Water tank and heater - Low pressure

[GHammer]

Hi all - We have an apartment in Cyprus and our water tank, pump and solar/electric heater system is on the flat roof. It's been fine since the building was built in 2018 but the other week we lost water pressure and I went onto the roof and saw the failure light was on, on the pump display. I reset the pump and it started working again but the water pressure was low, and seemed to get lower after a couple of minutes of use, also hot water supply was short. I got a plumber round who said there might have been an air-lock and he cleared it and he also said our main water tank was empty and not filling up due to a ball-cock being partially jammed. Whatever he did and charged us 30e for (good price) has got it working again like usual so all good so far.

I was just wondering if anyone can tell me if they know or can guess what the valve/hose does circled in the photos? The reason I am asking is I checked the other 12 water tanks on the roof and pumps for the other apartments and I would say half of them have this specific valve open and the other half closed. All the apartments are occupied and seem to have no issues, so I'm left baffled why some would be open and some would be closed and what the purpose is and why that would be. You also might be able to see from the photos that our water heat tank looks dented, happened by-itself, I'm guessing something to do with the solar heating the water to hot or something

 

[GHammer]

Or I guess another question would be, would a hose and valve be attached like the circled valve above if it was meant to be in the off position constantly? Still haven't got a clue why some of the tanks it would be closed and other tanks open.

 

[GHammer]

Is it a non return valve and therefore should always be open?