CMC · California Mechanical Code

How are expansion tanks sized and what are installation requirements for hot-water systems?

You must install an expansion tank on hot‑water heating systems and size it to accept the system’s thermal expansion using **Table 1004.4(2)** or the equations in **§ 1004.4**; install it per **§ 1004.1** so it’s securely supported (supports = 2× filled weight), accessible, drainable without emptying the system, and built to the strength/ASME rules in **§ 1004.3** when pressures exceed 30 psi.

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Last reviewed: July 6, 2026

What the code requires — 2-4 sentences

An expansion tank must be installed on hot‑water heating systems to control the pressure increase from thermal expansion; the general installation and support requirements are in § 1004.1. Minimum capacities for closed/forced systems must be taken from Table 1004.4(2) (or computed with the equations in § 1004.4) for forced and diaphragm tanks. The CMC also treats open tanks and closed‑type construction / hydrostatic test requirements separately in § 1004.2 and § 1004.3.

The single most important rule: provide an expansion tank sized to accept the full thermal expansion of the system and install it per the CMC sizing table or equation — see § 1004.4 and Table 1004.4(2).

Requirements in detail

Basic required function and tank type

  • The expansion tank is required to control increased pressure caused by thermal expansion in a hot‑water heating system — § 1004.1.
  • Tanks may be open or closed; closed tanks are required for closed hydronic systems (Chapter 10/12 references). Closed tanks must be airtight or use an approved air cushion consistent with system volume — § 1004.3.

Mounting, supports, accessibility, and drainage

  • Tanks must be securely fastened to or supported by the structure and supports must carry twice the weight of the tank when filled without placing strain on connecting piping — § 1004.1.
  • Expansion tanks must be accessible for maintenance and installed in accordance with the manufacturer’s instructions; closed tanks must allow draining without emptying the system — § 1004.2 / § 1209.2.

Closed‑type tank strength and ASME compliance

  • Closed‑type expansion tanks shall be designed for a hydrostatic test pressure of two‑and‑one‑half times the allowable working pressure of the system — § 1004.3.
  • Tanks for systems with maximum operating pressure greater than 30 psi (207 kPa) must comply with ASME BPVC Section VIII.1 — § 1004.3.

Open‑type tank placement and overflow

  • Open‑type expansion tanks must be located not less than 3 feet (914 mm) above the highest point of the system and must have an overflow sized not less than 1/2 the supply or 1 inch (25 mm) — § 1004.2.

Sizing rules (decision‑making)

  • For forced‑type and diaphragm‑type closed systems you must use Table 1004.4(2) or the equations in § 1004.4 to determine the minimum tank capacity. The table gives minimum tank gallons by system volume for diaphragm and non‑diaphragm tanks.

Decision‑relevant values (excerpt of Table 1004.4(2))

SYSTEM VOLUME (gallons) TANK CAPACITY — DIAPHRAGM TYPE (gallons) TANK CAPACITY — NON‑DIAPHRAGM TYPE (gallons) Code Reference
100 9 15 Table 1004.4(2) — § 1004.4
200 17 30 Table 1004.4(2) — § 1004.4
300 25 45 Table 1004.4(2) — § 1004.4
400 33 60 Table 1004.4(2) — § 1004.4
500 42 75 Table 1004.4(2) — § 1004.4
1000 83 150 Table 1004.4(2) — § 1004.4
2000 165 300 Table 1004.4(2) — § 1004.4

Notes accompanying table: values in Table 1004.4(2) are based on an average operating water temperature of 195°F (91°C), a fill pressure of 12 psig (83 kPa), and an operating pressure not to exceed 30 psig (207 kPa); system volume includes boiler, radiation and piping but not expansion tank — Table 1004.4(2) / § 1004.4.

When to use the equation

  • The Code provides alternative equations in § 1004.4 (Equation 1004.4(1) for forced‑type systems; Equation 1004.4(2) for diaphragm tanks) if you prefer to compute the minimum volume from system volume, operating temperature, fill pressure and operating pressure. Use the Table for quick sizing when system volume matches table entries.

Exceptions & special cases

  • If the system operating pressure is more than 30 psi, the expansion tank must meet ASME BPVC Section VIII.1 (this is a material / construction exception to ordinary tank rules) — § 1004.3.
  • Open‑type tanks are sized by capacity of the system and located at least 3 ft above the system’s highest point — they are not sized by Table 1004.4(2) (which is for closed/forced systems) — § 1004.2.
  • The Table 1004.4(2) notes its sizing assumptions (temp, fill pressure, max op pressure). If your project uses different fill or operating pressures or temperatures, compute with the equations in § 1004.4 rather than the table.

