Expansion Tank Calculator 🛢️

ASHRAE Diaphragm Tank Formula

v₁, v₂ are the specific volumes of water at the fill and maximum temperatures, α the linear expansion of the piping, and P₁, P₂ the absolute fill and maximum pressures at the tank. Water expands about 3.0% from 10 °C to 82 °C; steel piping takes up about 0.25% of that. The denominator is the acceptance factor: the larger the pressure band, the smaller the tank.

Tips

• Set the tank pre-charge to the fill pressure at the tank.
• Connect the tank at the pump suction (point of no pressure change) in hydronic systems.
• Estimate system volume from pipe contents plus boiler, chiller, coil and heater volumes.

Related: Hot Water Demand Calculator, Boiler Feed Pump Calculator.

FAQ

How do you size an expansion tank?

Use the ASHRAE formula: tank volume = system volume × net expansion fraction ÷ (1 − P1/P2), with absolute fill and maximum pressures. For 1,500 L heated from 10 °C to 82 °C between 150 and 270 kPa gauge, about 128 L (a 140 L standard tank).

What should the expansion tank pre-charge be?

Equal to the cold fill pressure at the tank, so the diaphragm starts to move as soon as the water expands.