CMC · California Mechanical Code

What overpressure protection is required for boilers and pressure vessels?

Boilers and pressure vessels in California must have listed pressure‑relief devices installed per manufacturer and the CMC: boilers per **§ 1005.1**, and pressure vessels per **§ 1113.0 / § 1113.1** (ASME BPVC sizing and Chapter 11 rules). Specific device type and installation depend on vessel volume (notably **3 cu ft** and **10 cu ft** thresholds), discharge location, and ASHRAE/ASME sizing rules; follow the cited sections for allowed devices, sizing, and piping requirements.

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Last reviewed: July 6, 2026

What the code requires — 2–4 sentences

In plain English: pressurized boilers and pressure vessels must be protected from excessive pressure by listed pressure-relief devices installed per the manufacturer and code. The California Mechanical Code requires a listed pressure relief valve for boilers and similar pressurized equipment (§ 1005.1) and requires pressure vessels to have overpressure protection in accordance with ASME BPVC Section VIII.1 and the CMC refrigeration provisions (§ 1113.0 and § 1113.1) .

The single most important rule: provide a listed pressure‑relief device sized and installed per the Code (and manufacturer) so the vessel cannot be overpressurized. See § 1005.1 and § 1113.1.

Requirements in detail

General obligation

  • Every pressurized vessel or boiler must have overpressure protection — typically a listed pressure relief valve — and it must be installed in accordance with the manufacturer’s instructions (§ 1005.1) .
  • For pressure vessels (including refrigerant-containing vessels that can be isolated by stop valves) the Code requires overpressure protection in accordance with ASME BPVC Section VIII.1; sizing of relief devices or fusible plugs follows CMC Section 1113.5 (and ASHRAE 15 as referenced) (§ 1113.0, § 1113.1) .

Type of device and volume thresholds

See the key decision dimensions below (volume thresholds, allowed device types, special installation rules).

Decision condition Required/allowed device Key requirement / limit Code Reference
Boilers & pressurized vessels (general) Listed pressure relief valve Installed per manufacturer § 1005.1
Internal gross volume ≤ 3 cu ft (0.1 m3) One or more pressure-relief devices or fusible plug permitted Fusible plug allowed only for ≤3 cu ft § 1113.2
Internal gross volume > 3 cu ft and < 10 cu ft One or more pressure-relief devices; fusible plugs prohibited Use relief devices sized per § 1113.5/ASHRAE 15 § 1113.2, § 1113.5
Internal gross volume ≥ 10 cu ft (0.28 m3) Rupture member(s) or dual pressure‑relief valves when discharging to atmosphere If dual valves used, a three‑way valve is required for testing/repair; single relief valve generally not permitted when discharging to lowside (§ 1113.3) unless exception applies § 1113.3, § 1113.6
Relief devices discharging to the lowside of system Single relief valve (not rupture member) not allowed on vessels ≥ 10 cu ft, except limited exception See Exception in § 1113.6 § 1113.3, § 1113.6
Sizing of discharge capacity Per ASHRAE 15 and CMC Section 1113.5 Relief device capacity must meet the minimum required discharge capacity § 1113.5
Discharge piping for relief valves Full‑size from valve outlet to flood level, no valves/obstructions, rated for temp, vented to safe location See multiple piping details and prohibited connections § 1005.2

Discharge piping and other installation controls

  • Discharge piping serving temperature or pressure relief valves must have no valves or isolating means, be not less than the valve outlet size, discharge full‑size to a safe termination, use materials rated for system temperature, and be arranged to avoid trapping or freezing and to prevent personal/structural harm (§ 1005.2) .
  • Other special discharge rules (splash shields, hazardous discharges to outside, vacuum relief, etc.) are in the same discharge section and must be followed where applicable (§ 1005.3–1005.5) .

