Hydronic Expansion Tank Calculator
Estimate water expansion and minimum diaphragm-tank volume from system volume, cold/hot temperatures and pressure limits.
Water expansion
2.96 %
Expanded water volume
2.96 gal
Tank acceptance fraction
32.7 %
Minimum tank shell volume
9 gal
Metric tank volume
34 L
Water systems only. Select a larger manufacturer-rated tank and verify precharge, acceptance volume, relief-valve margin and glycol correction where applicable.
Turn the calculation into a complete design workflow
Use the free result as a quick engineering check, then continue into MEPFlow for connected system design, sizing and project calculations.
How the Hydronic Expansion Tank Calculator Works
The calculator first estimates liquid-water density at the cold and hot design temperatures and uses the density ratio to estimate how much the system water volume expands as it is heated.
For a diaphragm tank precharged near the minimum system pressure, Boyle's law relates the available acceptance volume to minimum and maximum absolute pressure. The required shell volume is the expanded water volume divided by the tank acceptance fraction.
This implementation is for water systems only. Glycol mixtures have different expansion characteristics and should be sized with fluid-property data and the expansion-tank manufacturer's selection method.
Calculation basis
Tank Volume = Expanded Water ÷ [1 − Pmin(abs) / Pmax(abs)]
When to Use This Calculator
- Closed-loop heating systems
- Hydronic heat pumps
- Boiler expansion tanks
- Preliminary ASME tank sizing
Engineering Checks Before Final Design
- Water only; do not use the automatic expansion factor for glycol.
- Verify tank precharge and maximum pressure at the tank location.
- Select the next larger manufacturer-rated tank and verify acceptance volume.
Frequently Asked Questions
Why are pressures converted to absolute pressure?
Gas compression follows absolute pressure, so atmospheric pressure must be added to gauge pressure when applying Boyle's law.
What maximum pressure should I use?
Use a design maximum below the relief-valve opening pressure and consistent with the expansion-tank manufacturer's guidance.
Does system volume include the boiler and buffer tank?
Yes. Include all water volume connected to the closed system, including piping and equipment.
Can I use this for glycol?
No. Glycol expansion differs from water; use the manufacturer's fluid expansion data or a glycol-specific method.
Engineering References
MEPFlow uses public engineering references to document the calculation basis. Always verify the final design against the standard, code edition and manufacturer data applicable to your project.