Common mistakes

  • Relying on a manufacturer’s “tank blanketing” or nominal label without confirming the tank meets the minimum capacity from Table 1004.4(2) or the equations — the Code requires the minimum capacity be met (or exceeded).
  • Using a non‑ASME tank for a system with operating pressure > 30 psi. If >30 psi, ASME Section VIII.1 compliance is required — § 1004.3.
  • Mounting the tank on weak supports or piping that will carry the tank weight; supports must be sized to carry twice the filled weight — § 1004.1.
  • Installing a closed tank but putting a shutoff upstream so it can be isolated in normal operation; in hydronics the tank must be installed so it remains in service (see manufacturer's instructions and § 1209.2 for hydronic closed tanks).

Worked example — concrete scenario

Scenario: A forced hot‑water system has a calculated system water volume (boiler + piping + radiation) of 300 gallons. The system will operate near the table assumptions (avg. temperature ~195°F, fill pressure 12 psig, operating pressure ≤ 30 psig).

  • Using Table 1004.4(2), minimum tank capacities are: diaphragm tank = 25 gallons, non‑diaphragm = 45 gallons for a 300 gallon system. Choose the tank type based on application (diaphragm precharged tanks are common for potable/closed systems). § 1004.4 / Table 1004.4(2).

Installation steps you must follow under the CMC:

  1. Select a tank meeting or exceeding the 25‑gal diaphragm capacity (or 45‑gal non‑diaphragm) — § 1004.4.
  2. Confirm the tank is rated for the system pressure and — if system max operating pressure > 30 psi — that the tank complies with ASME BPVC Section VIII.1 — § 1004.3.
  3. Fasten or support the tank so supports can carry 2× the weight of the full tank (no strain on connecting piping) — § 1004.1.
  4. Install per the manufacturer’s instructions and provide accessibility and means to drain the tank without emptying the system — § 1209.2 / § 1004.2.

Related provisions

  • § 1004.1 — General requirement for expansion tanks (installation, supports, freezing protection).
  • § 1004.2 — Requirements for open‑type expansion tanks (height above system, overflow sizing).
  • § 1004.3 — Closed‑type tank design strength and ASME requirement for >30 psi.
  • § 1004.4 — Minimum capacity rules, equations and Tables (including Table 1004.4(2)).
  • § 1209.2 — Hydronics installation details: accessibility, shutoff device behavior, draining without emptying the system.
  • Chapter 10 / § 1005.0 — Relief valve and discharge piping requirements that interact with pressure control and overpressure protection.

Code references

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

  • CMC § 1209.2 High relevance — show source text

    Expansion tanks shall be of the closed type. Expansion tanks shall be rated for the pressure of the system. 1209.2 Installation. Expansion tanks shall be accessible for maintenance and shall be installed in accordance with the manufacturer’s installation instructions. Each expansion tank shall be equipped with a shutoff device that will remain open during operation of the hydronic system. Valve handles shall be locked open or removed to prevent from being inadvertently shut off. Provisions shall be made for draining the tank without emptying the system. Expansion tanks shall be securely fastened to or supported by the structure. Supports shall be capable of carrying twice the weight of the tank filled with water without placing a strain on connecting piping. Hot-water-heating systems incorporating hot water tanks or fluid relief columns shall be installed to prevent freezing under normal operating conditions. 1209.3 Closed-Type Tanks. Closed-type expansion tanks shall be designed for a hydrostatic test pressure of two and one-half times the allowable working pressure of the system. Expansion tanks for systems designed to operate at more than 30 pounds-force per square inch (psi) (207 kPa) shall comply with ASME BPVC Section VIII.1.

    1209.4 Sizing. Expansion tanks shall be sized to accept the full expansion volume of the fluid in the system. The minimum capacity of a closed-type expansion tank shall be sized in accordance with Section 1004.4.

    1210.0 Materials.

    1210.1 Piping, Tubing, and Fittings. Hydronic pipe and tubing shall comply with the applicable standards referenced in Table 1210.1 and shall be approved for use based on the intended purpose. Materials shall be rated for the operating