Exceptions & special cases

  • Fusible plugs: allowed only on vessels with internal gross volume ≤ 3 cu ft; they are explicitly prohibited for vessels >3 cu ft (§ 1113.2) .
  • Single relief valve on large vessels (≥ 10 cu ft): Normally not permitted when discharging into the lowside of the system, but an exception allows a single valve if all three conditions in the exception are met (lowside location, vessel designed for pumpdown with shutoff valves, other vessels separately protected) (§ 1113.3, § 1113.6 exception) .
  • Sizing and method references: The Code defers detailed sizing procedures to ASHRAE 15 (and ASME BPVC for construction), so where sizing is required you must follow those referenced standards in addition to CMC § 1113.5 .

Common mistakes

  • Installing a shutoff valve in the relief discharge line or otherwise isolating the relief device (disallowed by § 1005.2).
  • Using a fusible plug on vessels larger than 3 cu ft, which the Code forbids.
  • Using a single relief valve on a vessel ≥ 10 cu ft discharging to the lowside without meeting the exception conditions.
  • Failing to size relief devices per ASHRAE 15 / § 1113.5 or neglecting ASME BPVC requirements for pressure‑vessel construction and certification.
  • Undersizing or improperly routing discharge piping (must be at least valve outlet size, no traps, no threads at terminal, visible termination, etc.) (§ 1005.2).

Worked example — concrete scenario

Scenario: You have a refrigerant pressure vessel with an internal gross volume of 12 cubic feet (≈0.34 m3) that may be isolated by stop valves and will discharge to the atmosphere.

Step 1 — Device type:

  • Because the vessel is ≥ 10 cu ft, the CMC requires one or more rupture member(s) or dual pressure‑relief valves when discharging to atmosphere; if you install dual valves they must be piped with a three‑way valve to permit testing/repair (§ 1113.6) .

Step 2 — Sizing:

  • The relief devices must be sized to the minimum required discharge capacity per § 1113.5 using the methods in ASHRAE 15 (referenced by the Code) — do the ASHRAE calculation to determine flow capacity and select devices accordingly (§ 1113.5) .

Step 3 — Discharge piping:

  • Install discharge piping not less than the valve outlet size, full‑size to safe termination, with no isolating valves or traps, materials rated for temperature, and terminated so it will not cause personal injury or structural damage (§ 1005.2) .

If instead the vessel had been 2.5 cu ft, a fusible plug would be an allowable option; but at 12 cu ft fusible plugs are not permitted. (§ 1113.2)

Related provisions (CMC sections)

  • § 1005.1 — General requirement for listed pressure relief valve on pressurized vessels/boilers.
  • § 1005.2 — Discharge piping requirements for relief devices (no valves, size, termination rules).
  • § 1113.0 — Overpressure protection (introductory statement and ASME BPVC reference).
  • § 1113.1 — General requirements for pressure‑vessel overpressure protection and sizing reference to § 1113.5.
  • § 1113.2 — Type of protection by internal gross volume (≤3, >3 <10).
  • § 1113.3 — Restrictions when discharging into the lowside of the system.
  • § 1113.5 — Discharge capacity (sizing per ASHRAE 15).
  • § 1113.6 — Requirement for rupture members or dual valves and three‑way valve for ≥10 cu ft vessels.

Code references

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

  • CMC § 3.785 High relevance — show source text
    SYSTEM VOLUME2
    (gallons)    		 TANK CAPACITY
    DIAPHRAGM TYPE
    (gallons)    		 TANK CAPACITY NON-
    DIAPHRAGM TYPE
    (gallons)


    100    		 9 15


    200    		 17 30


    300    		 25 45

    400    		 33 60

    500    		 42 75

    1000    		 83 150

    2000    		 165 300

    For SI units: 1 gallon = 3.785 L

    Notes: 1 Based on an average operating water temperature of 195°F (91°C), a fill pressure of 12 psig (83 kPa), and an operating pressure of not more than 30 psig (207 kPa). 2 Includes volume of water in boiler, radiation, and piping, not including expansion tank.

    1005.0 Safety or Relief Valve Discharge.

    1005.1 General. Pressurized vessels or boilers shall be provided with overpressure protection by means of a listed pressure relief valve installed in accordance with the manufacturer’s

    installation instructions.

    1005.2 Discharge Piping. The discharge piping serving a temperature relief valve, pressure relief valve, or combination of both shall have no valves, obstructions, or means of isolation and provided with the following:

    (1) Not less than the size of the valve outlet and shall discharge full size to the flood level of the area receiving the discharge and pointing down.

