In an earlier contribution to this discussion,
Milleniumaire wrote:

Ian,

I tried what you suggested and that's exactly what happened. Having
set the pressure to 1.5 bar while it was "quiet" I then left it and
returned 15 minutes later to find the pressure over 3 bar and water
pouring out of the pipe at the bottom.

We have boiler insurance through Homeserve GB Ltd (because we have so
many problems with the boiler) and I'm not sure if the engineer who
visited worked directly for them or was on contract. I'm guessing
that they have a policy of attempting to replace the cheaper parts,
such as the pressure relief valve, before changing the biggies like
the expansion vessel.

I get the impression that the expansion vessel is quite an expensive
part and is difficult to change. Is this true do you know?

I'm guessing that the engineer doesn't really know what the problem is
otherwise he would have fixed it during his last visit. He is due
again next Tuesday at which time it will have been 3 weeks since we
first reported this problem to Homeserve. I'm afraid their service
leaves a lot to be desired and I'm tempted to have a chat with him to
try and persuade him to change the expansion vessel.

Thanks.

I would say that the expansion vessel was pretty certainly the prime
suspect.

However, it may not actually need replacing - it may just need re-charging
with air. A fairly simple DIY job as long as you've got slightly more than
half a clue.
--
Cheers,
Roger
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