Expansion Tank Sizing Calculator - Hydronic | ASHRAE

Free Online Expansion Tank Sizing Calculator

Size a diaphragm or bladder expansion tank for closed heating and chilled-water loops. Uses ASHRAE water-expansion and acceptance-factor method. Free, instant, no sign-up.

📐 Standard: ASHRAE

✅ Free to use

📄 PDF export

📱 Mobile friendly

🎈

Expansion Tank Sizing Calculator

Reference: ASHRAE

🎈 HVAC

Size a diaphragm or bladder expansion tank for closed heating and chilled-water loops. Uses ASHRAE water-expansion and acceptance-factor method. Free, instant, no sign-up.

Inputs

Results

ℹ️ About This Calculator

A closed hydronic system needs an expansion tank to absorb the increase in water volume as the loop heats up, keeping system pressure within safe limits. This calculator sizes a diaphragm (bladder) expansion tank from your system water volume, temperature range and fill/relief pressures using the ASHRAE method.

Water expands roughly 3-4% between cold fill and operating temperature. Without a correctly sized tank, that extra volume forces the relief valve to weep and lets air back into the system. The tank's acceptance factor - based on the ratio of absolute fill to relief pressure - determines how much of its shell volume is usable, so a 30 L expansion often needs a 50-60 L tank.

📐 Expansion Tank Formula (ASHRAE Diaphragm Method)

ASHRAE

Ve = Vs × (ρc / ρh − 1)
Vt = Ve / (1 − Pi_abs / Pf_abs)

Ve = expansion (thermal) volume, L
Vs = total system water volume, L
ρc, ρh = water density at cold-fill and hot temperature, kg/m³
Vt = minimum diaphragm tank volume, L
Pi_abs, Pf_abs = absolute fill and relief pressure (gauge + 1.013 bar)

🧮 Worked Example

Example: A 1000 L heating loop runs from a 10 °C fill to 82 °C, with a 1.5 bar fill pressure and 4 bar relief. Water density falls from 999.7 to 970.6 kg/m³, so Ve = 1000 × (999.7/970.6 − 1) ≈ 30 L. Absolute pressures are 2.51 and 5.01 bar, giving an acceptance factor of 1 − 2.51/5.01 = 0.50. Tank volume Vt = 30 / 0.50 ≈ 60 L, so the next standard 60 L diaphragm tank is selected.

❓ Frequently Asked Questions

What size expansion tank do I need for a closed loop? +

Size it from the system water volume and temperature swing. As a rule of thumb a heating loop needs a tank of about 6-8% of system volume, but always confirm with the acceptance-factor calculation, which accounts for your actual fill and relief pressures.

What is the acceptance factor of an expansion tank? +

The acceptance factor is 1 − (absolute fill pressure ÷ absolute relief pressure). It is the fraction of the tank's shell volume that can actually accept expanded water. A lower fill-to-relief ratio gives a higher acceptance factor and a smaller required tank.

Diaphragm vs compression expansion tank - what is the difference? +

A compression tank has air in direct contact with the water and must be much larger. A diaphragm (or bladder) tank separates air and water with a flexible membrane, so it is smaller and does not waterlog. This calculator sizes the diaphragm type.

Where should the expansion tank connect in the system? +

Connect it at the pump suction, near the point of no pressure change, ideally on the return before the pump. This keeps the pump adding pressure to the whole loop and avoids drawing the system below atmospheric pressure.

Does chilled water need an expansion tank too? +

Yes. Any sealed loop that changes temperature needs one. A chilled-water loop expands less than a heating loop because the temperature swing is smaller, so the tank is smaller - but it is still required to handle fill warm-up and pressure stability.