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Expansion Tank Acceptance Volume

Also known as expansion tank size · acceptance volume

Vt=Vs e1−P1P2V_t = \frac{V_s \, e}{1 - \dfrac{P_1}{P_2}}

Worked example: 4000 L loop, 250 L tank, 180/300 kPa abs → e = 2.5% — press Try an example to run it live, then adjust anything.

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Expansion Tank Acceptance Volume explained

P1P2eVtVs

Water is nearly incompressible, so a closed loop with nowhere to expand will simply lift the relief valve — about 1 % of volume growth is enough to take a system from 12 psi to 100 psi. The expansion tank gives that growth somewhere to go by compressing a captive air cushion. The bladder-tank equation says the tank must be big enough that the expanded water squeezes the air from P₁ to P₂ without exceeding P₂: Vt = Vs·e ÷ (1 − P₁/P₂), where e is the net expansion factor, roughly 2.4 % for water heated from 45 °F fill to 200 °F operating.

The trap that ruins more tanks than any other is gauge versus absolute pressure. Boyle's law needs absolute, so add 14.7 psi (101 kPa) to both readings before entering them: a 12 psig fill is 26.7 psia and a 30 psig relief is 44.7 psia. Do it in gauge and you undersize the tank by roughly half. Worked example: 1,000 gal of system water, e = 2.4 %, 26.7/44.7 psia gives Vt = 24 ÷ 0.4027 ≈ 59.6 gal of tank, and you buy the next size up. Second trap: the tank's air pre-charge must be set to the fill pressure before the system is filled, with the tank isolated. A factory 12 psi pre-charge dropped into a 25 psi fill leaves you with a tank that is already full of water and a relief valve that weeps every afternoon.

Expansion Tank Acceptance Volume formula

Vt=Vs e1−P1P2V_t = \frac{V_s \, e}{1 - \dfrac{P_1}{P_2}}
Where
  • VtV_t= Tank volume (L)
  • VsV_s= System water volume (L)
  • ee= Net expansion factor (%)
  • P1P_1= Fill pressure (absolute) (kPa)
  • P2P_2= Maximum pressure (absolute) (kPa)

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