I have an old style, conventional CH boiler (i.e. not combi/condenser), but the system has a pressure vessel in the loft and a mains refilling/pressurising stop-cock. There's a pressure gauge on the first floor next to the immersion tank and CH pump. The wall-mounted boiler is on the ground floor.
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 at 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?