Expansion Vessel losing pressure... but where from?

[MJNewton]

Our 150L Range Trubune UVC has a 12L Zilmet Hy-Pro expansion vessel connected to it, stock photo as follows:

 

The system is 18 years old and the last few years I've had to repressurise the EV (to 2 bar i.e. supply pressure of 2.2 bar less 0.2 bar as per the instructions) pretty much annually whenever I've started seeing dripping from the PRV. This last year, however, has seen me having to do this monthly and indeed even more recently I am doing it weekly! Clearly something amiss.

 

No water comes out of the schrader valve when pressed, so I am assuming this means the internal bladder is still intact. Speaking of the valve I assumed it was likely to be the issue and so I replaced its core but to no avail. Suspecting that my new core might not have seated well (it was a slightly different shape to the original) I have since also added a short (and, admittedly, very cheap) extension piece to the valve with its own sealing valve in it but still the air is gone within a week and the PRV passes during reheat.

 

Any thoughts as to where the air might be going? I've tried some soapy water around the valve (conveniently it sits in a recess under that black cap and so can hold a small pool of liquid around its base) but haven't spotted any leak. There's no signs of corrosion or other issue on the EV.

 

Maybe I should just spend £40 on a new EV and be done with it, but thought I throw this out there in case there's something I might be missing?

 

I'm going away on business tomorrow for a week and so will fight the temptation to do any too invasive today, but I have at least repressurised the EV once again which should cover the majority of the time I am gone.

Edited February 8 by MJNewton

[Chanmenie]

Have you represurised it, I think those ones need quite a bit of pressure, 3 bar as on the lable

If it’s not pressurised the expansion of the hot water will cause the overflow then the working pressure drops and you start again 

Edited February 8 by Chanmenie

[MJNewton]

Yeah, I pressurise it to 2 bar (supply pressure of 2.2 bar less 0.2 bar), and of course do this with the supply turned off and a tap open to allow the air to be admitted.

 

It wouldn't be so bad, but it takes just shy of a hundred cycles of my bike track pump to do it!

Edited February 8 by MJNewton

[Chanmenie]

1 minute ago, MJNewton said:

Yeah, I pressurise it to 2 bar (supply pressure of 2.2 bar less 0.2 bar), and of course do this with the supply turned off and a tap open to allow the air to be admitted.

 

It wouldn't be so bad, but it takes just shy of a hundred cycles of my bike track pump to do it!

The label says 3 bar so I’d do it to that. 
 

 

 

 

[MJNewton]

That's the pre-charge pressure (from the factory). The installation instructions specifically say this should be adjusted to suit the local supply pressure as per their formula.

 

Beside which though, even if I was under-pressuring the EV this wouldn't explain why I go from 2 bar to zero in a week?

Edited February 8 by MJNewton

[JohnMo]

Do you have depressed the UVC depressed and therefore the EV? Prior to pressuring the EV?

 

Maybe the bladder is at the end of its life, allowing air across but not water?

[MJNewton]

Yeah I make sure everything is depressurised (sitting at atmospheric pressure) prior to pressurising the EV. Once pumped up and monitoring the HW pressure I can see it rises from 2.2 bar to around 3 bar during/following reheat so it all appears to work fine... for a few days at least! It then peaks higher and higher - saw it at 5 bar this morning - and the PRV starts dripping.

 

It does feel like a bladder-related issue. Perhaps as you say air (only) permeability or given it is replaceable it sits sandwiched between two flanges at the bottom and maybe the seal on the air side of that interface is slightly passing. At any other time I'd probably 'nip up' those bolts you can see but I don't dare mess with anything that might make matters worse prior to me leaving tomorrow. Knowing my luck I'd end up misshaping the flange causing a leak on the water side too and I'm not sure what I'd do then!

 

When I'm back I think I'll just replace the EV. 18 years isn't too bad, and given how many times I've lost pressure I guess the bladder has really been exercised to the max and so it surely wouldn't last forever anyway. I might dunk the old EV in a bucket of soapy water and see if I can find a leak.

Edited February 8 by MJNewton

[Chanmenie]

1 hour ago, MJNewton said:

That's the pre-charge pressure (from the factory). The installation instructions specifically say this should be adjusted to suit the local supply pressure as per their formula.

 

Beside which though, even if I was under-pressuring the EV this wouldn't explain why I go from 2 bar to zero in a week?

Oh right mine didn’t say that, it has a set pressure.

 

yeah doesn’t explain where it going, you’d think the diaphragm was leaking but you said no water from the valve ? 

[MJNewton]

3 minutes ago, Chanmenie said:

yeah doesn’t explain where it going, you’d think the diaphragm was leaking but you said no water from the valve ? 

