How to use this calculator
- Enter system volume. Include boiler, piping, coils, heat exchangers and connected hydronic equipment.
- Set fluid expansion. Use an entered expansion percent, or switch to density ratio for a specific fluid and temperature range.
- Set pressure range. Enter cold fill pressure, relief setting, relief margin and tank precharge.
- Check selected tank. Enter the tank shell volume to see acceptance capacity, utilization and predicted maximum pressure.
- Verify boundaries. Confirm manufacturer tank acceptance, relief valve settings, equipment pressure limits, glycol data and local requirements.
How it works
The raw expansion volume is
V_e = V_s e,
where V_s is the closed-loop system volume and e
is the fluid expansion fraction. In density mode,
e = rho_c / rho_h - 1.
The working pressure limit is the relief valve setting minus the entered pressure margin. Tank acceptance uses absolute pressure. With precharge near cold fill pressure, the acceptance factor is approximately AF = 1 - P_fill,abs / P_work,abs.
Required tank shell volume is required acceptance divided by the acceptance factor. The selected-tank pressure check back-solves the gas side after the required expansion volume enters the selected tank.
Use adjacent hydronic and piping tools for the rest of the system: pipe volume, pipe pressure drop, pump NPSH and pipe insulation heat loss. For the exact sizing checklist, use the expansion tank size worksheet. For hydronic and boiler query wording, use the hydronic expansion tank calculator.
Worked example
Verified against the live calculator
With the defaults, a 100 gal hydronic loop with
3% fluid expansion and a 10% allowance needs
about 3.3 gal of acceptance. With 12 psi
cold fill, a 30 psi relief setting and a 3 psi
relief margin, the minimum tank volume is about 9.2 gal.
The entered 15 gal selected tank stays below the working
pressure limit in this screen.
Frequently asked questions
How do you size a hydronic expansion tank?
Find the closed-loop system volume, multiply by the fluid expansion fraction, add a safety allowance, then divide by the tank acceptance factor over the cold-fill to working-pressure range.
What is tank acceptance volume?
Acceptance volume is the amount of expanded fluid the tank can accept between the cold fill pressure and the maximum working pressure. It is usually less than the tank shell volume.
Should precharge match the cold fill pressure?
For most hydronic diaphragm tank screens, precharge is set near the cold fill pressure before the tank is connected and the system is filled. Final setup follows the tank and boiler manufacturer instructions.
Does this replace manufacturer tank sizing charts?
No. This is a formula screen for acceptance volume and pressure range. Final tank selection still needs manufacturer acceptance data, relief settings, equipment limits and local code requirements.
Method & assumptions
- System volume must include all connected closed-loop fluid volume.
- Pressure calculations use absolute pressure internally; inputs and outputs show gauge pressure.
- The relief pressure margin reserves room below the relief setting for operating tolerance and gauge uncertainty.
- Tank precharge is checked against cold fill pressure, but final precharge setup follows manufacturer instructions.
- Final selection still needs manufacturer tank acceptance charts, boiler and heat-exchanger pressure limits, relief valve requirements, glycol property data, potable versus hydronic rules, isolation/air-removal details, local code and commissioning checks.