In an earlier contribution to this discussion,
kmillar wrote:

I've just had a completely new system installed. The boiler is a BAXI
'System' 35/60 with pump and expansion vessel built in. We have 6
Radiators plus indirect cyclinder and a 3 way valve.

My question is this. As long as I leave the system running, ie HW
and/or CH demand on 'all day' the pressure sits between 1 and 1.5 bar
all week, or for as long as you keep it all running.

If I let the system go off, ie do not programme CH or HW to come on at
all (say over night), and allow everything to cool right down the
pressure drops right down to almost 0 by the morning.

I have been all over the system and cannot find any leaks and it only
takes a small amount of water through the filling loop to bring the
pressure back up.

My question is this. Why does the pressure only drop when the system
is allowed to cool right down? and could it be related to this... In
the commissioning stage of the install instructions for the boiler it
says to open the screw on the automatic air valve in the boiler. It
does not mention closing it again. I have checked and the screw is
definately loose (open). Is this correct? Or should it be closed once
the system is filled and bled?

1 - 1.5 bar is a bit low for a hot system. If you use the filling loop to
charge it up to 1 bar *cold*, does it *still* only get up to 1.5 hot and
then go back to zero when it cools?

If so - and possibly indicated by the fact that it only takes a small amount
of water to re-charge it - it sounds to me as if you could have a problem
with the pressure vessel. If this isn't working properly, it can't absorb
the expansion when the water gets hot - so pressure builds up (for a short
time which you might miss) to over 3 bar - whereupon the pressure relief
valve opens and lets some water out. You can test this by tying a plastic
bag round the end of the discharge pipe (which should go out through a wall
into fresh air) and seeing whether any water collects in the bag. If it
does, come back - and we'll tell you what to do next.

I would close the auto air valve once the system is fully bled. I doubt
whether this is the source of your problem unless there is any evidence of
*water* having come out through it.
--
Cheers,
Set Square
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