Expansion Tank Calculator

Why a closed system needs an expansion vessel

Water expands as it heats up. Between a cold fill at 10 °C and 90 °C it grows by about 3.6% in volume. In a 1,000-litre system that is 36 litres with nowhere to go. Without somewhere to absorb it, pressure would rise until the safety valve blows; when the system cools, pressure would fall low enough to draw in air at the high points. A membrane expansion vessel is a gas cushion that takes up the extra water when hot and gives it back when cold, keeping the pressure between two safe limits.

How the vessel is sized

The calculator follows the method of the European standard EN 12828 for closed heating systems, in simplified form:

e is the expansion of water between the fill and maximum temperatures, calculated from its density. V_wr is a small water reserve kept in the vessel to cover minor losses: 0.5% of the system volume, at least 3 litres. The pressures are gauge pressures in bar, and the “+ 1” converts them to absolute, because the gas cushion follows Boyle's law.

Pre-charge p₀ must at least balance the static height of the water above the vessel plus a margin: p₀ ≈ H / 10.2 + 0.2 bar. Too low, and the top of the system runs under-pressure and draws in air. Final pressure p_e is kept 0.5 bar below the safety valve setting, so the valve does not weep at full temperature.

A worked example

A 1,000-litre system, filled at 10 °C, runs up to 90 °C. The vessel sits 10 m below the highest radiator and the safety valve is set at 3 bar. Expansion is about 36 litres plus a 5-litre reserve. Pre-charge comes to about 1.2 bar and final pressure 2.5 bar. The required nominal volume is about 108 litres, so the next standard size, 140 litres, is the one to buy.

Notice how much the pressure window matters. If the same system were only 3 m tall, pre-charge would drop to about 0.5 bar and the required volume to about 71 litres, so an 80-litre vessel would do. Tall buildings with low safety valve settings need surprisingly large vessels.

Common mistakes on site

The most frequent fault is not the size but the pre-charge. Vessels ship at a factory pre-charge, often 1.5 bar, and are installed without adjusting it to the actual static height. The pre-charge must be set and checked with the vessel isolated and drained on the water side; reading it with the system full gives a false value. Check it once a year: membranes lose gas slowly, and a vessel that has lost its cushion shows up as pressure swinging between too low and the safety valve opening.

The second is forgetting the buffer tank. A 500-litre buffer in a heat pump or biomass system can double the water volume, and with it the expansion. When in doubt, add up the volumes from the data sheets rather than relying on the estimate per kW. The heating flow rate calculator and pipe size calculator help with the rest of the circuit.

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Frequently asked questions

Is this calculation valid for glycol systems?

Not directly. Glycol mixtures expand more than water, often 20 to 40 percent more depending on concentration. Use the manufacturer's expansion data for the mixture, or add a corresponding margin to the expansion volume.

Can I use this for domestic hot water or solar systems?

No. Potable hot water vessels must be suitable for drinking water and are sized with different rules, and solar systems must also absorb the volume of the collectors when the fluid boils off at stagnation. Use the method given by the vessel manufacturer for those applications.

What if the maximum temperature is above 100 °C?

The pre-charge must then also exceed the vapour pressure of the water at the maximum temperature, to prevent boiling. This simplified calculator is meant for systems up to about 100 °C; above that, follow EN 12828 in full or the vessel manufacturer's software.

Can I install a bigger vessel than calculated?

Yes. An oversized vessel keeps the pressure more stable and costs little extra. An undersized one lets the pressure climb to the safety valve setting. Rounding up to the next standard size is normal practice.