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

218 2025 CALIFORNIA MECHANICAL CODE

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]

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.