Repressurizing expansion vessel

[VAG_dude]

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

[Heliotrope Smith]

VAG_dude wrote in news:bd4932e4-0e04-473a-a550- [e-mail]:

It does not matter if the drain c*ck is open or closed when pressurising the expansion vessel as long as there is zero pressure in the system whilst this operation is carried out.

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[PeterC]

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!

[VAG_dude]

Cheers, turns out I had read a rather confusing guide!

[VAG_dude]

This boiler is a Vaillant Turbomax and I have just re-pressurised the EV and now it appears the diverter valve is not working as it should occasionally getting stuck in mid-position! Is this to be expected for a boiler that is 9 years old?

[dennis]

Sorry but it does matter. When you pressurise the expansion cylinder you want to leave it empty of water, if you pressurise it while full of water there will be no room for expansion afterwards.

That is if the system is full of water but at zero pressure the expansion vessel may still be full of water (there is nowhere for it to go), when you start to pump in air the pressure will go up but still leave no space for expansion.

You do need to ensure there is somewhere for the water that may be in the expansion vessel to go and that is the drain.

[PeterC]

Sorry, don't know. Mine is 10 - 11 yo and, touch wood...

[PeterC]

Yes, that's what I wondered, otherwise one is trying to compress water!