Expansion Tank Sizing Calculator

Inputs

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Total closed-loop fluid volume in boiler, piping, coils, heat exchangers and connected equipment.
gal
Use an entered expansion percent for most screens, or enter cold/hot densities for a specific fluid.
Fluid volume increase from cold fill temperature to maximum operating temperature.
%
Extra allowance added to the calculated expansion volume.
%
Gauge pressure at the expansion tank connection when the system is cold.
psi
Pressure relief setting. The calculator sizes below this by the entered margin.
psi
Margin below relief pressure reserved for operating tolerance and gauge uncertainty.
psi
Air-side tank precharge before the tank is connected to the filled system. It is usually set near cold fill pressure.
psi
Total shell volume of the tank being checked, not the catalog acceptance volume unless the manufacturer defines it that way.
gal

Results

Default result
Edit inputs
Minimum tank volume(Vt min)
9.173gal
Pass

Required acceptance volume divided by the tank acceptance factor.

Also computed

Required acceptance(Va req)Pass3.3gal

10.0% allowance added to raw fluid expansion.

Selected tank acceptance(Va sel)Pass5.396gal

Acceptance volume available between cold fill and working pressure limit for the entered precharge.

Selected tank utilization(U)Pass61.15%

Predicted max pressure(Pmax sel)Pass19.53psi

Back-solved from gas compression after the required acceptance volume enters the selected tank.

Pressure margin(dP)Pass7.47psi

Working pressure limit minus predicted selected-tank maximum pressure.

Fluid expansion used(e)3%

Entered fluid expansion percent.

Expansion tank preview: minimum tank volume 34.7 liters, selected tank utilization 61 percentExpansion tank sizingClosed-loop acceptance volume from fluid expansion and pressure rangeminimum tank volume34.7 Lselected max pressure19.5 psiUtilizationPressure marginScreen only. Final tanks need manufacturer acceptance charts, relief settings, boiler and code requirements, isolation, air removal and commissioning checks.
Method notes 4 notes
  • Raw fluid expansion is system volume times the entered expansion percent, or system volume times (cold density / hot density - 1) in density mode.
  • Tank acceptance uses absolute pressures. With precharge near cold fill pressure, acceptance factor is approximately 1 - Pfill_abs / Pwork_abs.
  • Selected-tank maximum pressure is back-solved with the ideal gas relation for the air side of a diaphragm or bladder tank.
  • This is a sizing screen only. Final selection needs manufacturer acceptance charts, relief settings, boiler and heat-exchanger limits, potable versus hydronic rules, isolation, air removal, glycol data, local code and commissioning checks.

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