Water Heater Thermal Expansion Tank
Potable closed-system expansion volume and required tank size from heater capacity, temperatures, and incoming pressure.
Example
You enter
- Water heater capacity (gal) 40
- Incoming pressure (psi) 60
- Relief setting (psi) 150
- Incoming water temp (°F) 50
- Set-point temp (°F) 120
- Acceptance factor (manufacturer) 0.46
You get
- Expansion volume 0.4537
- Required tank volume 0.986 gal
- Recommended standard tank 2 gal
Checked against published worked examples from: ASPE / ASME.
Details, formula, and sources
ScopePer ASPE PEDH Ch. 6 / ASME B40.1 steam tables; IPC 604.8 PRV note.
Formula or tableexpansion 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.
Source and editionASPE Plumbing Engineering Design Handbook (2nd ed.) Chapter 6; ASME B40.1 steam tables. IPC 2027 §604.8 (PRV, 80 psi) and §607.3 (thermal expansion control), both unchanged from 2021 (read 2026-10-17).
Free accessFree at aspe.org for table-of-contents excerpts; codes.iccsafe.org for the IPC TOC.
What governsEstimate. AHJ and licensed plumber / gas fitter govern. Verify against the IPC / IFGC edition adopted in your jurisdiction.
API field nameswater_heater_vol_gal, incoming_psi, relief_psi, incoming_F, setpoint_F, acceptance_factor, v_expansion_gal, v_tank_gal, recommended_gal
- Acceptance factor 0.46 for diaphragm-type tanks unless the user enters the manufacturer valueAmtrol / Watts diaphragm-tank cut sheets
- Standard tank sizes 2 / 4.4 / 8.5 / 14 / 20 gal diaphragm potable-expansion tankscommon North American residential sizes
- Water density ASME B40.1 steam-table values, linearly interpolated between bundled break pointsASME B40.1