In an earlier contribution to this discussion,
Bob Mannix wrote:


sounds like either the pressure relief valve that connects to the
overflow pipe has failed, or the expansion vessel. When the heating
is working how high does the pressure gauge go to. If it goes above
about 2.5 the expansion vessel has possible failed. In this case the
pressure relief valve lets out the over pressure water. When the
system goes cold pressure drops and then is to low to restart.

Try turning the pressure release valve (if you can) (this lets water
out) and letting it "thwack" back several times. Soemtimes they will
reseat.

That *might* work if the problem is with the relief valve - but it's far
more likely to be an issue with the expansion vessel - and the relief valve
is just doing its job.

If the problem *is* with the expansion vessel, it's not *necessarily*
curtains - it may just need re-pressurising on the air side.
--
Cheers,
Roger
______
Email address maintained for newsgroup use only, and not regularly
monitored.. Messages sent to it may not be read for several weeks.
PLEASE REPLY TO NEWSGROUP!