CMC · California Mechanical Code

How must safety/temperature/pressure-relief valve discharge piping be installed?

Your relief valve drain must never be valved or reduced; use piping at least the valve outlet size, use materials rated for the system, route it to a visible drain or outside through an air gap, terminate pointing down between 6 inches and 2 feet above the surface, and for refrigeration/pressure vessels confirm relief capacity per ASME as required by the CMC.

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Last reviewed: July 6, 2026

What the code requires — 2-4 sentences

The discharge piping for temperature, pressure, or combination temperature/pressure relief devices must be installed so it cannot be isolated, must be at least the size of the valve outlet, must discharge by gravity through an air gap to a visible drainage point or outdoors, and must terminate pointing downward at a location between 6 inches (152 mm) and 2 feet (610 mm) above the receiving surface. These basic rules are set out in § 1005.2 of the California Mechanical Code (CMC) ; equipment rating and relief-device capacity rules for refrigeration/pressure vessels are required to be determined in accordance with ASME and are summarized in § 1112.13 .

The single most important rule: the relief-device drain must remain open, full-size, and free‑flowing (no valves or traps) and must discharge so it will not cause injury or damage. See § 1005.2.

Requirements in detail

Basic installation constraints (what you cannot do)

  • The discharge piping shall have no valves, obstructions, or means of isolation between the valve and the termination point — it must remain permanently open (see § 1005.2).
  • The terminal end of the discharge pipe shall not be threaded. § 1005.2(6).
  • Discharge into a water-heater pan is prohibited. § 1005.2(7).

Minimum sizing and materials

  • The discharge pipe shall be not less than the size of the valve outlet and shall remain full-size to the flood level of the receiving area, pointing down (§ 1005.2(1)).
  • Materials must be rated for not less than the operating temperature of the system or comply with ASME A112.4.1 (§ 1005.2(2)).
  • For pressure-relief devices on refrigeration/pressure vessels, the device rating (discharge capacity) must be determined per ASME BPVC Section VIII.1, and piping between the valve and the protected part of the system must have an area at least equal to the inlet area of the valve (§ 1112.13) .

Discharge location and termination

  • Discharge shall be by gravity through an air gap into the drainage system or to the outside. The pipe end shall be no more than 2 feet (610 mm) and no less than 6 inches (152 mm) above the receiving ground or drain, and must point downward (§ 1005.2(3)).
  • The termination point shall be readily visible (so the discharge can be observed) (§ 1005.2(8)).

Backpressure, equivalent length, and refrigeration specifics

  • Where relief devices discharge to atmosphere, allowable design back pressure and the maximum equivalent length of discharge piping are controlled by the discharge-flow equations and limits in § 1112.12.4 and related subsections; these limit the piping length and sizing so the valve can deliver rated capacity (see § 1112.12 series). For rating and allowable backpressure, see § 1112.12 and § 1112.13.

Table — decision‑relevant dimensions/values

Requirement Value / Dimension When it applies Code Reference
Minimum pipe size Not less than valve outlet size All relief/discharge piping § 1005.2(1)
Discharge vertical termination ≥ 6 in (152 mm) and ≤ 2 ft (610 mm) above ground/drain; pipe pointing downward Termination to outside or into drain § 1005.2(3)
Terminal end condition Not threaded All discharge terminations § 1005.2(6)
Isolation devices Prohibited — no valves/obstructions Between valve and termination § 1005.2
Material temperature rating ≥ operating temperature or comply with ASME A112.4.1 Material selection § 1005.2(2)
Visibility Termination readily visible So discharge can be observed § 1005.2(8)
Device discharge capacity Determined per ASME BPVC Section VIII.1 Relief device sizing (refrigeration/pressure vessels) § 1112.13
Pipe area relative to valve inlet Pipe and fittings must have area ≥ valve inlet area Between valve and protected equipment § 1112.13

