On Fri, 20 Aug 2010 09:34:52 -0700 (PDT), VAG_dude wrote:

When you re-pressurise an expansion vessel in a boiler are you
supposed to leave the drain cock open once you have got the pressure
to 0 bar ?

By inference, on my Vaillant, with it open; i.e. it doesn't say close it.

"Release the pressure from the boiler as described in Section 7.1.4"
"Check that the internal pressure is... If the pressure is lower than this
the vessel should be repressurised ..."
Refit the valve cap"
"Repressurise boiler and heating system if necessary"

So, no explicit instruction to close the drain point.

I did mine a couple of days ago and it was 12psi with the drain open so no
action necessary, but it'd be interesting to see if it did make a
difference. I suppose that it depends on how much of the volume outside the
inflated part is occupied by air/water.

Had to search for a gauge - the one that I use nowadays starts at 50psi!
--
Peter.
The gods will stay away
whilst religions hold sway