Hydronic Expansion Tank

Required diaphragm tank volume from system volume, fill, and relief pressures.

Run the calculator

Example

You enter

System water volume (gal) 100
Fill temp (°F) 60
Max water temp (°F) 200
Fill pressure (psi) 12
Relief pressure (psi) 30

You get

Required tank volume 9.25 gal
Pre-charge (set air side) 12 psi

Checked against published worked examples from: ASHRAE Handbook (HVAC Systems and Equipment).

Details, formula, and sources

Formula or tableV_tank = V_sys × ((ρ_cold / ρ_hot) − 1) / (1 − P_initial_abs / P_final_abs). Absolute pressures (gauge + 14.7 psi); densities from a public water-density table.

Source and editionASHRAE Handbook (HVAC Systems and Equipment) by name; water-density table from NIST.

Free accessNIST data free at nist.gov; ASHRAE Handbook licensed.

What governsEstimate. AHJ and licensed plumber / gas fitter govern. Verify against the IPC / IFGC edition adopted in your jurisdiction.

API field namessystem_volume_gal, fill_temperature_F, max_temperature_F, fill_pressure_psi, relief_pressure_psi, tank_volume_gal, precharge_psi

Working temperatures user-supplied (cold fill, hot operating)user input
Final pressure the entered relief-valve setting (enter a lower value to keep a margin below it)ASHRAE Handbook by name