generally vessels are charged around 1 bar / 10psi. they have a valve to connect foot pump too same as car tyre valve and if you press the needle you should get air(could be low/flat) not water out. water means split diaphragm. if flat you would charge to around 1 bar with pump while holding prv open. remove pump and close prv top back up system to 1-1.5 bar.
Before doing this you could just observe how it performs- top to 1 bar. set heating away and check pressure and prv in 30mins time. whats it risen to with expansion? 1-2 good 2-2.5 could be bit undercharged check again in 30 mins more, just monitor it. normal home with 8 rads i would expect from 1 bar cold a rise up towards 2 bar hot as a rough example.
ste
October 2009