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    HYDRONICS

    TABLE 1210.1

    MATERIALS FOR HYDRONIC SYSTEM PIPING, TUBING, AND FITTINGS

    MATERIAL STANDARDS Col3
    MATERIAL PIPING/TUBING FITTINGS
    Copper/Copper Alloy ASTM B42, ASTM B43, ASTM B75,
    ASTM B88, ASTM B135, ASTM B2512,
    ASTM B302, ASTM B447    		 ASME B16.15, ASME B16.18, ASME B16.22,
    ASME B16.23, ASME B16.24, ASME B16.26,
    ASME B16.29, ASME B16.51, ASSE 1061,
    ASTM F3226, IAPMO PS 117
    Ductile Iron AWWA C115/A21.15,
    AWWA C151/A21.51    		 AWWA C110/A21.101,
    AWWA C153/A21.53
    Steel ASTM A53, ASTM A106, ASTM A254 ASME B16.5, ASME B16.9,
    ASME B16.11, ASTM A420, ASTM F3226,
    IAPMO IGC 353, IAPMO PS 117
    Stainless Steel ASTM A269, ASTM A312,
    ASTM A554, ASTM A778    		 ASTM F1476, ASTM F1548, ASTM F3226,
    IAPMO IGC 353,
  • CMC § 1003.2.1. High relevance — show source text

    be equipped with controls and limit devices in accordance with ASME CSD-1 or Table 1003.2.1.

    The Authority Having Jurisdiction shall have the authority to approve solid-fuel-fired boilers that comply with the safety requirements for automatic gas fired boilers or oil fired boilers.

    1003.3 Gauges. Steam boilers shall be provided with a pressure gauge and a water level glass. Water boilers shall be provided with a pressure gauge and a temperature gauge. Automatic boilers shall be equipped with the following gauges, as applicable :

    (1) Oil temperature

    (2) Oil suction pressure

    (3) High and low gas pressure

    (4) Stack temperature

    (5) Windbox pressure

    1003.4 Stack Dampers. Stack dampers on boilers fired with oil or solid fuel shall not close off more than 80 percent of the stack area. Operative dampers shall not be placed within a stack, flue, or vent of a gas-fired boiler.

    Exception: Automatic boilers with prepurge, automatic draft control, and interlock.

    1003.5 Welding. Welding on pressure vessels shall be done by certified welders in accordance with nationally recognized standards.

    1004.0 Expansion Tanks.

    1004.1 General. An expansion tank shall be installed in a hot-water-heating system as a means for controlling increased pressure caused by thermal expansion. Expansion tanks shall be of the closed or open type and securely fastened to or supported by the structure. Tanks shall be rated for the pressure of the system. Supports shall be capable of carrying twice the weight of the tank filled with water without placing a strain on connecting piping.

    Hot-water-heating systems incorporating hot water tanks or fluid relief columns shall be installed to prevent freezing under normal operating conditions.

    1004.2 Open-Type Expansion Tanks. Open type expansion tanks shall be located not less than 3 feet (914 mm) above the highest point of the system. Such tanks shall be sized based on the capacity of the system. An overflow with a diameter of not less than one-half the size of the supply or not less than 1 inch (25 mm) in diameter shall be installed at the top of the tank. The overflow shall discharge through an air gap into the drainage system.

    1004.3 Closed-Type Systems. Closed-type systems shall have an airtight tank or other approved air cushion that will be consistent with the volume and capacity of the system, and shall be designed for a hydrostatic test pressure of two and one-half times the allowable working pressure of the system. Expansion tanks for systems designed to operate at more than 30 pounds-force per square inch (psi) (207 kPa) shall comply with ASME BPVC Section VIII.1. Provisions shall be made

    for draining the tank without emptying the system.

    1004.4 Minimum Capacity of Closed-Type Tank. The minimum capacity for a gravity-type hot water system expansion tank shall be in accordance with Table 1004.4(1). The minimum capacity for a forced-type hot water system expansion tank shall be in accordance with Table 1004.4(2) or Equation 1004.4(1). The minimum capacity for a diaphragmtype hot water system expansion tank shall be in accordance with Table 1004.4(2) or Equation 1004.4(2).

    [Equation 1004.4(1)]

    Vt (forced-type) = [(0.00041] [t] [ - 0.0466) ] [V][s]

    (
  • 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 § 1211.2.2 High relevance — show source text

    1211.2.2 Polyethylene (PE) Plastic Pipe/Tubing. Polyethylene pipe and tubing bends shall have a bend radius in accordance with Table 1211.2.2. When a fitting or flange connection is present in the pipe bend, the minimum bend radius shall be one hundred times the pipe outside diameter (OD) for a distance of five times the pipe diameter on either side of the fitting location.

