ufhnoob

So we live in a rented property that has UFH, been moved in a month and there has been non stop dramas over the UFH working, we have had about 10 visits for someone to come out to look at it, long story short it is now working and has been sorted.. kinda! Yesterday they came to replace one of the sight glasses as they were not happy with how it was performing..(?) see picture attached, why is the one they replaced (the one circled) down and the rest are up? And is it normal for there to be no water in the one they replaced? Please explain in simple form as we are not clued up at all... On UFH! Thank you! :)