Expansion Tank Sizing Calculator

Estimate the minimum expansion tank size required for a closed hydronic heating system, based on ASHRAE sizing methodology.

System water volume (gallons)Boiler + piping + radiation content

Minimum fill temperature (°F)Typically 40°F

Maximum operating temperature (°F)System design temp

Tank fill / pre-charge pressure (psi)Typically 12 psi

Relief valve / max pressure (psi)Typically 30 psi

Enter valid numbers for all fields.

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Acceptance factor

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This calculator provides an estimate based on the standard ASHRAE expansion tank sizing formula (specific volume method). Always verify against the manufacturer's official sizing chart before ordering. Results assume a bladder/diaphragm-type tank.