Expansion vessel sizing
Free expansion vessel calculator to EN 12828 and DIN 4807-2 for heating, solar and DHW, with glycol. Size and pre-charge pressure, without signing up.
EN 12828 sizing for heating, solar and DHW — with glycol, pre-charge pressure and a safety-valve cross-check.
Expansion vessel sizing — method and FAQ
Sizing to EN 12828
The expansion vessel takes up the expansion of the water from the lowest to the highest operating temperature. Three system types are calculated: a closed heating or cooling circuit, solar thermal, and domestic hot water (potable).
Expansion is not read off a table but derived from the density of water after Kell (1975), so it answers for intermediate temperatures as well. It reproduces the DIN 4807-2 table to within 0.02 percentage points up to 90 °C; at 100 °C the deviation is 0.04 and at 110 °C 0.07, where the correlation is extrapolated beyond boiling.
With glycol, each component expands on its own. How much larger the expansion comes out depends on the temperature span and not on the concentration alone: 35 % propylene glycol gives roughly 30 % more from 10 to 80 °C, but 43 % from 10 to 60 °C and 23 % from 10 to 95 °C. That is why the ratio is shown worked out at your own temperatures instead of as a generic number.
The pre-charge pressure is half the answer
The right vessel at the wrong pre-charge behaves like the wrong vessel. That is why the pre-charge pressure is given the same prominence as the size. On a closed circuit and on a solar circuit it is computed from the static height, plus a vapour allowance where the temperature passes 100 °C, plus the margin you state. On a potable main it is not computed at all: the supply pressure sets it.
The margin up to the safety valve is not fixed: 0.5 bar for valves up to 3 bar, and 10 % of the set pressure above that — on a 6 bar valve, that is 0.6 bar. Without that margin the valve weeps every time the system heats up.
And the fill pressure is not "the pre-charge plus a bit". It is the pressure at which the vessel holds exactly the water seal it was sized for — it therefore depends on the size of vessel selected, and follows from Boyle's law. Every extra litre is taken off the expansion the vessel still has to accept.
Frequently asked questions
What expansion vessel does a heating system with 300 litres of water need?
On a closed circuit at 80 °C with a 3 bar safety valve and 10 m of static height, a 300-litre system leads to a 35-litre vessel with a 1.18 bar pre-charge. The height is not a detail: the same system needs 18 litres at ground level, 50 at 15 m and 140 at 20 m, where the pressure span is almost used up. That is why it is calculated rather than "read off" a generic table.
How much water is in my installation?
If you do not know it, the tool estimates it: pipework and underfloor loops from their geometry, emitters at indicative litres per kW — all of it visible and editable, so you can correct whichever figure you know better.
Why does a solar system need a different calculation?
Because at stagnation the collector reaches steam temperatures and the glycol-based fluid expands far more, while the safety valve is usually 6 bar. The same sizing rule as for heating would produce a dangerously small vessel.
Related tools
- Buffer tank sizing — Minimum volume from calorimetry — run time, defrost and hydraulic separation, not a litres-per-kW rule.
- Heat pump sizing for a dwelling — Heat loss, cooling load and required capacity — always with the operating conditions stated, so quotations become comparable.
- Solar collector sizing for domestic hot water — How many m² of collectors and how much storage for domestic hot water, and the share the sun covers each month — with your own certificate coefficients.