    »

    »

    1208.2 Mounting. The circulator or pump shall be installed in such a way that strain from the piping is not transferred to the circulator or pump housing. The circulator or pump shall be permitted to be directly connected to the piping, provided the piping is supported on each side of the circulator or pump. Where the installation of a circulator or pump will cause strain on the piping, the circulator or pump shall be installed on a mounting bracket or base plate or securely fastened to or supported by the structure with approved fastening devices. Where means for controlling vibration of a circulator or pump is required, an approved means for support and restraint shall be provided. 1208.3 Sizing. The selection and sizing of a circulator or pump shall be based on all of the following: (1) Loop or system head pressure, feet of head (m) (2) Capacity, gallons per minute (L/s) (3) Maximum and minimum temperature, °F (°C) (4) Maximum working pressure, pounds per square inch (kPa) (5) Fluid type

    1209.0 Expansion Tanks. 1209.1 General. An expansion tank shall be installed in each closed hydronic system to control system pressure due to thermal expansion and contraction. Expansion tanks shall be of the closed type. Expansion tanks shall be rated for the pressure of the system. 1209.2 Installation. Expansion tanks shall be accessible for maintenance and shall be installed in accordance with the manufacturer’s installation instructions. Each expansion tank shall be equipped with a shutoff device that will remain open during operation of the hydronic system. Valve handles shall be locked open or removed to prevent from being inadvertently shut off. Provisions shall be made for draining the tank without emptying the system. Expansion tanks shall be securely fastened to or supported by the structure. Supports shall be capable of carrying twice the weight of the tank filled with water without placing a strain on connecting piping. Hot-water-heating systems incorporating hot water tanks or fluid relief columns shall be installed to prevent freezing under normal operating conditions. 1209.3 Closed-Type Tanks. Closed-type expansion tanks shall be designed for a hydrostatic test pressure of two and one-half times the allowable working pressure of the system. Expansion tanks for systems designed to operate at more than 30 pounds-force per square inch (psi) (207 kPa) shall comply with ASME BPVC Section VIII.1.

    1209.4 Sizing. Expansion tanks shall be sized to accept the full expansion volume of the fluid in the system. The minimum capacity of a closed-type expansion tank shall be sized in accordance with Section 1004.4.

    1210.0 Materials.

    1210.1 Piping, Tubing, and Fittings. Hydronic pipe and tubing shall comply with the applicable standards referenced in Table 1210.1 and shall be approved for use based on the intended purpose. Materials shall be rated for the operating

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  • CMC § 610.4. High relevance — show source text

    ** Where static water pressure in the water supply piping exceeds 80 psi (552 kPa), an approved-type pressure regulator preceded by an adequate strainer shall be installed and the static pressure reduced to 80 psi (552 kPa) or less. Pressure regulators for potable water distribution systems shall comply with ASSE 1003 or AWWA C530. Pressure regulator(s) equal to or exceeding 1 [1] ⁄ 2 inches (40 mm) shall not require a strainer. Such regulator(s) shall control the pressure to water outlets in the building unless otherwise approved by the Authority Having Jurisdiction. Each such regulator and strainer shall be accessibly located aboveground or in a vault equipped with a properly sized and sloped boresighted drain to daylight, shall be protected from freezing, and shall have the strainer readily accessible for cleaning without removing the regulator or strainer body or disconnecting the supply piping. Pipe size determinations shall be based on 80 percent of the reduced pressure where using Table 610.4.

    An approved expansion tank shall be installed in the cold water distribution piping downstream of each such regulator to prevent pressure exceeding 80 psi from developing due to thermal expansion. Expansion tanks used in potable water systems intended to supply drinking water shall comply with NSF/ANSI/CAN 61. 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.

  • CMC § 1004.2 High relevance — show source text

    1004.2 Open-Type Expansion Tanks. Open type expansion tanks shall be located not less than 3 feet (914 mm) above the highest point of the system. Such tanks shall be sized based on the capacity of the system. An overflow with a diameter of not less than one-half the size of the supply or not less than 1 inch (25 mm) in diameter shall be installed at the top of the tank. The overflow shall discharge through an air gap into the drainage system.

    1004.3 Closed-Type Systems. Closed-type systems shall have an airtight tank or other approved air cushion that will be consistent with the volume and capacity of the system, and shall be designed for a hydrostatic test pressure of two and one-half times the allowable working pressure of the system. Expansion tanks for systems designed to operate at more than 30 pounds-force per square inch (psi) (207 kPa) shall comply with ASME BPVC Section VIII.1. Provisions shall be made

    for draining the tank without emptying the system.

    1004.4 Minimum Capacity of Closed-Type Tank. The minimum capacity for a gravity-type hot water system expansion tank shall be in accordance with Table 1004.4(1). The minimum capacity for a forced-type hot water system expansion tank shall be in accordance with Table 1004.4(2) or Equation 1004.4(1). The minimum capacity for a diaphragmtype hot water system expansion tank shall be in accordance with Table 1004.4(2) or Equation 1004.4(2).

