Where should the expansion tank be installed? On the primary side or on the secondary side? Both sides perhaps?
- x1 ?
- x2 ?
- x3 ?
- or x4 ?
If this somewhat confusing due to local terminology not disambiguated question is about a circulating heating loop, the expansion tank goes as close to the inlet of the (unseen) circulating pump as is practical. That makes the risk of cavitation less, and cavitation is Not Good for pumps.
Assuming the circulating pump is contained in the white and black unlabeled thing that's presumably a boiler with streamlined housing, because that's a terribly useful thing for a boiler to have, as they often travel at speeds of 100 kilometers an hour while sitting in your house, x4