Exceptions & special cases

  • Where operating temperature exceeds 212°F (100°C), the discharge pipe must be fitted with a splash shield or centrifugal separator to reduce injury and damage risk (see § 1005.3).
  • If a discharge would be hazardous inside a boiler room, steam or hazardous relief discharge must be routed to the outside of the boiler room or to an approved location (see § 1005.4).
  • For refrigeration and pressure vessels, maximum allowable back pressure and allowable equivalent discharge length are controlled by the formulas and limits in the § 1112.12 series; certain valve types (balanced, rupture disks, fusible plugs) have different allowed backpressure percentages and must be evaluated using those equations. See § 1112.12.3–1112.12.5 and § 1112.12.4 for calculation details.

If you have a situation where relief piping must run an extended distance, you must use the design equations in § 1112.12.4 to verify allowable equivalent length and backpressure — the code does not give a simple “maximum feet” number without that calculation.

Common mistakes

  • Installing a shutoff valve, gate, or union that can isolate the relief discharge — prohibited by § 1005.2.
  • Reducing the piping size downstream of the valve outlet (pipe must be at least as large as the outlet) — violates § 1005.2(1) and § 1112.13.
  • Threading the terminal end, which could allow a hose or obstruction to be attached — prohibited by § 1005.2(6).
  • Routing the discharge to a location where it can cause personal injury or structural damage — § 1005.2(4) requires discharge to avoid injury/damage.
  • Allowing discharge piping to be trapped or subject to freezing (this can block flow) — prohibited by § 1005.2(5).
  • For refrigeration/pressure vessels, failing to size devices per ASME BPVC Section VIII.1 and neglecting to confirm piping area equals or exceeds valve inlet area — required by § 1112.13.

Worked example — concrete scenario

Scenario: A building-installed storage water heater has a listed combination temperature-and-pressure relief valve with a 3/4‑inch outlet. The installer runs the relief piping down the side of the heater to outdoors.

How to apply the rules:

  1. Minimum pipe size: Use 3/4‑inch discharge piping (pipe must be not less than the valve outlet size) and maintain full-size run to the termination (§ 1005.2(1)).
  2. No isolation: Do not install any shutoff or valve on that discharge line — it must remain free-flowing (§ 1005.2).
  3. Material: Select piping material rated for the heater’s operating temperature (or comply with ASME A112.4.1) (§ 1005.2(2)).
  4. Termination: Run the pipe to terminate pointing downward, and place the end so it is between 6 inches and 2 feet above the exterior grade or drain and is readily visible (e.g., 12 inches above grade) (§ 1005.2(3),(8)).
  5. Terminal end: Ensure the terminal end is not threaded and that the route avoids traps and freezing locations (§ 1005.2(5),(6)).

If instead the device were a pressure-relief device on a refrigeration vessel, you would additionally confirm the relief device’s rated discharge capacity per ASME BPVC Section VIII.1 and ensure the discharge piping (and any common header) has an equivalent flow area at least equal to the valve inlet area as required by § 1112.13. For extended runs, perform the backpressure/length calculation in § 1112.12.4.

Related provisions

  • § 1005.3 — Splash shield required where operating temperature exceeds 212°F (100°C).
  • § 1005.4 — Hazardous discharges (route to outside of boiler room or approved location).
  • § 1112.12 — Discharge piping specifics for pressure-relief devices (connections, pipe size, maximum length, design back pressure equations).
  • § 1112.14 — Rating rules for rupture members and fusible plugs (when used).
  • § 1113.5 — (See Chapter 11 overpressure-sizing requirements for pressure vessels and relief sizing context.)

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 § 1005.3 High relevance — show source text

    (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.

    1006.1 General. An approved manual shutoff valve shall be installed upstream of all control devices on the main burner of a gas-fired boiler. The takeoff point for the gas supply to the pilot shall be upstream of the gas shutoff valve of the main burner and shall be valved separately. A union or other approved means of disconnect shall be provided immediately downstream of these shutoff valves.

