Expansion Tank Calculator

Plan system protection with accurate expansion volume checks. Set precharge and fill pressures with confidence. Print or export results for quick site reporting always.

Calculator

Total water volume in pipes, coils, and equipment.
°
Typical start-up or fill temperature.
° C
Maximum operating temperature.
Use custom factor for glycol blends or special fluids.
%
Enter total volume increase from cold to hot.
Static fill pressure at the tank connection point.
kPa
Usually set equal to fill pressure.
kPa
Nameplate pressure of the safety relief device.
kPa
Leave blank to use 90% of relief setting.
m
Improves atmospheric pressure estimate for absolute calculations.
L
Optional allowance for air removal, separators, or buffers.
%
Adds margin for uncertainty and future changes.
Controls displayed result units.

Example data table

Scenario System volume Cold → Hot Fill / Precharge Relief Safety Typical result
Hydronic heating loop 800 L 10°C → 90°C 150 kPa / 150 kPa 300 kPa 10% Min tank ≈ 98 L, choose 100 L standard
Medium chilled water system 1500 L 5°C → 30°C 200 kPa / 200 kPa 600 kPa 10% Min tank ≈ 55 L, choose 80 L standard
Glycol blend (custom factor) 500 gal 40°F → 180°F 20 psi / 20 psi 30 psi 15% Enter supplier factor; tank size increases vs water
Example results are indicative. Your actual selection depends on manufacturer acceptance ratings.

Formula used

  • 1 Thermal expansion factor (water): computed from density change between cold and hot temperatures using an empirical water density equation (0–100°C).
  • 2 Expansion volume: Vexp = Vsys × e, where e is the expansion factor (fraction).
  • 3 Required acceptance: Va = (Vexp + Vextra) × (1 + Safety%).
  • 4 Tank sizing (diaphragm/bladder): using ideal gas compression between fill and maximum pressure: Va = Vtank × Ppre(abs) × (1/Pfill(abs) − 1/Pmax(abs)). Therefore Vtank = Va / [Ppre(abs) × (1/Pfill(abs) − 1/Pmax(abs))].
  • 5 Absolute pressure: P(abs) = P(gauge) + Patm, with atmospheric pressure estimated from elevation.

How to use this calculator

  1. Enter total system fluid volume and select the correct unit.
  2. Provide cold and hot temperatures. Use realistic maximum operating temperature.
  3. Choose Auto (water) or enter a custom factor for glycol or special fluids.
  4. Enter fill pressure, precharge pressure, and relief valve setting.
  5. Optionally set max operating pressure. Leaving it blank uses 90% of relief.
  6. Add safety factor and any extra acceptance volume if required.
  7. Press Calculate. Download CSV or PDF for records.

Expansion tank sizing in closed-loop construction systems

An expansion tank protects a closed-loop heating or cooling system from excessive pressure caused by thermal expansion. As the fluid warms, the system’s effective volume increases. In a sealed network, that expanded volume must be absorbed. A diaphragm or bladder tank provides a compressible air cushion that accepts the expansion while keeping pressure below the relief valve setting.

This calculator estimates minimum tank size using practical inputs: total system fluid volume, temperature range (cold to hot), and pressure limits. For water, the expansion factor is computed from density change between the two temperatures. For glycol blends or specialty fluids, select the custom method and enter a supplier-provided expansion percentage for the same temperature range. The required acceptance volume is then adjusted by your safety factor and any extra acceptance allowance you include.

Tank sizing is based on ideal gas compression of the air side. The tool converts gauge pressures to absolute pressures by adding atmospheric pressure; entering site elevation refines that estimate for high-altitude work. The acceptance fraction between fill pressure and maximum operating pressure determines how much of the tank’s total volume is usable for acceptance. If you do not enter a maximum operating pressure, the calculator conservatively uses 90% of the relief setting.

Example dataset (hydronic loop): system volume 800 L, cold fill temperature 10°C, hot operating temperature 90°C. Set fill pressure to 150 kPa at the tank connection and set precharge to 150 kPa (verified with the tank isolated and drained). Set the relief valve to 300 kPa and leave maximum operating pressure blank. Add a 10% safety factor and set extra acceptance volume to 0. This case commonly returns a minimum tank size near 100 L, so a typical selection is a 100 L standard tank.

When choosing a product, remember that standard tank sizes are nominal. The acceptance available at your pressures may be lower than the total volume, so always confirm the acceptance rating and maximum temperature. If the project may expand later, consider a modest margin beyond the minimum result.

In practice, confirm the selected tank’s acceptance rating at your specific pressures and temperatures using the manufacturer’s curves. Install the tank at the correct point in the system, typically at the suction side of the circulating pump near the air separator, and include isolation and drain provisions for service. Verify relief device sizing and settings meet project specifications and local codes. Use this tool for preliminary design checks, documentation support, and commissioning verification.

FAQs

1) What is the difference between tank volume and acceptance volume?

