.5 kPa) are required to be equipped with overpressure protection by Section 1308.7, each overpressure protection device shall be adjusted to limit the gas pressure to each connected appliance as required by the appliance manufacturer’s installation instructions. [NFPA 54:5.8.2.2]
1308.8.2 Overpressure Protection Devices. Each overpressure protection device installed to meet the requirements of this section shall be capable of limiting the pressure to its connected appliance(s) as required by this section independently of any other pressure control equipment in the piping system. [NFPA 54:5.8.2.3]
1308.8.3 Detection of Failure. Each gas piping system for which an overpressure protection device is required by this section shall be designed and installed so that a failure of the primary pressure control device(s) is detectable. [NFPA 54:5.8.2.4]
1308.8.4 Flow Capacity. If a pressure relief valve is used to meet the requirements of this section, it shall
CMC § 1208.14.1 High relevance — show source text
1208.14.1 Special Local Conditions. Where local conditions include earthquake, tornado, unstable ground, or flood hazards, special consideration shall be given to increased strength and flexibility of piping supports and connections. [NFPA 54:5.13.2]
1208.15 Pressure Regulator and Pressure Control Venting. The venting of the atmospheric side of diaphragms in line pressure regulators, gas appliance regulators, and gas pressure limit controls shall be in accordance with all of the following:
(1) An independent vent pipe to the outdoors, sized in accordance with the device manufacturer’s instructions, shall be provided where the location of a device is such that a discharge of fuel gas will cause a hazard. For devices other than appliance regulators, vents are not required to be independent where the vents are connected to a common manifold designed in accordance with engineering methods to minimize backpressure in the event of diaphragm failure and such design is approved.
Exceptions:
(1) A regulator and vent limiting means combination listed as complying with CSA/ANSI Z21.80/CSA 6.22, shall not be required to be vented to the outdoors.
«
«
«
«
«
222 2025 CALIFORNIA PLUMBING CODE
), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.
FUEL GAS PIPING
(2) A listed gas appliance regulator factory equipped with a vent limiting device is not required to be vented to the outdoors.
(2) Materials for vent piping shall be in accordance with Section 1208.5 through Section 1208.5.12.5.
(3) The vent terminus shall be designed to prevent the entry of water, insects, and other foreign matter that could cause blockage.
(4) Vent piping shall be installed to minimize static loads and bending moments placed on the regulators and gas pressure control devices.
(5) Vents shall terminate not less than 3 feet (914 mm) from a possible source of ignition.
(6) At locations where a vent termination could be submerged during floods or snow accumulations, an antiflood-type breather vent fitting shall be installed, or the vent terminal shall be located above the height of the expected flood waters or snow.
(7) Vent piping from pressure regulators and gas pressure controls shall not be connected to a common manifold
that serves a bleed line from a diaphragm-type gas valve.
[NFPA 54:5.14]
TABLE 1208.13
MANUAL GAS VALVE STANDARDS
[NFPA 54: TABLE 5.11]
1209.0 Excess Flow Valve.
1209.1 General. Where automatic excess flow valves are installed, they shall be listed in accordance with ANSI Z21.93/CSA 6.30 and shall be sized and installed in accor dance with the manufacturers’ instructions. [NFPA 54:5.12]
1210.0 Gas Piping Installation.
CMC § 1112.14 High relevance — show source text
Where:
C = Rated discharge capacity expressed as mass flow of air, pounds per minute.
d = Smallest of the internal diameter of the inlet pipe, retaining flanges, fusible plug, and rupture member , inches.
For rupture members:
[Equation 1112.14(3)]
P1 = (rated pressure in psig x 1.1) + 14.7 psia
For fusible plugs:
P1 = Absolute saturation pressure corresponding to the stamped temperature melting point of the fusible plug or the critical pressure of the refrigerant used, whichever is smaller, pound-force per square inch atmosphere, psia. [ASHRAE 15:9.7.7]