    [Equation 1004.4(1)]

    Vt (forced-type) = [(0.00041] [t] [ - 0.0466) ] [V][s]

    (

    P P a a

    Pf Po )

    [Equation 1004.4(2)]

    Vt (diaphragm-type) = [(0.00041] [t] [ - 0.0466) ] [V][s]

    – [P][f]

    [1]

    ( Po )

    Where:

    Vt = Minimum volume of expansion tank, gallons. Vs = Volume of system, not including expansion tank, gallons.

    t = Average operating temperature, °F.

    Pa = Atmospheric pressure, pounds per square inch. Pf = Fill pressure, pounds per square inch. Po = Maximum operating pressure, pounds per square inch.

    For SI units: 1 gallon = 3.785 L

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    BOILERS AND PRESSURE VESSELS

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

  • CMC § 1002.2.2 High relevance — show source text

    1002.2.2 Installation. Tanks, piping, and valves for oil-burning boilers shall be installed in accordance with NFPA 31.

    1002.3 Electric Boilers. Electric boilers shall be listed and

    labeled in accordance with UL 834.

    1002.4 Solid-Fuel-Fired Boilers. Solid-fuel-fired boilers shall comply with UL 2523 and shall be installed in accordance with the manufacturer’s installation instruc tions.

    1002.5 Dual Purpose Water Heaters. Water heaters utilized for combined space- and water-heating applications shall comply with the standards referenced in Table 1203.2, and shall be installed in accordance with the manufacturer’s

    installation instructions.

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    BOILERS AND PRESSURE VESSELS

    1003.0 Detailed Requirements.

    1003.1 Safety Requirements. The construction of boilers and pressure vessels and the installation thereof shall be in accordance with minimum requirements for safety from structural and mechanical failure and excessive pressures as established by the Authority Having Jurisdiction in accordance with nationally recognized standards.

    1003.2 Controls. Required electrical, mechanical, safety, and operating controls shall carry the approval of an approved testing agency or be accepted by the Authority Having Jurisdiction. Electrical controls shall be of such design and construction as to be suitable for installation in the environment

    in which they are located.

    1003.2.1 Automatic Boilers. Automatic boilers shall

    be equipped with controls and limit devices in accordance with ASME CSD-1 or Table 1003.2.1.

    The Authority Having Jurisdiction shall have the authority to approve solid-fuel-fired boilers that comply with the safety requirements for automatic gas fired boilers or oil fired boilers.

    1003.3 Gauges. Steam boilers shall be provided with a pressure gauge and a water level glass. Water boilers shall be provided with a pressure gauge and a temperature gauge. Automatic boilers shall be equipped with the following gauges, as applicable :

    (1) Oil temperature

    (2) Oil suction pressure

    (3) High and low gas pressure

    (4) Stack temperature

    (5) Windbox pressure

    1003.4 Stack Dampers. Stack dampers on boilers fired with oil or solid fuel shall not close off more than 80 percent of the stack area. Operative dampers shall not be placed within a stack, flue, or vent of a gas-fired boiler.

    Exception: Automatic boilers with prepurge, automatic draft control, and interlock.

    1003.5 Welding. Welding on pressure vessels shall be done by certified welders in accordance with nationally recognized standards.

    1004.0 Expansion Tanks.

    1004.1 General. An expansion tank shall be installed in a hot-water-heating system as a means for controlling increased pressure caused by thermal expansion. Expansion tanks shall be of the closed or open type and securely fastened to or supported by the structure. Tanks shall be rated for the pressure of the system. Supports shall be capable of carrying twice the weight of the tank filled with water without placing a strain on connecting piping.

    Hot-water-heating systems incorporating hot water tanks or fluid relief columns shall be installed to prevent freezing under normal operating conditions.

  • CMC § 607.5 High relevance — show source text

    installation instructions. Such vent shall be screened with a cor rosion-resistant material of not less than number 24 mesh.

    607.5 Overflow. Tanks shall have not less than a 16 square inch (0.01 m [2] ) overflow that is screened with a corrosionresistant material of not less than number 24 mesh.

    607.6 Valves. Pressurized tanks shall be provided with a listed pressure-relief valve installed in accordance with the manufacturer’s installation instructions. The relief valve shall be discharged in accordance with Section 608.5. Where a potable water supply tank is located above the fixtures, appliances, or system components it serves, it shall be equipped with a vacuum relief valve that complies with ANSI Z21.22/CSA 4.4.