Tank volume is the total physical size. Acceptance volume is the usable water volume the tank can absorb between fill and maximum pressure. Acceptance is always lower than total volume and depends on precharge and pressure limits.

2) Should precharge equal the fill pressure?

In many closed-loop designs, yes. Setting precharge close to the fill pressure at the tank connection helps maximize acceptance and stabilizes pressure. Always set and verify precharge with the system isolated and at zero gauge pressure.

3) Why does the calculator use absolute pressure?

The air side follows gas compression rules that depend on absolute pressure. Gauge pressure ignores atmospheric pressure, which would distort acceptance calculations. The tool adds an atmospheric estimate (optionally corrected by elevation) for more reliable sizing.

4) What maximum operating pressure should I use?

Use a realistic upper limit during normal operation, staying below the relief setting. If unsure, leaving it blank applies a conservative default of 90% of the relief pressure to reduce nuisance discharge risk.

5) How do I size for glycol or special fluids?

Select the custom factor option and enter the fluid’s total volumetric expansion percentage across your temperature range. Use the supplier’s data sheet. Glycol mixtures typically expand more than water, so the tank often increases.

6) What does “extra acceptance volume” represent?

It is an optional allowance for additional volume that the tank may need to absorb beyond pure thermal expansion, such as air removal effects, buffer requirements, or anticipated system modifications. Use it sparingly and justify it in design notes.

7) Can I rely on the suggested standard tank size?

It is a convenience recommendation based on common sizes. Always verify the manufacturer’s acceptance rating at your pressures and temperatures, check diaphragm limits, and confirm compliance with local codes and project specifications before purchase.

Article: Expansion tanks for closed-loop construction services

Expansion tanks protect closed-loop heating and cooling circuits by absorbing the extra water volume created as the fluid warms. Without a properly sized tank, pressure can rise rapidly, triggering relief valve discharge, nuisance alarms, or premature component wear. On construction projects, that often means repeated call-backs, water losses, and difficulty maintaining stable system pressure during commissioning.

Sizing starts with the total system volume and the operating temperature range. Water becomes less dense as it heats, so the same mass occupies more space. This calculator estimates the expansion factor automatically for water using cold and hot temperatures. If your loop contains glycol or another blend, select the custom factor option and enter the supplier’s stated percentage increase over your temperature span.

Pressure inputs determine how much of the tank’s physical volume is usable as acceptance volume. A diaphragm or bladder tank contains a precharged gas cushion. When the system warms, expanding fluid compresses that gas. The accepted volume between the fill pressure and the maximum operating pressure depends on absolute pressures, so the tool adds an atmospheric pressure estimate (improved by entering site elevation).

Use the example table as a quick benchmark. For the hydronic heating loop case (800 L, 10°C to 90°C, 150 kPa fill and precharge, 300 kPa relief), the calculated minimum tank volume lands near a 100 L selection. That aligns with the practical approach on site: choose the next available standard size that meets or exceeds the required minimum, then confirm the manufacturer’s acceptance rating at your operating pressures.

Add a safety factor when system volume is uncertain, when future branches may be added, or when temperature control could exceed design values. Also consider air separators, make-up connections, and expansion tank location. Finally, document inputs and results in your QA records by exporting CSV or PDF, then verify final selection against project specifications and local codes.

Tip: Keep precharge close to fill pressure for best acceptance performance.

FAQs

1) What is “acceptance volume”?

The acceptance volume is the fluid volume the tank can absorb between fill pressure and maximum operating pressure. It is always less than the tank’s physical volume because part of the tank is occupied by the gas cushion.

2) Should precharge equal fill pressure?

In many closed loops, set precharge approximately equal to the fill pressure at the tank connection. If precharge is too high, acceptance drops; if too low, the tank may partially fill at cold start.

3) Why does the calculator use absolute pressure?

Gas compression follows absolute pressure. The tool converts gauge pressures by adding atmospheric pressure, then sizes the tank using the change between fill and maximum operating pressures for consistent results across elevations.

4) What if I have glycol in the system?

Select the custom factor method and enter the supplier’s expansion percentage for your cold and hot temperatures. Glycol typically expands more than water, so the required acceptance and tank size often increase.

5) What maximum operating pressure should I use?

Use a realistic maximum pressure that remains below the relief valve setting. If unknown, the calculator uses 90% of the relief setting as a conservative default to reduce the risk of nuisance discharge.

6) How do I choose a standard tank size?

Choose the next available standard tank size above the calculated minimum. Then confirm manufacturer acceptance ratings at your fill and maximum pressures, and verify connection size, temperature rating, and code compliance.

7) Why add a safety factor?

A safety factor covers uncertain system volume, future extensions, temperature overshoot, or measurement error. It is a practical construction allowance that reduces the chance of relief valve lift and improves operational stability.

Engineering note: This tool supports preliminary sizing for closed-loop systems. For large plants, consult design standards, tank acceptance curves, and local regulations.

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