For SI units:1 pound per minute = 0.00756 kg/s
1113.0 Overpressure Protection.
1113.1 General. Pressure vessels shall be provided with overpressure protection in accordance with ASME BPVC Section VIII.1. Pressure vessels containing liquid refrigerant that are capable of being isolated by stop valves from other parts of a refrigerating system shall be provided with overpressure protection. Pressure relief devices or fusible plugs shall be sized in accordance with Section 1113.5. [ASHRAE 15:9.7.1, 9.7.2]
»
Unless the maximum allowable back pressure ( P0 ) is specified by the relief valve manufacturer, the following maximum allowable back pressure values shall be used for P0, where P is the set pressure and Pa is atmospheric pressure at the nominal elevation of the installation (see Table 1112.12.4):
(1) For conventional relief valves: 15 percent of set
pressure:
P0 = (0.15• P )+ Pa [Equation 1112.12.4(2)] (2) For balanced relief valves: 25 percent of set pres sure:
P0 = (0.25• P )+ Pa [Equation 1112.12.4(3)]
(3) For rupture disks alone, fusible plugs, or pilot-operated relief devices: 50 percent of set pressure:
P0 = (0.50• P )+ Pa [Equation 1112.12.4(4)]
For fusible plugs, P shall be the saturated absolute pressure for the stamped temperature melting point of the fusible plug or the critical pressure of the refrigerant used, whichever is smaller. [ASHRAE 15:9.7.9.3.1, 9.7.9.3.2]
TABLE 1112.12.4
ATMOSPHERIC PRESSURE AT NOMINAL INSTALLATION ELEVATION ( Pa )
[ASHRAE 15: TABLE 9-2]
CMC § 1104.1 High relevance — show source text
Permissible . . . . . . . . . . . . . . . . . . . . . . . . Table 1104.1
Pressure-limiting devices . . . . . . . . . . . . . . . . . . . 1111.0
Pressure-relief devices . . . . . . . . . . . . . . 1112.0, 1112.9,
1112.10, 1112.11
1113.4, 1113.5, 1115.4
Probability . . . . . . . . . . . . . . . 1103.2, 1103.2.1, 1103.2.2
Protection from
mechanical damage . . . . . . . . . . . . . . . . . . . 1109.4.1
Requirements for . . . . . . . . . . . . . . . . . . . . . . . . . 1104.0
Self-contained mechanical
refrigeration systems . . . . . . . . . . . . . . . . . . . . . 933.3
Self-contained refrigerators
and freezers . . . . . . . . . . . . . . . . . . . . . . . . . . . 933.1
Unit coolers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 933.2
Ventilation of room . . . . . . . . . . . . . . . . . . 1105.5, 1107.0
REFRIGERATORS
Clearance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 927.2
Factory-built . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 933.1
Ventilating kit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 927.3
REGULATOR VENT PIPING . . . . . . . . . . . . . . . . 1308.4.4.1
REGULATORS
Combustion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1009.0
Gas appliance. . . . . . . . . . . . . . . . . . . . . . . . . . . . 923.5
Gas-pressure regulators . . . . . . . . . . . . . . . . . . . 1007.0
Vent piping . . . . . . . . . . . . . . . . . . . . . . . . . . 1308.4.4.1
RETURN DUCT SIZING . . . . . . . . . . . . . . . . . . . . . . . 601.2.1
RISER HEAT PIPE
Definition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220.0
ROOFTOP TERMINATIONS
CMC § 1112.12.4 High relevance — show source text
»
Unless the maximum allowable back pressure ( P0 ) is specified by the relief valve manufacturer, the following maximum allowable back pressure values shall be used for P0, where P is the set pressure and Pa is atmospheric pressure at the nominal elevation of the installation (see Table 1112.12.4):
(1) For conventional relief valves: 15 percent of set
pressure:
P0 = (0.15• P )+ Pa [Equation 1112.12.4(2)] (2) For balanced relief valves: 25 percent of set pres sure:
P0 = (0.25• P )+ Pa [Equation 1112.12.4(3)]
(3) For rupture disks alone, fusible plugs, or pilot-operated relief devices: 50 percent of set pressure:
P0 = (0.50• P )+ Pa [Equation 1112.12.4(4)]
For fusible plugs, P shall be the saturated absolute pressure for the stamped temperature melting point of the fusible plug or the critical pressure of the refrigerant used, whichever is smaller. [ASHRAE 15:9.7.9.3.1, 9.7.9.3.2]
TABLE 1112.12.4
ATMOSPHERIC PRESSURE AT NOMINAL INSTALLATION ELEVATION ( Pa )
[ASHRAE 15: TABLE 9-2]
ELEVATION ABOVE SEA LEVEL,
FEETPOUNDS PER SQUARE INCH,
ABSOLUTE (Pa)
014.7
50014.4
100014.2
150013.9
200013.7
250013.4
300013.2
350012.9
400012.7
450012.5
500012.2
600011.8
700011.3
800010.9
900010.5
100010.1 For SI units: 1 foot = 304.8 mm, 1 pound-force per square inch = 6.8947 kPa
1112.12.5 Simultaneous Operation. When outlets of two or more relief devices or fusible plugs, which are
TABLE 1112.12.4
ATMOSPHERIC PRESSURE AT NOMINAL INSTALLATION ELEVATION ( Pa )
[ASHRAE 15: TABLE 9-2]
For SI units: 1 foot = 304.8 mm, 1 pound-force per square inch = 6.8947 kPa
244 2025 CALIFORNIA MECHANICAL CODE
), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.
