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On Fri, 22 Oct 2004 22:29:52 GMT, (Jim T) wrote:

On Fri, 22 Oct 2004 20:57:08 +0100, "Ed Sirett"
wrote:

See the FAQ.
Given that most combi boilers have an internal vessel capable of
coping with the size of most typical systems[1]. I would say that the
vessel has failed.

It does sound like it.... Anyone know how much a new expn vessel for
one of these might cost? Are they difficult to replace? As I mentioned
I've never delved into that part of the boiler before. I gather the
expansion vessell is situated at the very back of the boiler, behind
the main firebox, and I think it's integral with the boiler chassis

As a work around you can drain a radiator (or 2)in a less used (or less
important to heat) room and then refill without bleeding that radiator.
This will likely provide enough cushion to enable you to have heating
until you can replace or install a good vessel.

Ed, I had wondered about that! Nice to hear someone coroborate what my
logic had suggested... I could to that, and thus postpone this work
for as long as possible....

Unless the manufacturers state otherwise, precharge the vessel to 0.75 bar,
and fill the system initially to 1.0.

The 0.75 bar is about right. The boiler's manual says they pre-charge
them to 0.7 bar.

[1] Retro style radiators or larger systems nearing the 24kW heating
output, may need additional vessels to limit the presure rises.

This is a modern system with normal B&Q-style rads. It's a small 3-bed
house. The manual says the expansion vessel is suitable for systems up
to 145 litres capacity. I've never really measured the capacity - only
guesstimated it roughly, on the basis of the size of the house and the
number and size of the rads - and that was years ago when I designed
the system... the details of which have escaped my memory.

Did you manage to sort this out?

The reason I ask is because I also have a Britony 80 combi boiler, and
have been having exactly the same problem with it over the past month
or so. I too replaced the pressure relief valve believing it to be
faulty, but it made no difference. Yesterday morning I depressurized
the system, checked the expansion vessel pressure & found it to have
dropped to below 6 psi when it should be around 10 psi (roughly 0.7
bar). I topped the pressure up with a footpump, and it seems much
improved, although there is still a very occasional drip from the
relief valve.

The pressure seems more stable now, there was previously nearly a 1bar
pressure difference between the system hot and cold. This seems to
have dropped to a difference of less than 0 .5 bar between hot and
cold.

Nick

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