 

That's right - no water coming out at all!

 

I'm just going to swap the EV. £40 seems a worthwhile gamble to fix it, which surely it will! 

[dpmiller]

you could always try tightening the bolts on the end flange a wee bit, likewise if there's a nut retaining the valve body. But I wouldn't try too hard, at £40...

[MJNewton]

Don't tempt me to do anything before I leave for the week! 😂 

[Chanmenie]

5 hours ago, MJNewton said:

 

That's right - no water coming out at all!

 

I'm just going to swap the EV. £40 seems a worthwhile gamble to fix it, which surely it will! 

I agree for £40 it’s worth a try as it seems the most likely culprit 

[Nickfromwales]

1 hour ago, Chanmenie said:

I agree for £40 it’s worth a try as it seems the most likely culprit 

Either that or the PRedV is 'letting by'?

 

9 hours ago, MJNewton said:

supply pressure of 2.2 bar

If that's what you get daytime, network can peak at the early hours too, so I'd also consider a strip / clean / inspection of the PRedV on the control group.

 

Defo start at the EV, as these things don't last forever and you've had a bloody good run out of it, just odd to not be getting water on the secondary side of the bladder.

 

I assume you'll mothball this while away and turn the stopcock off until your return?

[MJNewton]

The PRedV is actually fixed at 3 bar, its the incoming supply pressure that doesn't seem to go above 2.2 bar - we're in a small town fed by a water tower and we're at the top of a hill too so low pressure is fairly normal around here. Even if the PRedV was passing a higher pressure whilst that might explain the PRV opening we would still have the EV charge air disappearance mystery. Good point about day/night variation though - I must take a look when the opportunity arises. 

 

Stopcock has to stay open as the family aren't coming with me (business trip)! I've recharged the EV though, am satisfied that the PRV does work and will safely (and calmly) discharge if required. I might also set the reheat window to a time when my wife can open a hot water tap for a few seconds to relieve any excess pressure - that's what I do now when I discover the PRV dripping and the EV charge gone.

 

 

Edited February 8 by MJNewton

[MJNewton]

Thanks everyine for pondering this with me, and sharing your thoughts. Will report back once things progress. 

Edited February 8 by MJNewton

[John Carroll]

Suggest removing the plastic cap over the schrader valve and spray some soapy water all round the valve, these sometimes leak where they exit from the cylinder itself.

Edited February 9 by John Carroll

[Nickfromwales]

On 08/02/2025 at 12:48, MJNewton said:

Any thoughts as to where the air might be going? I've tried some soapy water around the valve (conveniently it sits in a recess under that black cap and so can hold a small pool of liquid around its base) but haven't spotted any leak. There's no signs of corrosion or other issue on the EV.

👌👍

[MJNewton]

Thought it worth posting back as whilst not the most exciting of subjects you never know if it might help someone else out in the future.

 

As per the discussion I tried nipping up the flange bolts but to no avail - air still leaking out over a matter of days. By chance someone nearby was ripping out their perfectly working identical cylinder due to the installation of a heat pump so I bought the expansion vessel off him, and the cylinder too as these things are always worthwhile having a spare of, right?!

 

 

 

With the replacement EV fitted everything is now working properly.

 

So, what was wrong with my old one? Well, I removed the flange plate and pulled out the bladder - the 19th century football appearance immediately pointed the finger towards corrosion being present:

 

 

Sure enough it was clearly visible inside vessel:

 

 

And most crucially at a couple of points around the neck too which corresponded to where I'd seen bubbles emerging from once the vessel was out and I could dunk it in a bucket of water:

 

 

 

So problem now solved and all that's now left is to work out where best to story the spare cylinder, the acquisition of which means I will almost certainly now never need it!

 

Thanks all for the discussion - really appreciated.

Edited March 2 by MJNewton

[Nickfromwales]

23 minutes ago, MJNewton said:

So problem now solved and all that's now left is to work out where best to story the spare cylinder, the acquisition of which means I will almost certainly now never need it!

On marketplace here!! If your existing unit is stainless steel it'll outlive us both.

 

Great to see this resolved, and thanks for posting to wrap it up  

 

18 years is pretty good value for money tbh, but quite annoying to see how thin the footprint of the seal at the flange becomes at the bolt holes!

[MJNewton]

22 minutes ago, Nickfromwales said:

If your existing unit is stainless steel it'll outlive us both.

 

It was more in case it ever got damaged, or overrun with scale. I am planning on checking and descaling ours, and I am mindful that removing the immersion heater - which has been in place for 18 years - could lead to damage but hopefully the duplex steel is a lot stronger than copper where perhaps this risk is more likely?

Edited March 2 by MJNewton