REFRIGERATION
CMC § 12.13.1 High relevance — show source text
«
«
«
«
«
«
164 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
ances requiring chimney draft for operation; single-firebox boilers equipped with conversion burners with inputs greater than 400 000 Btu/h (117 kW); appliances equipped with blast, power, or pressure burners that are not listed for use with draft hoods; and appliances designed for forced venting. [NFPA 54:12.13.1]
802.12.1 Installation. A draft hood supplied with or forming a part of a listed vented appliance shall be installed without alteration, exactly as furnished and specified by the appliance manufacturer. [NFPA 54:12.13.2]
If a draft hood is not supplied by the appliance manufacturer where one is required, a draft hood shall be installed, be of a listed or approved type, and, in the absence of other instructions, be of the same size as the appliance flue collar. Where a draft hood is required with a conversion burner, it shall be of a listed or approved type. [NFPA 54:12.13.2.1]
802.12.2 Draft Control Devices. Where a draft con trol device is part of the appliance or is supplied by the appliance manufacturer, it shall be installed in accordance with the manufacturer’s instructions. In the
absence of manufacturer’s instructions, the device shall be attached to the flue collar of the appliance or as near to the appliance as practical. [NFPA 54:12.13.3]
802.12.3 Additional Devices. Appliances requiring controlled chimney draft shall be permitted to be equipped with listed double-acting barometric draft regulators installed and adjusted in accordance with the manufacturer’s instructions. [NFPA 54:12.13.4]
802.12.4 Location. Draft hoods and barometric draft
regulators shall be installed in the same room or enclosure as the appliance in such a manner as to prevent any difference in pressure between the hood or regulator and the combustion air supply. [NFPA 54:12.13.5]
802.12.5 Positioning. Draft hoods and draft regulators shall be installed in the position for which they were designed with reference to the horizontal and vertical planes and shall be located so that the relief opening is not obstructed by any part of the appliance or adjacent construction. The appliance and its draft hood shall be located so that the relief opening is accessible for checking vent operation. [NFPA 54:12.13.6]
802.12.6 Clearance. A draft hood shall be located so
that its relief opening is not less than 6 inches (152 mm) from any surface except that of the appliance it serves and the venting system to which the draft hood is connected. Where a greater or lesser clearance is indicated on the appliance label, the clearance shall not be less than that specified on the label. Such clearances shall not be reduced. [NFPA 54:12.13.7]
802.13 Manually Operated Dampers. A manually operated damper shall not be placed in any appliance vent connector. Fixed baffles and balancing baffles shall not be classified as manually operated dampers. Balancing baffles shall be mechanically locked in the desired position before
CMC § 304.8 Medium relevance — show source text
For SI units: 1 foot = 304.8 mm, 1 pound-force per square inch = 6.8947 kPa
1112.12.5 Simultaneous Operation. When outlets of two or more relief devices or fusible plugs, which are
TABLE 1112.12.4
ATMOSPHERIC PRESSURE AT NOMINAL INSTALLATION ELEVATION ( Pa )
[ASHRAE 15: TABLE 9-2]
For SI units: 1 foot = 304.8 mm, 1 pound-force per square inch = 6.8947 kPa
244 2025 CALIFORNIA MECHANICAL CODE
), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.