    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:

  • CMC § 608.3 High relevance — show source text

    The expansion tank shall be properly sized, securely fastened to the structure, and installed in accordance with the manufacturer’s installation instructions and listing. Systems designed by a licensed plumbing contractor or registered design professionals shall be permitted to use approved pressure relief valves in lieu of expansion tanks provided such relief valves have a maximum pressure relief setting of 100 psi (689 kPa) or less. 608.3 Expansion Tanks, and Combination Tempera- ture and Pressure-Relief Valves. A water system provided with a check valve, backflow preventer, or other normally closed device that prevents dissipation of building pressure back into the water main, independent of the type of water heater used, shall be provided with an approved, listed, and adequately sized expansion tank or other approved device having a similar function to control thermal expansion. Prepressurized water expansion tanks shall comply with IAPMO/ANSI Z1088. Such expansion tank or other approved device shall be installed on the building side of the check valve, backflow preventer, or other device and shall be sized, securely fastened to the structure, and installed in accordance with the manufacturer’s installation instructions.

    A water system containing storage water heating equipment shall be provided with an approved, listed, adequately sized combination temperature and pressure-relief valve, except for listed nonstorage instantaneous heaters having an inside diameter of not more than 3 inches (80 mm). Each such approved combination temperature and pressure-relief valve shall be installed on the water-heating device in an approved location based on its listing requirements and the manufacturer’s installation instructions. Each such combination temperature and pressure-relief valve shall be provided with a drain in accordance with Section 608.5.

    Exception: An expansion tank shall not be required for an instantaneous non-storage water heater. 608.4 Pressure Relief Valves. Each pressure relief valve shall be an approved automatic type with drain, and each such relief valve shall be set at a pressure of not more than 150 psi (1034 kPa). No shutoff valve shall be installed between the relief valve and the system. 608.5 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 be 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

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    WATER SUPPLY AND DISTRIBUTION

    (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) The discharge termination point shall be readily observable.

  • 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 § 1205.3 High relevance — show source text

    Note: [HCD 1 & HCD 2] PEX-AL-PEX is not adopted in the California Plumbing Code for use in potable water supply and distribution systems. (2) Copper tubing shall be tested at not less than 80 psi (552 kPa). 1205.3 Flushing. Heating and cooling sources, system piping and tubing shall be flushed after installation with water or a cleaning solution. Cleaning and flushing of the heating and cooling sources shall comply with the manufacturer’s instructions. The cleaning solution shall be compatible with all system components and shall be used in accordance with the manufacturer’s instructions.

    1206.0 Pressure and Safety Devices. 1206.1 General. Each closed hydronic system shall be protected against pressures exceeding design limitations with not less than one pressure relief valve. Each closed section of the system containing a heat source shall have a relief valve located so that the heat source is not capable of being isolated from a relief device. Pressure relief valves shall be installed in accordance with their listing and the manufacturer’s installation instructions.

    1206.2 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 be 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) The discharge termination point shall be readily visible.

    1207.0 Heating Appliances and Equipment. 1207.1 General. Heating appliances, equipment, safety and operational controls shall be listed for its intended use in a hydronic heating system and installed in accordance with the manufacturer’s installation instructions.

    1207.2 Boilers. Boilers and their control systems shall comply with Section 1002.0. 1207.2.1 Condensing Boilers. A condensing boiler, in which the heat exchanger and venting system are designed to operate with condensing flue gases, shall be permitted to be connected directly to the panel heating system without a protective mixing device. 1207.2.2 Noncondensing Boilers. Where the heat exchanger and venting system are not designed to operate with condensed flue gases, the boiler shall be permitted to connect directly to the panel heating system where protected from flue gas condensation. The operating temperature of the boiler shall be more than the fluid temperature in accordance with the manufacturer’s instructions. The minimum return-water temperature to the heat source shall comply with Section 1201.5.

  • CMC § 3663.3 High relevance — show source text

    (n) A high oil temperature interlock for oil or liquid fuel requiring preheating.

    (o) The burner oil pump shall automatically not operate or rotate while the alternate fuel is firing.

