Heating

Expansion vessel

Vn

14,9l

Size

18l

The expansion vessel sizes a diaphragm vessel to DIN EN 12828. System volume, maximum flow temperature, static height and the safety-valve set pressure give the expansion, the pre-charge and the next standard size.

Vn

14,9

l

Size

18

l

n
2,24%
Ve
3,4l
VWR
3,0l
VA
150l
p0
1,00bar
pa
1,30bar
pe
2,50bar

Set the pre-charge on the vessel before filling. Follow the manufacturer's data.

System volume

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How it's calculated

The expansion vessel follows DIN EN 12828. The expansion factor is n = density at 10 °C divided by the density at the maximum flow temperature, minus one. Expansion volume is Ve = VA · n. The water reserve is VWR = max(0.005 · VA, 3 l). Pre-charge is p0 = H/10 + 0.2 bar, and at least 0.7 bar. Final pressure pe is 0.5 bar below the safety-valve setting, or 90 % of it when the valve is above 5 bar. Fill pressure is pa = p0 + 0.3 bar. Nominal volume is Vn = (Ve + VWR) · (pe + 1) / (pe − p0). The next standard size from 8 l is selected.

Worked example

150 l, 70 °C, static height 8 m, safety valve 3 bar: Ve 3.4 l, p0 1.0 bar, pe 2.5 bar, Vn 14.9 l, next size 18 l.

FAQ

How do I estimate the system volume?
As a guide, 10 l per kW for radiators and 20 l per kW for underfloor heating. The content from the pipe schematic is better.
Why is the water reserve at least 3 litres?
The standard keeps a small reserve in the vessel, even when 0.5 % of the volume would be less.
When is the safety valve set too low?
When the final pressure is not at least 0.3 bar above the pre-charge. The height then does not fit the set pressure.
When is the pre-charge set?
On the vessel before filling. The manufacturer's data take precedence.