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Calculations:
Worked Example:
Positioning the vessel
Pre-charge pressure of the vessel
[1] Expansion Volume (EV) =
[1] Expansion Volume (EV) =
[2] Reserve Volume (RV), 25% = 3.88 x 1.25 = 4.85 litres
[2] Reserve Volume (RV), 25% = EV x 1.25
[3.1] Required Gross Volume (L) =
[3.1] Required Gross Volume (L) =
[3.2] Required Gross Volume (L) =
[3.2] Required Gross Volume (L) =
= 3.88 litres
= 7.7 litres ~ 8 litres
~ (if pre - charge = 0.5 bar)
~ (if pre - charge = 1.0 bar)
= 9.7 litres ~ 10 litres
200 x 1.94
100
4.85
0.63
4.85
0.5
Using the above calculations and an example system volume of 200 litres
• A 8L expansion vessel pre-charged to 0.5 bar is sufficient.
• A 10L expansion vessel pre-charged to 1.0 bar is sufficient.
NOTE: if any of the system data values change a different vessel would be required.
It is recommended to position the vessel below the heat pump on the primary return pipe.
Deciding the pre-charge pressure of the vessel is part of the vessel sizing procedure. If the vessel is sized
at 0.5 bar and the difference in height (H) is zero, then the vessel must be pre-charged to 0.5 bar. If the
difference in height increases or decreases, then an additional charge pressure will be required. Use the
formula below to calculate the additional charge and add this to the pre-charge pressure:
Pg (bar) =
System Volume x 1.94
100
RV
0.63
RV
0.5
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