Expansion Tank Sizing Calculator

📈 System Parameters

System Fluid Volume (gal) Total fluid volume in piping, coils, and equipment

Minimum (Fill) Pressure (psig) Static fill pressure at tank location; typically 10–20 psig

Maximum System Pressure (psig) Maximum operating pressure at the tank — typically the relief setting less 10% (or 5 psi), corrected for any height difference

Minimum System Temp (°F) Fill (lowest) temperature; typically 40–60°F

Maximum System Temp (°F) Highest temperature the fluid reaches: HHW design supply (180–200°F); for chilled water, the idle loop at room ambient (about 90–100°F), not the CHW supply

Fluid Type Glycol % by volume

Piping Material Pipe growth credits the 3αΔT term

📊 Sizing Results

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Required Tank Volume (gal)

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Net Expansion = Required Acceptance (gal)

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Acceptance Factor (%)

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Net Expansion Factor (%)

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Pre-charge Pressure (psig)

Selection Note: Select the next standard tank size above the calculated required volume. Add 10–20% safety factor for systems with significant pipe headers or coil volume uncertainty.

📚 Fluid Expansion Factors (v₂/v₁ − 1, from a 50°F fill)

Temp Range (°F)	Water	30% EG	50% EG	30% PG	50% PG
50 → 100	0.0066	0.0107	0.0120	0.0126	0.0151
50 → 140	0.0168	0.0221	0.0244	0.0255	0.0301
50 → 180	0.0303	0.0361	0.0395	0.0413	0.0477
50 → 200	0.0382	0.0441	0.0480	0.0501	0.0575
50 → 220	0.0469	0.0533	0.0577	0.0602	0.0685

Gross fluid expansion, before the piping-growth credit. Water from saturated-water specific volume; glycol from Dow typical density data for inhibited fluids (DOWTHERM SR-1 ethylene glycol, DOWFROST HD propylene glycol), % by volume. These are the same numbers the calculator uses. Confirm glycol data with your fluid supplier.

About This Calculator

This calculator sizes the diaphragm or bladder expansion tank for a closed hydronic loop — hot water, chilled water, or glycol — with the ASHRAE closed-tank method. Enter the system fluid volume, fill and maximum pressures at the tank, the lowest and highest fluid temperatures, the fluid, and the piping material. The tool returns the required tank volume, the net expansion volume the tank must accept, the acceptance factor, the net expansion factor, and the pre-charge pressure.

Expansion comes from the fluid’s density at the two temperatures (water tables, or Dow data for ethylene and propylene glycol), less the growth of the steel or copper piping. For a chilled-water loop, size on the warmest temperature the idle loop reaches — typically room ambient — not the chilled-water supply. Select the next standard tank size above the calculated requirement, and add a safety factor where header and coil volumes are uncertain.

Formula & Method

Fluid expansion	v₂/v₁ − 1 = ρ_min ÷ ρ_max − 1
Net expansion	e = (v₂/v₁ − 1) − 3α(t₂ − t₁)
Net expansion volume	V_e = V_system × e
Acceptance factor	1 − (P_fill,abs ÷ P_max,abs)
Required tank	V_t = V_e ÷ [1 − (P_fill,abs ÷ P_max,abs)]

This is the ASHRAE closed diaphragm/bladder-tank relation (ASHRAE HVAC Systems and Equipment, hydronic systems chapter): V_t = V_s[(v₂/v₁ − 1) − 3αΔt] ÷ (1 − P₁/P₂). Specific volumes v₁, v₂ are at the lowest and highest temperatures — water from saturated-water tables (the same table the AIM Works app uses), ethylene and propylene glycol from Dow typical density data for inhibited fluids by volume percent, interpolated in temperature. α is the linear expansion coefficient of the piping (steel 6.5×10⁻⁶, copper 9.5×10⁻⁶ per °F); choosing “ignore” drops the credit and sizes slightly larger. Pressures are converted to absolute by adding 14.696 psi before forming the ratio. Pre-charge pressure is set equal to the fill pressure at the tank.

Frequently Asked Questions

How is the required expansion tank volume calculated?

First the net expansion volume is found by multiplying system fluid volume by the fluid’s expansion between the lowest and highest temperatures (the ratio of specific volumes minus one), less the 3αΔt growth of the piping. That volume is then divided by the acceptance factor, 1 minus the ratio of absolute fill pressure to absolute maximum pressure, to give the required diaphragm or bladder tank volume. This is the ASHRAE closed-tank method.

What pre-charge pressure should the tank be set to?

For a diaphragm or bladder tank, the air-side pre-charge is set equal to the system fill (minimum) pressure at the tank location before the system is filled. This tool reports the pre-charge equal to the fill pressure you enter. Setting pre-charge to fill pressure means the diaphragm just starts to accept water as the system warms above its cold fill condition.

Why does glycol require a larger expansion tank?

Glycol solutions expand more than water over the same temperature rise, so the tank must accept more fluid. This tool uses Dow’s density data for each glycol mix rather than a fixed multiplier, because the ratio changes with temperature: from 50°F to 200°F, 30 percent ethylene glycol expands about 15 percent more than water and 50 percent propylene glycol about 50 percent more, while for a chilled-water loop warming from 50°F to 100°F a glycol mix can expand 60 to 130 percent more than water.

What is the difference between tank volume and acceptance volume?

Total tank volume is the full physical size of the tank shell. Acceptance volume is the amount of expanded system fluid the tank can actually take in between fill pressure and maximum pressure, equal to tank volume times the acceptance factor. You size the tank so that its acceptance volume is at least the net expansion volume, then select the next standard tank size above the calculated requirement and confirm the manufacturer’s maximum acceptance volume for that model.

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