    (2) Materials shall be rated at not less than the operating temperature of the system and approved for such use or shall comply with ASME A112.4.1.

    (3) Discharge pipe shall discharge independently by gravity through an air gap into the drainage system or outside of the building with the end of the pipe not exceeding 2 feet (610 mm) and not less than 6 inches (152 mm) above the ground and pointing downwards.

    (4) Discharge in such a manner that does not cause personal injury or structural damage.

    (5) No part of such discharge pipe shall be trapped or subject to freezing.

    (6) The terminal end of the pipe shall not be threaded.

    (7) Discharge from a relief valve into a water heater pan shall be prohibited.

    (8) Discharge to a termination point that is readily visible.

    1005.3 Splash Shield. Where the operating temperature exceeds 212°F (100°C), the discharge pipe shall be installed with a splash shield or centrifugal separator.

    1005.4 Hazardous Discharge. Where the discharge from safety valves is capable of being hazardous, discharge of steam inside the boiler room, such discharge shall be discharged to the outside of the boiler room. Discharges from relief valves on industrial boilers shall be discharged to an approved location.

    1005.5 Vacuum Relief Valve. Hot-water heating systems that are subjected to a vacuum while in operation or during shutdown shall be protected with a vacuum relief valve that complies with ANSI Z21.22/CSA 4.4. Where the piping configuration, equipment location, and valve outlets are located below the boiler elevation, the system shall be equipped with a vacuum relief valve at the highest point.

    1006.0 Shutoff Valves.

  • 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 § 1112.12.4. High relevance — show source text

    ( P [2] 0 P 22 ) d - ln ( P0P2 )

    f - C [2] r 6 - f

    2025 CALIFORNIA MECHANICAL CODE 243

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    REFRIGERATION

    expected to operate simultaneously, connect to a common discharge pipe, the common pipe shall be sized large enough to prevent the back pressure at each pressure-relief device from exceeding the maximum allowable back pressure in accordance with Section 1112.12.4.

    [ASHRAE 15:9.7.9.3.3]

    1112.13 Rating of Pressure-Relief Device. The rated discharge capacity of a pressure-relief device expressed in pounds of air per minute (kg/s), shall be determined in accordance with ASME BPVC Section VIII.1. Pipe and fittings between the pressure-relief valve and the parts of the system it protects shall have not less than the area of the pressurerelief valve inlet area. [ASHRAE 15:9.7.6]

    1112.14 Rating of Rupture Members and Fusible Plugs. The rated discharge capacity of a rupture member or fusible plug discharging to the atmosphere under critical flow conditions, in pounds of air per minute (kg/s), shall be determined in accordance with the following formulas:

    C = 0.64 P l d [2] [Equation 1112.14(1)]

    d = 1.25 √ C / P l [Equation 1112.14(2)]

    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):

  • CMC § 3.785 High relevance — show source text
    INSTALLED EQUIVALENT
    DIRECT RADIATION2
    (square feet)    		 TANK CAPACITY
    (gallons)

    Up to 350    		 18

    Up to 450    		 21

    Up to 650    		 24


    Up to 900    		 30


    Up to 1100    		 35


    Up to 1400    		 40


    Up to 1600    		 2 to 30


    Up to 1800    		 2 to 30

    Up to 2000    		 2 to 35

    Up to 2400    		 2 to 40

    For SI units: 1 gallon = 3.785 L, 1 square foot = 0.0929 m [2]

    Notes: 1 Based on a two-pipe system with an average operating water temperature of 170°F (77°C), using cast-iron column radiation with a heat emission rate of 150 British thermal units per square foot hour [Btu/(ft [2] - h)] (473 W/m [2] ) equivalent direct radiation. 2 For systems that exceed 2400 square feet (222.9 m 2 ) of installed equivalent direct water radiation, the required capacity of the cushion tank shall be increased on the basis of 1 gallon (4 L) tank capacity per 33 square feet (3.1 m [2] ) of additional equivalent direct radiation.