    608.0 Water Pressure, Pressure Regulators, Pres- sure Relief Valves, and Vacuum Relief Valves. 608.1 Inadequate Water Pressure. Where the water pressure in the main or other source of supply will not provide a residual water pressure of not less than 15 pounds force per square inch (psi) (103 kPa), after allowing for friction and other pressure losses, a tank and a pump or other means that will provide said 15 psi (103 kPa) pressure shall be installed. Where fixtures, fixture fittings, or both are installed that, require a residual pressure exceeding 15 psi (103 kPa), that minimum residual pressure shall be provided.

    608.2 Excessive Water Pressure. Where static water pressure in the water supply piping exceeds 80 psi (552 kPa), an approved-type pressure regulator preceded by an adequate strainer shall be installed and the static pressure reduced to 80 psi (552 kPa) or less. Pressure regulators for potable water distribution systems shall comply with ASSE 1003 or AWWA C530. Pressure regulator(s) equal to or exceeding 1 [1] ⁄ 2 inches (40 mm) shall not require a strainer. Such regulator(s) shall control the pressure to water outlets in the building unless otherwise approved by the Authority Having Jurisdiction. Each such regulator and strainer shall be accessibly located aboveground or in a vault equipped with a properly sized and sloped boresighted drain to daylight, shall be protected from freezing, and shall have the strainer readily accessible for cleaning without removing the regulator or strainer body or disconnecting the supply piping. Pipe size determinations shall be based on 80 percent of the reduced pressure where using Table 610.4.

    An approved expansion tank shall be installed in the cold water distribution piping downstream of each such regulator to prevent pressure exceeding 80 psi from developing due to thermal expansion. Expansion tanks used in potable water systems intended to supply drinking water shall comply with NSF/ANSI/CAN 61. 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.

  • California Mechanical Code High relevance — show source text

    |100 psf|1 hr
    23 min|||7|1, 2|11/3| |F/C-4-RC-9|4″|4″ deep (4370 psi);1/4″ reinforcement bars
    at 6″ pitch with3/4″ cover;1/4″ main rein-
    forcement bars at 4″ pitch perpendicular
    with1/2″ cover; 13′1″ span restrained.|150 psf|2 hrs|||7|1, 3|2| |F/C-4-RC-10|4″|4″ thick (5140 psi) deck;1/4″ reinforce-
    ment bars at 71/2″ pitch with7/8″ cover;3/8″
    main reinforcement bars at 33/4″ pitch
    perpendicular with1/2″ cover; 13′1″ span
    restrained.|140 psf|1 hr
    16 min|||7|1, 5|11/4| |F/C-4-RC-11|4″|4″ thick (4000 psi) concrete deck;
    3″ × 11/2″ × 4 lbs R.S.J.; 2′6″ C.R.S.; flush
    with top surface; 4″ × 6″ x 13 SWG mesh
    reinforcement 1″ from bottom of slab; 6′6″
    span restrained.|150 psf|2 hrs|||7|1, 3|2| |F/C-4-RC-12|4″|4″ deep (2380 psi) concrete deck;
    3″ × 11/2″ × 4 lbs R.S.J.; 2′6″ C.R.S.; flush
    with top surface; 4″ × 6″ x 13 SWG mesh
    reinforcement 1″ from bottom surface;
    6′6″ span restrained.|150 psf|1 hr
    3 min|||7|1, 2|1| |F/C-4-RC-13|41/2″|41/2″ thick (5200 psi) deck;1/4″ reinforce-
    ment bars at 71/4″ pitch with7/8″ cover;3/8″
    main reinforcement bars at 33/4″ pitch
    perpendicular with1/2″ cover; 13′1″ span
    restrained.|140 psf|2 hrs|||7|1, 3|2| |F/C-4-RC-14|41/2″|41/2″ deep (2525 psi) concrete deck;1/4″
    reinforcement bars at 71/2″ pitch with7/8″
    cover;3/8″ main reinforcement bars at
    33/8″ pitch perpendicular with1/2″ cover;
    13′1″ span restrained.|150 psf|42 min|||7|1, 5|2/3| |F/C-4-RC-15|41/2″|41/2″ deep (4830 psi) concrete deck;
    11/2″ × No.

  • CMC § 1004.4 High relevance — show source text

    [Equation 1004.4(1)]

    Vt (forced-type) = [(0.00041] [t] [ - 0.0466) ] [V][s]

    (

    P P a a

    Pf Po )

    [Equation 1004.4(2)]

    Vt (diaphragm-type) = [(0.00041] [t] [ - 0.0466) ] [V][s]

    – [P][f]

    [1]

    ( Po )

    Where:

    Vt = Minimum volume of expansion tank, gallons. Vs = Volume of system, not including expansion tank, gallons.

    t = Average operating temperature, °F.

