Expansion vessel. Rookie mistake

[Tim+]

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.

Fuck 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.

Tim

[nothanks]

Thanks for owning-up; it might save me from making the same mistake when the day (inevitably) arrives.

[newshound]

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).

[Roger Hayter]

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

[Tim+]

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

[newshound]

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.

[John Rumm]

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:

[Tim+]

Certainly would have been nice to have! Must look out for one.

Tim

[SH]

a hobby mains powered compressor with an air hose and a tyre inflator guage makes this easy.....

[newshound]

Only for unvented DHW though. For combis, or "system" boiler with vented DHW you need "my" method. I must admit I had not noticed when I posted earlier that Tim has unvented DHW.

[Pamela]

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

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

[Tim+]

I can?t see that it would be worth the bother. Presumably you had an external one fitted because changing the internal one was bothersome.

Tim

[Pamela]

That's correct: it was filling and emptying in an erratic way. It's still in series with the new expansion tank in the water circuit.

[Mathew Newton]

On Wednesday, 7 April 2021 at 23:37:35 UTC+1, John Rumm wrote: .

For what it's worth I note that the instructions for my vessel say to pump it up to 0.2 bar less than the static pressure. I've always assumed it is to give a slight positive bias in the direction of intended travel so as to minimise stresses when it returns to the at-rest position.

[John Rumm]

It depends a bit on what the PRV on the inlet is set to (and the water pressure if its less than the PRV setting)

[Mathew Newton]

Yes, I'd take the lower of the two as the reference.