    TABLE 1004.4(2) EXPANSION TANK CAPACITIES FOR FORCED HOT WATER SYSTEMS [1]

    SYSTEM VOLUME2
    (gallons)    		 TANK CAPACITY
    DIAPHRAGM TYPE
    (gallons)    		 TANK CAPACITY NON-
    DIAPHRAGM TYPE
    (gallons)


    100    		 9 15


    200    		 17 30


    300    		 25 45

    400    		 33 60

    500    		 42 75

    1000    		 83 150

    2000    		 165 300

    For SI units: 1 gallon = 3.785 L

    Notes: 1 Based on an average operating water temperature of 195°F (91°C), a fill pressure of 12 psig (83 kPa), and an operating pressure of not more than 30 psig (207 kPa). 2 Includes volume of water in boiler, radiation, and piping, not including expansion tank.

    1005.0 Safety or Relief Valve Discharge.

    1005.1 General. Pressurized vessels or boilers shall be provided with overpressure protection by means of a listed pressure relief valve installed in accordance with the manufacturer’s

    installation instructions.

    1005.2 Discharge Piping. The discharge piping serving a temperature relief valve, pressure relief valve, or combination of both shall have no valves, obstructions, or means of isolation and provided with the following:

    (1) Not less than the size of the valve outlet and shall discharge full size to the flood level of the area receiving the discharge and pointing down.

    (2) Materials shall be rated at not less than the operating temperature of the system and approved for such use or shall comply with ASME A112.4.1.

  • CMC § 1112.13 High relevance — show source text

    1112.13 Rating of Pressure-Relief Device. . . . . . . . . . . . . . . . . . . . . . 244

    1112.14 Rating of Rupture Members and Fusible Plugs . . . . . . . . . . . . . . . . 244

    1113.0 Overpressure Protection. . . . . . . . 244

    1113.1 General. . . . . . . . . . . . . . . . . . . . . 244

    1113.2 Type of Protection . . . . . . . . . . . . 245

    1113.3 Discharging into Lowside of System . . . . . . . . . . . . . . . . . . . . . 245

    1113.4 Parallel Pressure-Relief Devices . . 245

    1113.5 Discharge Capacity . . . . . . . . . . . 245

    1113.6 Three-Way Valve . . . . . . . . . . . . . 245

    1114.0 Special Discharge Requirements. . 245

    1114.1 General. . . . . . . . . . . . . . . . . . . . . 245

    1114.2 Design Requirements . . . . . . . . . . 245

    1114.3 Testing . . . . . . . . . . . . . . . . . . . . . 245

    1115.0 Labeling and Identification . . . . . . 245

    1115.1 General. . . . . . . . . . . . . . . . . . . . . 245

    1115.2 Volume and Type . . . . . . . . . . . . . 245

    1115.3 Permanent Sign . . . . . . . . . . . . . . 245

    1115.4 Marking of Pressure-Relief Devices. . . . . . . . . . . . . . . . . . . . . 245

    1115.5 Nameplate . . . . . . . . . . . . . . . . . . 245

    1116.0 Testing of Refrigeration Equipment . . . . . . . . . . . . . . . . . . 246

    1116.1 Factory Tests . . . . . . . . . . . . . . . . 246

    1116.2 Field Tests . . . . . . . . . . . . . . . . . . 246

    1116.3 Test Gases . . . . . . . . . . . . . . . . . . 246

    1116.4 Declaration . . . . . . . . . . . . . . . . . . 246

    1116.5 Brine Systems . . . . . . . . . . . . . . . 247

    1117.0 Refrigerant-Containing Pressure Vessels . . . . . . . . . . . . . . . . . . . . . 247

    1117.1 Inside Dimensions 6 Inches

    or Less . . . . . . . . . . . . . . . . . . . . . 247

    1117.2 Inside Dimensions More than

    6 Inches . . . . . . . . . . . . . . . . . . . . 247

    1117.3 Pressure Vessels for 15 psig or Less . . . . . . . . . . . . . . . . . . . . . 247

  • 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

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    2025 CALIFORNIA MECHANICAL CODE 281

