In an earlier contribution to this discussion,
neil f wrote:

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.

There's no need to drain the system - just remove it and replace it with a
new one of the same or greater capacity.

Are you sure that there are no other leaks, and that the pressure relief
valve isn't faulty - or, indeed, that the pressure vessel doesn't simply
need re-charging with air? There are a few things which I would try before
lashing out on a new one - such as checking the charge pressure with a car
tyre pressure gauge with the water system unpressurised. This needs to be
about 0.7 bar (~10psi). If it isn't, charge it up with a foot pump, then
re-pressurise the water system and see whether it *still* loses pressure.

[OK, you *know* you've got a (slight?) leak from the vessel - but this may
or may not be the root cause of your loss of pressure, so it would be best
to do a bit more diagnosis].
--
Cheers,
Roger
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