They are not that expensive, but first I would try pumping it back up/
repressurising it. The problem with some boilers is that to renew the
expansion vessel you need to remove the boiler
Now you know what the problem is you can amaze the engineer with your
knowledge when he appears and charge him £80 call out and £ 80 per hour plus
vat ( isn't that the norm when it comes to heating engineers )( not me)

Good Luck

"Milleniumaire" wrote in message
ups.com...
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.

IAN CAPEL wrote:
The first thing you need to do is to stop it leaking from the " overflow
pipe " which I guess is the pressure relief pipe. As it is leaking
underneath the boiler that could be the reason for no water from it
outside.
The pressure relief valve may need renewing. Also I would look at the
expansion vessel on the boiler. To see if it is faulty you would need to
fill the boiler to about 1.5bar, turn on the heating and see if the
pressure
rises to 3bar in about 15 mins or less. That would indicate that the
expansion vessel is knackered taking the pressure too high, opening the
pressure relief valve, losing your water in the system, hence " a leak "

"Milleniumaire" wrote in message
oups.com...
I have a Saunier Duval Thelia Twin 28E combination boiler which was
installed when the house was built seven years ago. Over the last 3/4
years it has caused us a lot of problems usually due to lack of
pressure but for the last 8 months (since it was last repaired) it has
been okay.

Now it is loosing pressure so quickly that it needs to be topped up
every 3/4 hours. I have placed a bucket underneath and this is
catching water leaking from an overflow pipe under the boiler. There
is also an overflow pipe outside but this is dry. The boiler only
seems to leak when I initially top up the pressure, it then settles
down, but after 3/4 hours has lost so much pressure that it cuts out.

I have found that if I turn down the CH thermostat so that the boiler
is off and ensure no hot water is being used, then I allow water in to
top the pressure up, the pressure still falls slightly over a short
period e.g. 15 minutes even though the boiler isn't being used. During
this period no water is seen to be coming out of the overflow pipe
under the boiler. Initially, this suggested to me that I had a leak in
the CH system and so checked all the radiators and pipes in the house
but couldn't see any evidence of this. The only pipes I can't check
are those under the downstairs floorboards. I would expect to see damp
patches in the ceiling if pipes between the floors were leaking.
Besides, if there was a leak in the CH system then surely this would
result in the highest radiators (in the loft) having "cold patches"
where air had replaced the lost water. This is not the case, in fact,
I have bled all radiators and none had any significant amount of air in
them. Does this suggest that the CH system is NOT leaking and that the
problem is definately with the boiler?

We have already had an engineer replace a part last week (after waiting
for 2 weeks for the replacement part to arrive) but this hasn't made
the slightest bit of difference. Sorry, but I don't know what part he
changed!

Are there any more tests I can perform to try to confirm that this is
definately a boiler problem and not caused by leaking radiators/pipes?
The boiler is covered by an insurance policy, but the radiators and
pipes aren't so I want to make sure that the engineer doesn't try to
blame a leaking system for the fault rather than a faulty boiler on his
next visit.

If it is the boiler, any idea what might be causing it?

Thanks.