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    FUEL GAS PIPING

    TABLE 1308.12

    MANUAL GAS VALVE STANDARDS

    [NFPA 54: TABLE 5.11]

  • CMC § 304.8 High 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

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    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 § 1308.8 High relevance — show source text

    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

    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:

  • CMC § 1007.0 Medium relevance — show source text

    1007.0 Gas-Pressure Regulators.

    1007.1 General. An approved gas-pressure regulator shall be installed on gas-fired boilers where the gas supply pressure is exceeding that at which the main burner is designed to operate. A separate approved gas-pressure regulator shall be installed to regulate the gas pressure to the pilot or pilots.

    2025 CALIFORNIA MECHANICAL CODE 219

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    BOILERS AND PRESSURE VESSELS

    1008.0 Low-Water Cutoff.

    1008.1 General. Hot water boilers and steam boilers shall be installed with a low-water cutoff. A coil-type boiler or a water-tube boiler that requires forced circulation to prevent overheating of the coils or tubes shall be installed with a flowsensing device in the outlet piping in lieu of the low-water cutoff. The low-water cutoff or the flow sensing device shall be installed so as to prevent damage to the boiler and to permit testing of the fuel-supply cutoff without draining the heating system. The low-water cutoff shall shut off the energy source at a water level setpoint that is in accordance with the boiler manufacturer’s instructions.

    1009.0 Combustion Regulators – Safety Valves.

    1009.1 General. The following requirements shall be retroactive:

    (1) Hot-water-heating boilers, other than manually fired, shall be equipped with two temperature combustion regulators in series. Steam-heating boilers, other than manually fired, shall be equipped with a pressure combustion regulator and a low-water cutoff. (See Section 1008.0)

    (2) Boilers and pressure vessels shall be provided with the required number, size, and capacity of safety or relief valves to ensure positive relief of overpressure in accordance with nationally recognized standards, as applicable. Valves so employed shall be constructed, sealed, and installed in accordance with nationally recognized standards, as applicable.

    1010.0 Clearance for Access.

    1010.1 General. Where boilers are installed or replaced, clearance shall be provided to allow access for inspection, maintenance, and repair. Passageways around all sides of boilers shall have an unobstructed width of not less than 18 inches (457 mm). Clearance for repair and cleaning shall be permitted to be provided through a door or access panel into another area, provided the opening is of sufficient size.

    Exception: Subject to the approval of the Authority Having Jurisdiction, boilers shall be permitted to be installed with a side clearance of less than 18 inches (457 mm), provided that the lesser clearance does not inhibit inspection, maintenance, or repair.

    1010.2 Power Boilers. Power boilers having a steam-generating capacity in excess of 5000 pounds per hour (lb/h) (0.6299 kg/s) or having a heating surface in excess of 1000 square feet (92.9 m [2] ) or input in excess of 5 000 000 Btu/h (1464 kW) shall have a clearance of not less than 7 feet (2134 mm) from the top of the boiler to the ceiling.

    1010.3 Steam-Heating Boilers, Hot Water Boilers, and Power Boilers. Steam-heating boilers and hot-water-heating boilers that exceed one of the following limits:

    (1) 5 000 000 Btu/h input (1464 kW)

  • CMC § 1306.1 Medium relevance — show source text

    1306.1 Disconnection . . . . . . . . . . . . . . . . 276

    1306.2 Notice . . . . . . . . . . . . . . . . . . . . . . 276

    1306.3 Capped Outlets. . . . . . . . . . . . . . . 276

    1307.0 Temporary Use of Gas . . . . . . . . . 276

    1307.1 General. . . . . . . . . . . . . . . . . . . . . 276

    1308.0 Gas Piping System Design, Materials, and Components . . . . . 276

    1308.1 Installation of Piping System . . . . 276

    1308.2 Interconnections Supplying Separate Users. . . . . . . . . . . . . . . 276

    1308.3 Sizing of Gas Piping Systems . . . 277

    1308.4 Acceptable Piping Materials and Joining Methods . . . . . . . . . . 277

    Table 1308.3.1 Approximate Gas Input for Typical Appliances . . . . . . . . . . . . 277

