On 07/04/2021 22:01, Tim+ wrote:
Roger Hayter wrote:
On 7 Apr 2021 at 18:47:18 BST, "newshound"
wrote:

On 07/04/2021 18:07, Tim+ wrote:
Our mains pressure HW system has an external expansion vessel.

Lately its 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. Wont 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.

Tim

With the bag failed you would get water out of the shraeder valve,
especially if you remove the insert completely.

Remember that for repressurising, you should really have the hot water
circuit completely depressurised so that the diaphragm can force all the
water out back into the system. To save that faff, on my system I have
an isolating valve to the hot water circuit and a drain valve for the
vessel itself. Obviously, you need to remember to open the isolating
valve once you have repressurised the vessel (and really you should do
that when the heating is off).

Otherwise you would need to pressurise it up to about 4 bar or more which
would be possible but difficult with a bicycle pump.


A track pump can do 100psi (7ish bar) with no difficulty. Problem is small
stroke volume compared to size of reservoir and trying to use it standing
on a stool in the top of the wiring cupboard! ;-)

Tim

Not if you have one of the little Lidl tyre pumps running of their 20v
lithium batteries! (Actually I have not checked what pressure it gets up
to, but it was very handy the other day using the football filler
"hypodermic" to blow out the jets of the carb on my two stroke hedge
trimmer). It does have quite a reasonable volumetric flow, certainly
enough for an expansion vessel.