Water Heater Thermal Expansion Tank
Potable closed-system expansion volume and required tank size from heater capacity, temperatures, and incoming pressure. Per ASPE PEDH Ch. 6 / ASME B40.1 steam tables; IPC 604.8 PRV note.
Formula and source
expansion factor = (rho_cold − rho_hot) / rho_hot from public steam-table densities; V_expansion = heater volume × factor; V_tank = V_expansion / acceptance factor; pre-charge = incoming pressure. Recommended size = smallest standard diaphragm tank (2 / 4.4 / 8.5 / 14 / 20 gal) ≥ required volume.
ASPE Plumbing Engineering Design Handbook (2nd ed.) Chapter 6; ASME B40.1 steam tables. IPC 2021 §604.8 (PRV) and §607 (thermal expansion control).
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