    (p) A pressure-relief valve shall be provided between safety shutoff valves and between pump and safety valves where an integral valve is used with a pump.

    (q) A separate relief device is required on each transfer

    pump.

    (3) One million to 2 499 999 Btu/h (293 kW to 732 kW) input per burner.

    (a) One approved manual shutoff valve lever handle.

    (b) One approved fuel-oil filter, installed on the supply piping.

    (c) Two safety shutoff valves in series with a combined flame failure response and valve closing time not to exceed 5 seconds with strainer directly before the valves.

    (d) Programmed electronic flame safeguard including proven low-fire start, manual reset lockout, 100 percent shutoff (both pilot and main burner), and a separately supervised and proven pilot.

    Flame-sensing systems utilizing a UV scanner shall prove pilot and interrupt ignition spark prior to main burner valves being energized.

    (e) Two controls, one operating and one high limit, activated by temperature or pressure.

    (f) Burners relying on mechanical means to provide air for combustion shall have actual proof-of-air interlock device.

    (g) Power burners shall include proven prepurge of not less than 60 seconds at high-fire damper settings. This prepurge shall occur before every burner cycle, regardless of reason.

    (h) Installations with dampered combustion air openings shall prove damper open position before trial for burner ignition.

    (i) Vent dampers and flue dampers shall be interlocked to prevent burner ignition unless safely open.

    (j) One high oil or liquid fuel-pressure interlock, reset from flame safeguard or manually.

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    APPENDIX C

    (k) Where hot water or steam, two low water cutoffs.

    (l) An atomizing medium proving switch.

    (m) A low oil temperature switch for oil or liquid fuel requiring preheating.

    (n) A high oil temperature interlock for oil or liquid fuel requiring preheating.

    (o) The burner oil pump shall automatically not operate or rotate while the alternate fuel is firing.

    (p) A pressure-relief valve shall be provided between safety shutoff valves and between pump and safety valves where an integral valve is used with a pump.

    (q) A separate relief device is required on each transfer

    pump.

    (r) One low oil or liquid fuel-pressure interlock, reset from flame safeguard or manually.

    (s) Burners with automatic controls, prepurge, proofof-closure, modulation, or postpurge shall not use relays external to the flame safeguard to accomplish these functions.

    (4) Two million five hundred thousand to 12 499 999 Btu/h (733 kW to 3663.3 kW) per burner.

    (a) One approved manual shutoff valve lever handle.

    (b) One approved fuel-oil filter, installed on the supply piping.

    (c) Two safety shutoff valves in series, with a combined flame failure response and valve closing time not to exceed 5 seconds with strainer directly before the valves.

  • CMC § 1204.8 High relevance — show source text

    1204.8 Identification of Chemical Additives. In systems where chemical additives are used, documentation including the following information shall be readily accessible and maintained onsite:

    (1) Concentrations

    (2) Maintenance requirements

    (3) Maintenance log

    (4) Material Safety Data Sheet (SDS)

    For SI units: 1000 British thermal units per hour = 0.293 kW

    1205.0 Installation, Testing, and Inspection.

    1205.1 Operating Instructions. Operating and maintenance information shall be provided to the building owner. 1205.2 Pressure Testing. System piping and components shall be tested with a pressure of not less than one and one-half times the operating pressure but not less than 100 psi (689 kPa). Piping shall be tested with water or air except that plastic pipe

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    HYDRONICS

    shall not be tested with air. Test pressures shall be held for a period of not less than 30 minutes with no perceptible drop in pressure. These tests shall be made in the presence of the Authority Having Jurisdiction.

    Exceptions: (1) For PEX, PP-R, PP-RCT, PEX-AL-PEX, PE-RT, and PEAL-PE piping systems, testing with air shall be permitted where authorized by the manufacturer’s instructions for the PEX, PP-R, PP-RCT, PEX-AL-PEX, PE-RT, and PE-ALPE pipe and fittings products, and air testing is not prohibited by applicable codes, laws, or regulations outside this code.

