unvented water cylinder

[Combi]

I have only ever installed these, so havn't got much fault finding experience. The water pressure out of the hot taps was low, the pressure in the vessel was low so I pumped it up to 3.5 bar, turned the hot water tap on pressure was ok for 30 seconds then it droped, water then came out the prv, faulty expansion vessel?

 

[AWheating]

clean strainer, measure incomming mains presure before and after press red valve to make sure its working correctly, adjust expansion pressure when system pressure is removed as a starting point.

 

[Ermintrude]

You've only installed combis or unvented

 

[townfanjon]

.
Water to cylinder must be turned off and hot tap/taps open before you re-charge vessel.
is it all the hot taps ?