On Sun, 24 Nov 2019 02:32:41 -0800 (PST), Lee Nowell wrote:

2. It is because the expansion vessels have run out of air. I appear to
have 2. One in the inbound supply side and one on the circuit coming out
of the boilers. Some people seem to say to just check them and if low
too them up (not sure to what) with a tyre inflator (presume the little
compressor thing I use to pump tyres up). Others say this is very
dangerous. Also TBH not sure what the purpose of either is and therefore
which one is likely to be it

Water expands when heated, the expansion vessels give some where for
this increase in volume to go. They are simply a chamber with a
stretchy membrane across with the water one side and trapped air the
other.

If safe, I could do both of course.

You have two as the boiler circuit is seperate to the domestic HW.
Both being sealed both need expansion vessels. As we are looking at
the HW side then it's only the expansion vessel on that we need to
worry about. If it has failed then the pressure can get *very* high
and this could force water out of places it shouldn't come out of,
like that pressure reducing valve.

The vessel should have car tyre like valve press the little bobble in
the middle air should hiss out, no wet. If wet the vessel has failed
and needs replacing. Not 100% fool proof if the valve isn't at the
bottom but they normally are... If dry it may of lost pressure on the
air side but this isn't the normal failure mode. To repressurise I
think you'd need to de-pressure the wet side and pump up the air side
to a preset value dependant on that particular vessel, that figure
might be on the label.

--
Cheers
Dave.