    Note: [HCD 1 & HCD 2] PEX-AL-PEX is not adopted in the California Plumbing Code for use in potable water supply and distribution systems. (2) Copper tubing shall be tested at not less than 80 psi (552 kPa). 1205.3 Flushing. Heating and cooling sources, system piping and tubing shall be flushed after installation with water or a cleaning solution. Cleaning and flushing of the heating and cooling sources shall comply with the manufacturer’s instructions. The cleaning solution shall be compatible with all system components and shall be used in accordance with the manufacturer’s instructions.

    1206.0 Pressure and Safety Devices. 1206.1 General. Each closed hydronic system shall be protected against pressures exceeding design limitations with not less than one pressure relief valve. Each closed section of the system containing a heat source shall have a relief valve located so that the heat source is not capable of being isolated from a relief device. Pressure relief valves shall be installed in accordance with their listing and the manufacturer’s installation instructions.

    1206.2 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 be 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 § 904.5 High relevance — show source text

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    INSTALLATION OF SPECIFIC APPLIANCES

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    904.5 Temperature or Pressure Limiting Devices. Steam and hot water boilers, respectively, shall be provided with approved automatic limiting devices for shutting down the burner(s) to prevent boiler steam pressure or boiler water temperature from exceeding the maximum allowable working pressure or temperature. Safety limit controls shall not be used as operating controls. [NFPA 54:10.3.5]

    904.6 Low-Water Cutoff. All water boilers and steam boilers shall be provided with an automatic means to shut off the fuel supply to the burner(s) if the boiler water level drops below the lowest safe water line. In lieu of the low-water cutoff, water tube or coil-type boilers that require forced circulation to prevent overheating and failure shall have an approved flow sensing device arranged to shut down the boiler when the flow rate is inadequate to protect the boiler against overheating. [NFPA 54:10.3.6]

    904.7 Steam Safety and Pressure Relief Valves. Steam and hot water boilers shall be equipped, respectively, with listed or approved steam safety or pressure relief valves of appropriate discharge capacity and conforming with ASME requirements. A shutoff valve shall not be placed between the relief valve and the boiler or on discharge pipes between such valves and the atmosphere. [NFPA 54:10.3.7]

    904.7.1 Discharge. Relief valves shall be piped to discharge near the floor. [NFPA 54:10.3.7.1]

    904.7.2 Size. The entire discharged piping shall be at least the same size as the relief valve discharge piping.

    [NFPA 54:10.3.7.2]

    904.7.3 End Connections. Discharge piping shall not contain threaded end connection at its termination point.

    [NFPA 54:10.3.7.3]

    904.8 Refrigeration Coils. The installation of refrigeration coils shall comply with the following requirements:

    (1) A refrigeration coil shall not be installed in conjunction with a forced air furnace where circulation of cooled air is provided by the furnace blower, unless the blower has

    sufficient capacity to overcome the external static pressure resistance imposed by the duct system and refrigeration coil at the air flow rate for heating or cooling, whichever is greater.

    (2) Furnaces shall not be located upstream from refrigeration coils, unless the refrigeration coil is designed or equipped so as not to develop excessive temperature or

    pressure.

    (3) Refrigeration coils shall be installed in parallel with or on the downstream side of central furnaces to avoid condensation in the heating element, unless the furnace has been specifically listed for downstream installation. With a parallel flow arrangement, the dampers or other means used to control flow of air shall be sufficiently tight to prevent any circulation of cooled air through the furnace.

    (4) Means shall be provided for disposal of condensate and to prevent dripping of condensate on the heating element.

    [NFPA 54:10.3.9]

    904.9 Cooling Units Used with Heating Boilers. Boilers, where used in conjunction with refrigeration systems, shall be installed so that the chilled medium is piped in parallel with the heating boiler with appropriate valves to prevent the chilled medium from entering the heating boiler.

  • CMC § 304.8 Medium relevance — show source text

    (2) The floor drain shall be sized to handle the flow rate from a single broken tube in any refrigerantcontaining heat exchanger.