    Table 1308.4.6.2 Specifications for Threading Metallic Pipe . . . . . . . . . . . . . . . . . 278

    1308.5 Gas Meters . . . . . . . . . . . . . . . . . . 280

    1308.6 Gas Pressure Regulators . . . . . . . 280

    1308.7 Overpressure Protection. . . . . . . . 281

    2025 CALIFORNIA MECHANICAL CODE

    xlii

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

    TABLE OF CONTENTS

    1308.8 Pressure Limitation

    Requirements . . . . . . . . . . . . . . . . 281

    1308.9 Overpressure Protection Devices. . . . . . . . . . . . . . . . . . . . . 281

    1308.10 Backpressure Protection . . . . . . . 282

    1308.11 Low-Pressure Protection . . . . . . . 282

    1308.12 Shutoff Valves . . . . . . . . . . . . . . . 282

    Table 1308.12 Manual Gas Valve Standards. . . . 282

    1308.13 Expansion and Flexibility . . . . . . . 282

    1308.14 Pressure Regulator and Pressure Control Venting . . . . . . . 282

    1309.0 Excess Flow Valve . . . . . . . . . . . . 283

    1309.1 General. . . . . . . . . . . . . . . . . . . . . 283

    1310.0 Gas Piping Installation . . . . . . . . . 283

    1310.1 Piping Underground . . . . . . . . . . . 283

    1310.2 CSST Piping Systems . . . . . . . . . 284

    1310.3 Installation of Aboveground Piping . . . . . . . . . . . . . . . . . . . . . . 284

    Table 1310.3.5.1 Support of Piping . . . . . . . . . . . . . 285

  • CMC § 2.5.1.4 Medium relevance — show source text

    0 1 2 3 4 5 6 7

    8 9 10

    FIRE-RESISTANCE RATING (HOURS)

    TABLE 2.5.1.4
    STEEL COLUMNS—CONCRETE ENCASEMENTS
    MINIMUM DIMENSION 10″ TO LESS THAN 12″    		 Col2 Col3 Col4 Col5 Col6 Col7 Col8 Col9 Col10
    ITEM
    CODE    		 MINIMUM
    DIMENSION    		 CONSTRUCTION DETAILS PERFORMANCE PERFORMANCE REFERENCE NUMBER REFERENCE NUMBER REFERENCE NUMBER NOTES REC.
    HOURS
    ITEM
    CODE    		 MINIMUM
    DIMENSION    		 CONSTRUCTION DETAILS LOAD TIME PRE-BMS-92 BMS-92 POST-BMS-92 POST-BMS-92 POST-BMS-92
    C-10-SC-1 10″ 10″ × 12″ concrete encased steel column;
    8″ × 6″ × 35 lbs “H” beam; protection: gravel
    aggregate concrete (3640 psi); 6″ × 4″—13 SWG
    mesh, 1″ below column surface.    		 90
    tons    		 3 hrs
    7 min    		 7 1, 2 3
    C-10-SC-2 10″ 10″ × 16″ column; 8″ × 6″ × 35 lbs “H” beam;
    protection: clay brick concrete (3630 psi);
    6″ × 4″—13 SWG mesh, 1″ below column
    surface.    		 90
    tons    		 4 hrs
    6 min    		 7 2 4
    C-10-SC-3 10″ 10″ × 12″ column; 8″ × 6″ × 35 lbs “H” beam;
    protection: crushed stone and sand concrete
    (3930 psi); 6″ × 4″—13 SWG mesh, 1″ below
    column surface.    		 90
    tons    		 3 hrs
    17 min    		 7 2 31/4
    C-10-SC-4 10″ 10″ × 12″ column; 8″ × 6″ × 35 lbs “H” beam;
    protection: crushed basalt and sand concrete
    (4350 psi); 6″ × 4″—13 SWG mesh, 1″ below
    column surface.    		 90
    tons    		 3 hrs
    22 min    		 7 2 31/3
    C-10-SC-5 10″ 10″ × 12″ column; 8″ × 6″ × 35 lbs
    “H” beam; protection: gravel aggregate
    concrete (5570 psi); 6″ × 4″—13 SWG mesh.    		 90
    tons    		 3 hrs
    39 min    		 7 2 31/2
    C-10-SC-6 10″ 10″ × 16″ column; 8″ × 6″ × 35 lbs “I” beam;
    protection: gravel concrete (4950 psi);
  • CMC § 1308.8 Medium relevance — show source text