    Pa = Atmospheric pressure, pounds per square inch. Pf = Fill pressure, pounds per square inch. Po = Maximum operating pressure, pounds per square inch.

    For SI units: 1 gallon = 3.785 L

    218 2025 CALIFORNIA MECHANICAL CODE

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

    BOILERS AND PRESSURE VESSELS

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

    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]

  • CMC § 1002.3 High relevance — show source text

    1002.3 Electric Boilers . . . . . . . . . . . . . . . 217

    1002.4 Solid-Fuel-Fired Boilers . . . . . . . . 217

    1002.5 Dual Purpose Water Heaters . . . . 217

    1003.0 Detailed Requirements . . . . . . . . . 218

    1003.1 Safety Requirements . . . . . . . . . . 218

    1003.2 Controls . . . . . . . . . . . . . . . . . . . . 218

    1003.3 Gauges . . . . . . . . . . . . . . . . . . . . . 218

    1003.4 Stack Dampers . . . . . . . . . . . . . . . 218

    1003.5 Welding. . . . . . . . . . . . . . . . . . . . . 218

    1004.0 Expansion Tanks . . . . . . . . . . . . . 218

    1004.1 General. . . . . . . . . . . . . . . . . . . . . 218

    1004.2 Open-Type Expansion Tanks . . . . 218

    1004.3 Closed-Type Systems . . . . . . . . . 218

    1004.4 Minimum Capacity of ClosedType Tank. . . . . . . . . . . . . . . . . . . 218

    Table 1004.4(1) Expansion Tank Capacities for Gravity Hot Water Systems . . . . . 219

    Table 1004.4(2) Expansion Tank Capacities for Forced Hot Water Systems . . . . . 219

    1005.0 Safety or Relief Valve Discharge . . 219

    1005.1 General. . . . . . . . . . . . . . . . . . . . . 219

    1005.2 Discharge Piping . . . . . . . . . . . . . 219

    1005.3 Splash Shield . . . . . . . . . . . . . . . . 219

    1005.4 Hazardous Discharge . . . . . . . . . . 219

    1005.5 Vacuum Relief Valve . . . . . . . . . . 219

    1006.0 Shutoff Valves . . . . . . . . . . . . . . . 219

    1006.1 General. . . . . . . . . . . . . . . . . . . . . 219

    1007.0 Gas-Pressure Regulators . . . . . . . 219

    1007.1 General. . . . . . . . . . . . . . . . . . . . . 219

    1008.0 Low-Water Cutoff . . . . . . . . . . . . . 220

    1008.1 General. . . . . . . . . . . . . . . . . . . . . 220

    1009.0 Combustion Regulators Safety Valves . . . . . . . . . . . . . . . . 220

    1009.1 General. . . . . . . . . . . . . . . . . . . . . 220

    1010.0 Clearance for Access . . . . . . . . . . 220

    1010.1 General. . . . . . . . . . . . . . . . . . . . . 220

  • CMC § 0.6 High relevance — show source text

    (a) be balanced to maintain adequate system temperatures; and (b) have flow velocities that do not exceed 0.6 m/s (2 ft/s) (see Table 2); and (c) use only PEX tubing designated for hot, chlorinated water recirculation systems and rated for the maximum percentage of time during which the system is intended to be operated at elevated temperatures, in accordance with ASTM F876. 10.7 Installation Testing

    Installation of PEX water distribution systems may be tested with air when

    (a) expressly allowed in the written instructions of the manufacturers of all plastic pipe and fittings installed at the time the PEX piping system is being tested; and (b) compressed air or other gas testing is not prohibited by the authority having jurisdiction.

    Note: LB = C × √ (D × Δ L)

    where

    LB = length of flexible arm

    C = material constant (12 for PEX)

    D = nominal outside diameter of tubing

    Δ L = thermal expansion length

    FIGURE 1

    EXPANSION ARMS

    (See Sections 6.4.1 and 7.3)

    2025 CALIFORNIA PLUMBING CODE 459

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

    IS 31

    TABLE 1

    CALCULATION OF FLUSH TANK AND FLUSH VALVE FIXTURE UNITS

    (See Section 10.6.3)