    (3) Either of the following:

    (a) The Authority Having Jurisdiction finds it acceptable that the working fluid, corrosion inhibitor, and other additives used in this type of refrigeration system are permitted to infrequently be discharged to the sewer system, or

    (b) A catch tank sized to handle the expected discharge shall be installed and equipped

    For SI units: 1 foot = 304.8 mm, 1 pound-force per square inch =

    6.8947 kPa, 1 pound per minute = 0.00756 kg/s

    with a normally closed drain valve and an overflow line to drain. [ASHRAE 15:9.7.8.4.1]

    1112.12 Discharge Piping. The piping used for pressure-relief device discharge shall be in accordance with Section 1112.12.1 through Section 1112.12.5. [ASHRAE 15:9.7.9]

    1112.12.1 Piping Connection. Piping connected to the discharge side of a fusible plug or rupture member shall have provisions to prevent plugging of the pipe upon operation of a fusible plug or rupture member.

    [ASHRAE 15:9.7.9.1]

    1112.12.2 Pipe Size. The size of the discharge pipe from the pressure-relief device or fusible plug shall be not less than the outlet size of the pressure-relief device or fusible plug. [ASHRAE 15:9.7.9.2]

    1112.12.3 Maximum Length. The maximum length of the discharge piping installed on the outlet of pressure-relief devices and fusible plugs discharging to the atmosphere shall be determined in accordance with Section 1112.12.4 and Section 1112.12.5. {ASHRAE 15:9.7.9.3}

    1112.12.4 Design Back Pressure. The design back pressure due to flow in the discharge piping at the outlet of pressure relief devices and fusible plugs, discharging to atmosphere, shall be limited by the allowable equivalent length of piping determined in accordance with Equation 1112.12.4(1).

    [Equation 1112.12.4(1)]

    L= 0.2146 - d [5] ( P [2] 0 P 22 ) d - ln ( P0P2 )

    f - C [2] r 6 - f

    Where:

    L = Equivalent length of discharge piping, feet.

    Cr = Rated capacity as stamped on the pressure relief device in pounds per minute (lb/min), or in standard cubic feet per minute (SCFM) multiplied by 0.0764, or as calculated in Section 1112.14 for a rupture member or fusible plug, or as adjusted for reduced capacity due to piping in accordance with the manufacturer of the device, or as adjusted for reduced capacity due to piping as estimated by an approved method.

    f = Moody friction factor in fully turbulent flow.

    d = Inside diameter of pipe or tube, inches.

    ln = Natural logarithm.

    P2 = Absolute pressure at outlet of discharge piping, psia.

  • CMC § 9.9.4 Medium relevance — show source text

    [ASHRAE 15:9.9.4]

    240 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

    1112.0 Pressure-Relief Devices.

    1112.1 General. Refrigeration systems shall be protected by a pressure-relief device or other approved means to safely relieve pressure due to fire or abnormal conditions. [ASHRAE 15:9.4.1] 1112.2 Positive Displacement Compressor. A positive displacement compressor with a stop valve in the discharge connection shall be equipped with a pressure-relief device that is sized, and with a pressure setting, in accordance with the compressor manufacturer to prevent rupture of the compressor or to prevent the pressure from increasing to more than 10 percent above the maximum allowable working pressure of any other component located in the discharge line between the compressor and the stop valve or in accordance with Section 1113.5, whichever is larger. The pressure-relief device shall discharge into the low-pressure side of the system or in accordance with Section 1112.11.

    Exception: Hermetic refrigerant motor-compressors that are listed and have a displacement not more than 50 cubic feet per minute (1.42 m [3] /min).

    The relief device(s) shall be sized based on compressor flow at the following conditions:

    (1) For compressors in single-stage systems and high-stage compressors of other systems, the flow shall be calculated based on 50°F (10°C) saturated suction temperature at the compressor suction.

