I have a conventional CH boiler (it's an old Thorn Apollo 50/65B, not a
modern combi/condenser type), and piping the system is standard apart from a
pressure vessel in the loft and a mains refilling/pressurising stop-cock
instead of the usual F/E tank. There's a pressure gauge one floor down next
to the immersion tank and CH pump. The wall-mounted boiler is another floor
down again.

The system no longer maintains pressure and I can see from tell-tale stains
that the globe-type pressure vessel is leaking around its circumferential
seal. As it's high above the top of the system, can I just replace this
vessel with a similar unit and then repressurise with the mains inlet? Or
will I have to drain the system right down and refill? There is no pressure
left in the system - due to the leak it is at normal atmospheric.

While old, the system is otherwise very reliable. The pressure vessel was
included originally as we thought we might convert the loft and add a
radiator, but that never happened so a pressurised system wasn't really
necessary after all.

-neil f.