It's probably nothing special, just a junction between the boiler flow pipe, cold feed and expansion pipe, and the vent pipe. You can make a similar junction up yourself using 22mm tees or even 28mm tees and reducers if you want the slight advantage of larger tube (water moves more slowly and entrained air separates more easily).
The point of the Ventair is to replace the normal arrangement described above on open vented systems. Similar products have been available for many years although rarely used. The vent pipe connection should be vertical.
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