On 23:37 7 Apr 2021, John Rumm said:
On 07/04/2021 18:07, Tim+ wrote:


Our mains pressure HW system has an external expansion vessel.

Lately it's not being doing its job and water has been trickling
through the pressure relief valve whenever it was heating up.

I had tried unsuccessfully to recharge it with my bicycle track
pump but I ended up convincing myself that the internal rubber
bag had failed.

Anyhow, took the bull by the horns today and depressurised the HW
system and undid the pipe connection. I then unscrewed the
securing jubilee clips that hold it to the wall bracket.

**** me but the holding it up with one hand in the top of the
airing cupboard and trying to lift it off the pipe work was not
happening! Managed to undo another pipe joint and to free the
unit and then dragged it out of the cupboard.

Turned out of course that it was still full of water and at about
30cm wide and 40 cm tall it would hold around 28L!

With hindsight I should obviously have let air into the top of
the tank to allow it to drain. Won't make that mistake again!

The bag actually seemed fine and once reinstalled and
re-pressurised to 3 bar there was no evidence of any leak.

The normal procedure for checking and re-pressurising is to:

Turn off mains cold inlet to cylinder. Open a hot tap somewhere -
water will drain until any water in the expansion vessel is
expelled.

Now check the pressure at the shrader valve. If it's less than
(typically) 3 bar, add air until it's at 3 bar.

Close the hot tap, and open the inlet valve to the cylinder.

I added some notes to the maintenance section:

http://wiki.diyfaq.org.uk/index.php/...DHW#Servicing_
requirements

This reminds me ... not long ago I was asking here about a failing
expansion vessel inside my boiler.

http://al.howardknight.net/?ID=161797506000

I had a new one fitted in the external pipework. Should the failed
expansion vessel inside the boiler have been isolated or perhaps
drained?