    NOMINAL TUBING
    SIZE    		 FLOW VELOCITY: 3.0 m/s (10 ft/s) Col3 Col4 FLOW VELOCITY: 2.4 m/s (8 ft/s) Col6 Col7
    NOMINAL TUBING
    SIZE    		 FLOW VOLUME,
    L/min (gpm)    		 FLUSH TANK
    FIXTURE UNITS    		 FLUSH VALVE
    FIXTURE UNITS    		 FLOW,
    L/min (gpm)    		 FLUSH TANK
    FIXTURE UNITS    		 FLUSH VALVE
    FIXTURE UNITS
    1⁄2 20.8 (5.5) 6 — 16.7 (4.4) 4 —
    3⁄4 41.6 (11.0) 15 — 33.3 (8.8) 11 —
    1 68.9 (18.2) 26 — 55.3 (14.6) 20 —
    11⁄4 103.0 (27.2) 46 10 82.5 (21.8) 33 5
    11⁄2 143.5 (37.9) 77 24 114.7 (30.3) 54 13
    2 246.1 (65.0) 200 91 196.8 (52.0) 135 52
    3 533.0 (140.8) 590 495 426.2 (112.6) 443 310

    TABLE 2

    TUBING SIZES, FLOWS, AND FRICTION LOSSES FOR HOT-WATER RECIRCULATION SYSTEMS

  • 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 § 938.1 Medium relevance — show source text

    938.1 General. . . . . . . . . . . . . . . . . . . . . 214

    939.0 Sauna Heaters . . . . . . . . . . . . . . . 214

    939.1 Electric Sauna Heaters. . . . . . . . . 214

    CHAPTER 10 BOILERS AND

    PRESSURE VESSELS . . . . . . . . 215

    1001.0 General. . . . . . . . . . . . . . . . . . . . . 217

    1001.1 Applicability. . . . . . . . . . . . . . . . . . 217

    1001.2 Boiler Rooms and Enclosures . . . 217

    1001.3 Air for Combustion

    and Ventilation . . . . . . . . . . . . . . . 217

    1001.4 Drainage . . . . . . . . . . . . . . . . . . . . 217

    1001.5 Mounting. . . . . . . . . . . . . . . . . . . . 217

    xxxvii

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

    TABLE OF CONTENTS

    1001.6 Chimneys or Vents . . . . . . . . . . . . 217

    1002.0 Standards . . . . . . . . . . . . . . . . . . . 217

    1002.1 General. . . . . . . . . . . . . . . . . . . . . 217

    1002.2 Oil-Burning Boilers . . . . . . . . . . . . 217

    1002.3 Electric Boilers . . . . . . . . . . . . . . . 217

    1002.4 Solid-Fuel-Fired Boilers . . . . . . . . 217

    1002.5 Dual Purpose Water Heaters . . . . 217

    1003.0 Detailed Requirements . . . . . . . . . 218

    1003.1 Safety Requirements . . . . . . . . . . 218

    1003.2 Controls . . . . . . . . . . . . . . . . . . . . 218

    1003.3 Gauges . . . . . . . . . . . . . . . . . . . . . 218

    1003.4 Stack Dampers . . . . . . . . . . . . . . . 218

    1003.5 Welding. . . . . . . . . . . . . . . . . . . . . 218

    1004.0 Expansion Tanks . . . . . . . . . . . . . 218

    1004.1 General. . . . . . . . . . . . . . . . . . . . . 218

    1004.2 Open-Type Expansion Tanks . . . . 218

    1004.3 Closed-Type Systems . . . . . . . . . 218

    1004.4 Minimum Capacity of ClosedType Tank. . . . . . . . . . . . . . . . . . . 218

    Table 1004.4(1) Expansion Tank Capacities for Gravity Hot Water Systems . . . . . 219

    Table 1004.4(2) Expansion Tank Capacities for Forced Hot Water Systems . . . . . 219

Frequently asked questions

What section of the CMC requires an expansion tank?

The CMC requires an expansion tank for hot‑water heating systems in § 1004.1; sizing rules and minimum capacities are in § 1004.4 and Table 1004.4(2).

Can I use the table or the equation to size a tank?

Yes — for forced and diaphragm types you may use Table 1004.4(2) for standard cases or use the equations provided in § 1004.4 when system conditions differ from the table assumptions.

Do closed expansion tanks need special construction?

Closed tanks must be designed for a hydrostatic test pressure of 2.5× the system allowable working pressure; and if the system operates at >30 psi, the tank must comply with ASME BPVC Section VIII.1 — § 1004.3.

Where must an open expansion tank be located?

An open‑type expansion tank must be located not less than 3 feet (914 mm) above the highest system point and have an overflow at least 1/2 the supply size or 1 inch — § 1004.2.

How strong must the tank support be?

Supports must be capable of carrying twice the weight of the tank when full without placing strain on connecting piping — § 1004.1.

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