Repressurising central heating when there are 3 gauges

[Michael001]

Hi I have an unvented, indirect system that is heated via a communal boiler located in the basement of my apartment block. Whilst the hot water and central heating are working I think the radiator heat is not very even. I understand that the pressure for most boilers (and so presumably central heating?) should be approx 1 to 1.5 bar when cold. I am confused due to the 3 gauges I can see, 0 bar at the expansion vessel and 1.4 and 2.0 bar on the cold and hot entry points the heat exchange unit. Should I just ignore the heat exchange unit, and using the filling loop would have no impact on it?
From online guidance I have seen topping up the pressure is with reference to the display on the boiler or a gauge below an expansion vessel, however those systems did not have communal boilers. Should the central heating work at all if the pressure is really 0 bar?

 

[ShaunCorbs]

The gauge on the red expansion vessel is the one for your heating system pressure

And unless there's a low pressure sensor then yes

 

[Michael001]

Thanks very much Shaun. Just to be sure, am I right in saying that the filling loop hose should be connected between the two "Plumber only do not touch" signs?! The central heating system being on the left and the cold mains on the right of the picture. I would have thought homeowners should be educated on how to top up pressure rather than deterred by the signs on my system

 

[ShaunCorbs]

Yep there the ones

Don't suppose you have the braided link ?

 

[Michael001]

Yes I do indeed, thanks again for your help

 

[Stigster]

It is possible that the pressure gauge has gone faulty and is just stuck at zero. Something to be aware of as you top up the heating system.

 

[Michael001]

The pressure gauge seems to be working, but it appears that the expansion vessel has lost its charge. After repressurising the system / bleeding radiators I turned on the central heating and the pressure rose 2 bar to 3 bar so I turned it off. I understand PRV should kick in at this point anyway.
I pressed the schrader valve on the vessel and no air/nitrgoen or water came out. Assuming the expansion vessel doesn't require replacement I understand that I should recharge it when empty to ensure the correct pressure. As I don't see valves to isolate the vessel is there a straightforward way of ensuring it's empty without draining the entire system? Or would it be sufficient to reduce the system pressure to 0 bar?
Also what is the correct pressure for the air - I have seen various numbers quoted: 0.2 bar below system pressure / the same as system pressure / a kingspan video says 1.5 bar. The same as system pressure seems logical to me

 

[Ric2013]

Assuming the vessel is okay, open a radiator bleed valve or similar fully while pumping up the vessel, though you'll lose some water. This ensures the system isn't under pressure as you put pressure in the vessel, measured at the Shraeder valve. If the vessel is bad, you'll fill the system with air.

After pumping up to pressure, a bubble of spit on the Shraeder valve will confirm it isn't leaking - I always do this on my car tyres, ever since I had a tyre slowly run flat during a long drive due to a faulty valve core.