Calculator Inputs
Example Data Table
This example is calculated using the same formulas on this page.
| System Volume (L) | Fill Temp (°C) | Max Temp (°C) | Fill Pressure (bar) | Max Pressure (bar) | Precharge (bar) | Safety (%) | Expansion Factor (e) | Expansion Volume (L) | Min Tank Volume (L) | Recommended Size |
|---|---|---|---|---|---|---|---|---|---|---|
| 1,200 | 10 | 90 | 1.5 | 3.0 | 1.2 | 10 | 0.035812 | 47.27 | 143.62 | 150 L |
Formula Used
1) Fluid expansion volume: Ve = Vs × e × (1 + S) where Vs is total system volume, e is the fractional expansion, and S is the safety factor (as a fraction).
2) Diaphragm/bladder tank sizing (ideal gas, isothermal compression of the air cushion): Vt = Ve / (P0 × (1/Pc − 1/Pm)) using absolute pressures.
3) Auto expansion factor for water uses the density ratio: e ≈ ρ(Tfill)/ρ(Tmax) − 1. For glycol, an adjustable multiplier is applied for a quick estimate.
How to Use This Calculator
- Choose your unit system and enter total system volume.
- Enter fill temperature and maximum operating temperature.
- Select fluid type. Use custom factor when you have verified data.
- Enter cold fill pressure, max allowable pressure, and tank precharge.
- Click Calculate Tank Size to view results above the form.
- Use the CSV/PDF buttons to export a shareable report.
- Pick the next nominal tank size above the minimum volume.
System Volume and Temperature Inputs
Expansion tank sizing begins with a reliable estimate of total system volume and the temperature rise from fill to peak operation. For hot-water heating, a fill condition near 10 °C to 20 °C is common, while design temperatures may reach 80 °C to 95 °C. The calculator converts these inputs into a fractional expansion factor and then into a required acceptance volume, which is the portion the tank must absorb. When system volume is uncertain, use equipment schedules and pipe takeoffs, then add a practical allowance for coils, headers, and ancillary components.
Pressure Limits and Safety Margin
Pressure settings define how much the air cushion can compress. The cold fill pressure should satisfy static height and pump inlet requirements, while the maximum allowable pressure must stay below the relief valve setpoint and equipment ratings. A safety factor is applied to cover estimating error, air removal behavior, and operational variability. Many projects use 5% to 15%, but the right value depends on how complete the system inventory is at the sizing stage.
Precharge Setup for Proper Acceptance
Precharge should be set with the tank isolated and the water side at zero pressure. For best acceptance, precharge is typically slightly below the cold fill pressure so the tank starts accepting expansion early in the heating cycle. If precharge exceeds the cold fill pressure, the bladder may not move until the system pressure rises further, which can reduce usable acceptance. The calculator flags this condition so you can adjust precharge or revise the fill pressure target.
Fluid Selection and Expansion Factor Control
Water expansion is temperature-driven and is estimated here from a density ratio model across the specified temperature range. For glycol mixtures, expansion can differ materially by concentration and temperature, so the calculator offers a quick adjustment and a custom-factor option. For critical plants, use mixture datasheets or manufacturer curves and enter the verified expansion factor. Doing so tightens the acceptance volume estimate and helps avoid undersizing in high-temperature or high-glycol applications.
Example Data for a Typical Closed Loop
The following example aligns with the built-in table and illustrates how each input drives the result:
- System volume: 1200 L
- Fill / Max temperature: 10 °C / 90 °C
- Pressures: fill 1.5 bar(g), max 3.0 bar(g), precharge 1.2 bar(g)
- Safety factor: 10%
- Result: expansion factor and minimum tank volume computed automatically.
FAQs
1) What volume should I enter for “total system volume”?
Use the sum of boiler, piping, coils, and accessories. If you only have pipe lengths, estimate pipe volume by size and add equipment volumes from schedules, then include a small allowance for unknowns.
2) Why must max allowable pressure be higher than fill pressure?
The tank works by compressing the air cushion as the system heats. If maximum pressure is not above fill pressure, there is no pressure range for compression, so the tank cannot accept expansion safely.
3) How do I choose the precharge pressure?
Set precharge with the tank isolated and drained on the water side. A common target is slightly below the cold fill pressure so acceptance begins early and bladder movement remains stable.
4) Should I use the auto expansion factor or a custom factor?
Auto works well for water across typical heating ranges. Use custom when you have verified mixture data, unusual temperatures, or a manufacturer-specified factor that better matches your project fluid behavior.
5) How does glycol concentration affect sizing?
Glycol mixtures can expand more than water and may require additional acceptance volume. This calculator applies a simplified adjustment, but you should confirm with the glycol datasheet or tank manufacturer sizing tables.
6) What safety factor is appropriate?
Typical design practice uses 5% to 15%. Use the higher end when system volume is estimated early, when future additions are likely, or when operating temperatures may exceed initial assumptions.
7) Why does the recommended size exceed the calculated minimum?
Tanks come in standard nominal sizes. The recommendation selects the next size above the minimum to ensure adequate acceptance volume and to accommodate real-world tolerances, precharge variability, and installation conditions.