1004.4 Minimum Capacity of Closed-Type Tank — 2025 California Mechanical Code

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

218 2025 CALIFORNIA MECHANICAL CODE

BOILERS AND PRESSURE VESSELS

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

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]

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.