REFRIGERATION
1113.2 Type of Protection. Pressure vessels with an internal gross volume of 3 cubic feet (0.1 m [3] ) or less shall use one or more pressure relief devices or a fusible plug. Pressure vessels of more than 3 cubic feet (0.1 m [3] ) but less than 10 cubic feet (0.28 m [3] ) internal gross volume shall use one or more pressure relief devices. Fusible plugs shall not be used.
[ASHRAE 15:9.7.2.1, 9.7.2.2]
1113.3 Discharging into Lowside of System. For pressure-relief valves discharging into the lowside of the system, a single relief valve (not rupture member) of the required relieving capacity shall not be used on vessels of 10 cubic feet (0.28 m [3] ) or more internal gross volume except under the conditions permitted in Section 1112.11.3. [ASHRAE 15:9.7.3]
1113.4 Parallel Pressure-Relief Devices. Two or more
pressure-relief devices in parallel to obtain the required capacity shall be considered as one pressure-relief device. The discharge capacity shall be the sum of the capacities required for each pressure vessel being protected.
1113.5 Discharge Capacity. The minimum required discharge capacity of the pressure-relief device or fusible plug for a pressure vessel shall be determined in accordance with ASHRAE 15.
1113.6 Three-Way Valve. Pressure vessels of 10 cubic feet (0.28 m [3] ) or more internal gross volume shall use one or more rupture member(s) or dual pressure-relief valves where discharging to the atmosphere. Dual pressure-relief valves shall be installed with a three-way valve to allow testing or repair. Where dual relief valves are used, the valve shall comply with Section 1113.5.
Exception: A single relief valve shall be permitted on pressure vessels of 10 cubic feet (0.28 m [3] ) or more internal gross volume where in accordance with the following conditions:
(1) The relief valves are located on the lowside of the sys tem.
(2) The vessel is provided with shutoff valves designed to allow pumpdown of the refrigerant charge of the pressure vessel.
(3) Other pressure vessels in the system are separately protected in accordance with Section 1113.1. [ASHRAE 15:9.7.2.3]
1114.0 Special Discharge Requirements.
CMC § 1307.3 Medium relevance — show source text
ways.
(3) Medical device applications directly related to respiration.
(4) Power for medical devices used directly on patients.
(5) Calibration of medical devices intended for Section 1307.3(1) through Section 1307.3(4).
(6) Simulation centers for the education, training, and assessment of health care professionals. [NFPA 99:5.1.3.5.2]
1307.4 Materials. Materials used in central supply systems shall meet the following requirements:
(1) In those portions of systems intended to handle oxygen at gauge pressures greater than 350 pounds-force per square inch (psi) (2413 kPa), interconnecting hose shall contain no polymeric materials.
(2) In those portions of systems intended to handle oxygen or nitrous oxide material, construction shall be compatible with oxygen under the temperatures and pressures to which the components can be exposed in the containment and use of oxygen, nitrous oxide, mixtures of these gases, or mixtures containing more than 23.5 percent oxygen.
(3) If potentially exposed to cryogenic temperatures, materials shall be designed for low temperature service.
(4) If intended for outdoor installation, materials shall be installed per the manufacturer’s requirements. [NFPA 99:5.1.3.5.4]
1308.0 Pressure-Regulating Equipment.
1308.1 Where Required. Pressure-regulating equipment shall be installed in the supply main upstream of the final linepressure valve. Where multiple piping systems for the same gas at different operating pressures are required, separate pressure-regulating equipment, relief valves, and source shutoff valves shall be provided for each pressure.
1308.2 Pressure Relief Valves. All pressure relief valves shall meet the following requirements:
278 2025 CALIFORNIA PLUMBING CODE
), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.
HEALTH CARE FACILITIES AND MEDICAL GAS AND MEDICAL VACUUM SYSTEMS
(1) They shall be of brass, bronze, or stainless steel construction.
(2) They shall be designed for the specific gas service.
(3) They shall have a relief pressure setting not higher than the maximum allowable working pressure (MAWP) of the component with the lowest working pressure rating in the portion of the system being protected.
(4) They shall be vented to the outside of the building, except that relief valves for compressed air systems having less than 3000 cubic feet (84 950 L) at STP shall be permitted to be diffused locally by means that will not restrict the flow.
(5) They shall have a vent discharge line that is not smaller than the size of the relief valve outlet or ¾ NPS (20 mm), whichever is larger.
