Expansion Tank Acceptance Volume

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: V_t = V_s·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 V_t = 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.