    (2) For low-stage or booster compressors in compound refrigerating systems, the compressors that are capable of running only where discharging to the suction of a high-stage compressor, the flow shall be calculated based on the saturated suction temperature equal to the design operating intermediate temperature.

    (3) For low-stage compressors in cascade systems, the compressors that are located in the lower-temperature stage(s) of cascade systems, the flow shall be calculated based on the suction pressure being equal to the pressure setpoint of the pressure-relieving devices that protect the lowside of the stage against overpressure.

    Exceptions: For Section 1112.2(1), Section 1112.2(2), and Section 1112.2(3), the discharge capacity of the relief device shall be permitted to be the minimum regulated flow rate of the compressor where the following conditions are met:

    (1) The compressor is equipped with capacity regulation.

    (2) Capacity regulation actuates to a flow at not less than 90 percent of the pressure-relief device setting.

    (3) A pressure-limiting device is installed and set in accordance with the requirements of Section 1111.0.

    [ASHRAE 15:9.8] 1112.3 Liquid-Containing Portions of Systems. Liquid-containing portions of systems, including piping, that is isolated from pressure-relief devices required elsewhere, and that develops pressures exceeding their working design pressures due to temperature rise, shall be protected by the installation of pressure-relief devices. 1112.4 Evaporators. Heat exchanger coils located downstream, or upstream within 18 inches (457 mm), of a heating source and capable of being isolated shall be fitted with a

  • CMC § 1112.11.3 Medium relevance — show source text

    1112.11.3 Internal Relief. Pressure-relief valves

    designed to discharge from a higher-pressure vessel into a lower pressure vessel internal to the system shall comply with the following:

    (1) The pressure-relief valve that protects the higherpressure vessel shall be selected to deliver capacity in accordance with Section 1113.5 without exceed ing the maximum allowable working pressure of the higher-pressure vessel accounting for the change in mass flow capacity due to the elevated backpressure.

    (2) The capacity of the pressure-relief valve protecting the part of the system receiving a discharge from a pressure-relief valve protecting a higher-pressure vessel shall be not less than the sum of the capacity required in Section 1113.5 plus the mass flow capacity of the pressure-relief valve discharging into that part of the system.

    (3) The design pressure of the body of the relief valve used on the higher-pressure vessel shall be rated for operation at the design pressure of the higher-pressure vessel in both pressure-containing areas of the valve. [ASHRAE 15:9.7.8.3]

    1112.11.4 Discharge Location, Special Require- ments. Additional requirements for pressure relief device discharge location and allowances shall apply for specific refrigerants in accordance with Section 1112.11.4.1. [ASHRAE 15:9.7.8.4]

    1112.11.4.1 Water (R-718). Where water is the only refrigerant, discharge to a floor drain shall be permitted where all of the following conditions are

    met:

    (1) The pressure-relief device set pressure shall not exceed 15 psig (103 kPag).

    (2) The floor drain shall be sized to handle the flow rate from a single broken tube in any refrigerantcontaining heat exchanger.

    (3) Either of the following:

    (a) The Authority Having Jurisdiction finds it acceptable that the working fluid, corrosion inhibitor, and other additives used in this type of refrigeration system are permitted to infrequently be discharged to the sewer system, or

    (b) A catch tank sized to handle the expected discharge shall be installed and equipped

    For SI units: 1 foot = 304.8 mm, 1 pound-force per square inch =

    6.8947 kPa, 1 pound per minute = 0.00756 kg/s

    with a normally closed drain valve and an overflow line to drain. [ASHRAE 15:9.7.8.4.1]

    1112.12 Discharge Piping. The piping used for pressure-relief device discharge shall be in accordance with Section 1112.12.1 through Section 1112.12.5. [ASHRAE 15:9.7.9]

    1112.12.1 Piping Connection. Piping connected to the discharge side of a fusible plug or rupture member shall have provisions to prevent plugging of the pipe upon operation of a fusible plug or rupture member.