(6) Where two or more relief valves discharge into a common vent line, the internal cross-sectional area of the common line shall be not less than the aggregate cross-sectional area of all relief valve vent discharge lines served.
(7) They shall not discharge into locations creating potential hazards.
(8) They shall have the discharge terminal turned down and screened to prevent the entry of rain, snow, or vermin.
(9) They shall be designed in accordance with ASME B31.3.
[NFPA 99:5.1.3.5.6.1]
CMC § 10-09 Medium relevance — show source text
Mechanical fans and all other electric devices must be explosion proof and meet the requirements of the NFPA-70: National Electric Code for Class I, Division 1, Group D locations.
Mechanical fans and detection equipment must be continuously monitored in case of failure. Alarms for trouble and failure must be installed in accordance with NFPA-72, National Fire Alarm Code.
Prepared by: AAJ7 PG&E Internal Information, SL2 © 2013 Pacific Gas & Electric Company. All Rights Reserved. Page 5 of 11
Gas Meter Room J-16
Rev. #03a 10-09-13
Natural Gas Regulator Requirements
- Natural Gas Regulators
Typically PG&E will install natural gas service regulators and overpressure protection devices outside of a meter room. Applicants/customers who want to install gas service regulators and overpressure protection devices inside of a meter room must provide PG&E with the written justification as part of the application. The preferred gas riser, meter and regulator location is outside and adjacent to the building being served. On an exception basis, gas meters and regulators may be installed in a specially designed gas meter room. PG&E must approve all gas meter and regulator installations and the gas meter room design in advance of any construction. If acceptable to PG&E, the following additional conditions apply:
A. Each gas service regulator installed within a building must be located as near as practical to the point of the service line entrance into the meter room, and as specified by PG&E.
B. PG&E will specify materials and designs for any overpressure protection devices needed, as outlined in Items 2 and 3 below.
- Regulator Vent Lines
A. Regulators and any additional overpressure protection equipment installed indoors must be vented to the outdoors. The customer is required to provide holes (penetrations) through walls or ceilings for these vents. In rare situations where the meter room location is not adjacent to an outside wall, the customer is required to install the vent piping from the meter room to the outside wall (PG&E will determine pipe size and location). PG&E will be responsible for connecting the vent piping to the regulator or overpressure protection device.
B. PG&E will position gas regulators to minimize the length of the regulator vent lines and to ensure adequate venting capacity.
- “ −C. PG&E will specify regulator vent lines, in accordance with Gas Design Standard H 93, Regulator Vent Lines
- Regulator Vent Locations
A. Service regulator vents must terminate in a safe outside location that complies with the following criteria:
(1) The regulator vent must not terminate near any sources of ignition or openings into the building. The regulator vent must be 36 inches horizontally from sources of ignition and openings into the building, and this clearance area will extend 10 feet above and 36 inches below the regulator vent termination.
(2) A minimum lateral distance of 8 feet from a forced air intake. This includes the intake vents for the gas meter room.
(3) Within any location that is under display platforms or show windows in commercial buildings, including any permanent, elevated, display floors or platforms associated with the window.
(4) Within any location that is under building overhangs, where the overhang is likely to direct venting gas into a building opening.
Frequently asked questions
When must a regulator vent go outdoors?
If the regulator location is such that a discharge of fuel gas would cause a hazard, an independent vent pipe to the outdoors is required; otherwise a common manifold is allowed only for non‑appliance regulators when engineered and approved. § 1308.14
Can I reduce the vent size after the relief valve outlet?
No — the code requires the discharge stack or vent line to be at least the same size as the outlet of the pressure relieving device. § 1308.9.6
May multiple regulator vents be combined into one vent?
Only for devices other than appliance regulators, and only if a common manifold is designed using engineering methods to minimize backpressure and is approved. Appliance regulator vents must not rely on that manifold allowance. § 1308.14
What if fittings between the system and the relief valve are long or have multiple elbows?
Fittings, pipe, and openings between the system and the relieving device must be sized and installed so they do not cause valve hammering or impair relief capacity — avoid unnecessary restrictions and consult the device rating. § 1308.9.7
Who determines regulator vent size?
The vent for the atmospheric side of the diaphragm shall be sized in accordance with the device manufacturer’s instructions, per § 1308.14.
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