EXPANSION VESSEL SIZE CALCULATOR
What size expansion tank do I need for my closed hydronic system?
What size expansion vessel do I need for my closed hydronic system?
This calculator sizes a bladder (diaphragm) type expansion vessel for a closed heating or cooling hydronic system, following the ASHRAE method. Work through the inputs in order:
Select the vessel type. For a sealed system, choose "Closed · Diaphragm" — the common type, with a bladder separating the air and water.
Select the fluid. Choose Water, or Ethylene/Propylene glycol if the system is dosed. If you select a glycol, enter its concentration as a percentage of the total system volume.
Choose your units (metric or imperial). The examples below are in metric.
Enter the total system volume. This is all the water in the system — pipework, coils, heat exchangers, equipment and any buffer or storage tanks — in litres (L). If the volume is unknown, switch to "Estimate from kW" and enter the chiller or boiler duty instead.
Enter the highest and lowest water temperatures the vessel will experience:
Heating system: the lower temperature is the ambient fill temperature (e.g. 10°C) and the higher temperature is the operating supply water temperature (e.g. 80°C).
Chilled water system: the lower temperature is the design chilled water supply temperature (e.g. 6°C) and the higher temperature is the ambient/off temperature (e.g. 35°C).
Dual-temperature (hot/chilled) system: the lower temperature is the chilled water supply temperature (e.g. 6°C) and the higher temperature is the heating design supply temperature (e.g. 80°C). The preset buttons (Heating, Chilled, Condenser) will fill typical ranges automatically.
Enter the operating (fill) pressure — the normal system pressure, set by the make-up unit or RPZ valve.
Enter the relief valve set point — the highest pressure in the system, i.e. the PRV setting at the expansion vessel location. For lower-pressure systems a 300 kPa PRV is typical. The relief setting must be above the operating pressure.
Enter the static head above the vessel — the pressure due to the height of water above the tank location. Convert metres of water to kPa by multiplying by 9.81 (e.g. 15 m × 9.81 ≈ 150 kPa). Each pressure can be tagged as gauge or absolute; gauge is the default.
Enter the safety margin. A minimum of 15% is recommended.
The calculator then returns the minimum required expansion vessel volume and recommends the next standard manufacturer size. It also shows the recommended pre-charge pressure — set the vessel's air-side charge to this value before filling the system — along with a full calculation breakdown.