    [ASHRAE 15:9.7.9.1]

    1112.12.2 Pipe Size. The size of the discharge pipe from the pressure-relief device or fusible plug shall be not less than the outlet size of the pressure-relief device or fusible plug. [ASHRAE 15:9.7.9.2]

  • CMC § 904.2.1 Medium relevance — show source text

    obtained from the outdoors. The closet shall be for the exclusive use of the central heating furnace or low-pressure boiler.

    (2) Central heating furnaces and low-pressure boilers shall be of the direct vent type.

    904.2.1 Self-Closing Doors. Self-closing doors shall swing easily and freely, and shall be equipped with a self-closing device to cause the door to close and latch each time it is opened. The closing mechanism shall not have a hold-open feature.

    904.2.2 Gasketing. Gasketing on gasketed doors or frames shall be furnished in accordance with the published listings of the door, frame, or gasketing material manufacturer.

    Exception: Where acceptable to the Authority Having Jurisdiction, gasketing of noncombustible or limitedcombustible material shall be permitted to be applied to the frame, provided closing and latching of the door are not inhibited.

    904.3 Clearance. Central heating furnaces and low-pressure boilers shall be provided with clearances in accordance with Section 904.3.1 through Section 904.3.7.

    904.3.1 Listed Units. Listed central heating furnaces and low-pressure boilers shall be installed with clearances in accordance with the terms of their listings and the manufacturer’s installation instructions. {NFPA 54:10.3.3.1}

    904.3.2 Unlisted Units. Unlisted central heating furnaces and low-pressure boilers shall be installed with clearances from combustible material not less than those specified in Table 904.3.2. [NFPA 54:10.3.3.2]

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    904.5 Temperature or Pressure Limiting Devices. Steam and hot water boilers, respectively, shall be provided with approved automatic limiting devices for shutting down the burner(s) to prevent boiler steam pressure or boiler water temperature from exceeding the maximum allowable working pressure or temperature. Safety limit controls shall not be used as operating controls. [NFPA 54:10.3.5]

    904.6 Low-Water Cutoff. All water boilers and steam boilers shall be provided with an automatic means to shut off the fuel supply to the burner(s) if the boiler water level drops below the lowest safe water line. In lieu of the low-water cutoff, water tube or coil-type boilers that require forced circulation to prevent overheating and failure shall have an approved flow sensing device arranged to shut down the boiler when the flow rate is inadequate to protect the boiler against overheating. [NFPA 54:10.3.6]

    904.7 Steam Safety and Pressure Relief Valves. Steam and hot water boilers shall be equipped, respectively, with listed or approved steam safety or pressure relief valves of appropriate discharge capacity and conforming with ASME requirements. A shutoff valve shall not be placed between the relief valve and the boiler or on discharge pipes between such valves and the atmosphere. [NFPA 54:10.3.7]

    904.7.1 Discharge. Relief valves shall be piped to discharge near the floor. [NFPA 54:10.3.7.1]

Frequently asked questions

Can I put a shutoff valve on a T&P discharge line?

No. The CMC forbids any valve, obstruction, or other means of isolation on discharge piping for temperature/pressure-relief valves — the line must remain open and full‑size (§ 1005.2).

How high should the discharge end be above grade?

The discharge end must be no less than 6 inches (152 mm) and no more than 2 feet (610 mm) above the ground or receiving surface, and must point downward (§ 1005.2(3)).

Can the discharge terminate into a pan under the water heater?

No. Discharge into a water‑heater pan is expressly prohibited (§ 1005.2(7)).

What determines the rated discharge capacity for a pressure‑relief device?

The rated discharge capacity for pressure‑relief devices is determined in accordance with ASME BPVC Section VIII.1; the CMC requires this method for sizing and requires piping area to be at least the area of the valve inlet (§ 1112.13).

If the relief discharge is steam or otherwise hazardous, do I still terminate outside?

Yes — if the discharge is capable of being hazardous, it must be discharged to a safe, approved location (for example, outside the boiler room) so it does not create harm or structural damage (§ 1005.4).

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California Mechanical Code