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

    TABLE OF CONTENTS

    1308.8 Pressure Limitation

    Requirements . . . . . . . . . . . . . . . . 281

    1308.9 Overpressure Protection Devices. . . . . . . . . . . . . . . . . . . . . 281

    1308.10 Backpressure Protection . . . . . . . 282

    1308.11 Low-Pressure Protection . . . . . . . 282

    1308.12 Shutoff Valves . . . . . . . . . . . . . . . 282

    Table 1308.12 Manual Gas Valve Standards. . . . 282

    1308.13 Expansion and Flexibility . . . . . . . 282

    1308.14 Pressure Regulator and Pressure Control Venting . . . . . . . 282

    1309.0 Excess Flow Valve . . . . . . . . . . . . 283

    1309.1 General. . . . . . . . . . . . . . . . . . . . . 283

    1310.0 Gas Piping Installation . . . . . . . . . 283

    1310.1 Piping Underground . . . . . . . . . . . 283

    1310.2 CSST Piping Systems . . . . . . . . . 284

    1310.3 Installation of Aboveground Piping . . . . . . . . . . . . . . . . . . . . . . 284

    Table 1310.3.5.1 Support of Piping . . . . . . . . . . . . . 285

    1310.4 Concealed Piping in Buildings . . . 285

    1310.5 Piping in Vertical Chases . . . . . . . 286

    1310.6 Maximum Operating Pressure in Buildings . . . . . . . . . . . . . . . . . . 286

    1310.7 Appliance Overpressure Protection . . . . . . . . . . . . . . . . . . . 286

    1310.8 Gas Pipe Turns. . . . . . . . . . . . . . . 286

    1310.9 Drips and Sediment Traps . . . . . . 287

    1310.10 Outlets . . . . . . . . . . . . . . . . . . . . . 287

    1310.11 Manual Gas Shutoff Valves . . . . . 287

    1310.12 Prohibited Devices . . . . . . . . . . . . 288

    1310.13 Systems Containing Gas-Air Mixtures Outside the Flammable

    Range . . . . . . . . . . . . . . . . . . . . . . 288

    1310.14 Systems Containing Flammable Gas-Air Mixtures. . . . . . . . . . . . . . 288

    1311.0 Electrical Bonding and Grounding. . . . . . . . . . . . . . . . . . . 289

    1311.1 Pipe and Tubing other than CSST . . . . . . . . . . . . . . . . . . . . . . 289

    1311.2 Bonding of CSST Gas Piping . . . . 289

    1311.3 Arc-Resistant Jacketed CSST . . . 289

Frequently asked questions

What is meant by a "listed" pressure relief valve?

A "listed" valve is one tested and listed by a recognized listing agency and installed per the manufacturer's instructions; the CMC requires a listed pressure relief valve for pressurized vessels/boilers (§ 1005.1).

Can I use a fusible plug on any size refrigeration vessel?

No — fusible plugs are allowed only for vessels with internal gross volume ≤ 3 cu ft; they are prohibited for vessels larger than 3 cu ft (§ 1113.2).

Are shutoff valves allowed between a relief valve and the vessel?

No — discharge piping serving relief valves must have no valves, obstructions, or means of isolation per § 1005.2.

If a vessel is 15 cu ft and discharges to the lowside, can one relief valve be used?

Generally no for vessels ≥ 10 cu ft discharging to the lowside — a single relief valve (not a rupture member) is not allowed except under the narrow exception listed in § 1113.6 (conditions must be met).

Where do I find how to size the relief valve?

The CMC directs sizing of pressure‑relief devices for pressure vessels to § 1113.5 and ASHRAE 15; follow those procedures to determine required discharge capacity.

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Overpressure Protection, Relief Devices and Discharge

California Mechanical Code