From our own experience and from researching the different international regulations on solar energy, it’s recommended that when the fluid volume between the collector and the expansion vessel is less than 50% of the “wet side” volume (between the extended diaphragm and the vessel inlet) the use of an additional tank is recommended. This approach is covered by European Directive VDI 6002.
The VSG vessel provides a method of heat dissipation above the expansion vessel, protecting the membrane from excessive temperatures. The correctly sized additional tank also allows a decrease in the temperature of the solar liquid in the expansion system.
It is worth remembering that this option is rarely used when the heating system is placed on the roof due to the fact that the piping runs are shorter than normal.
capacity: from 5-105 litres
Installation and Operations Manual