Unvented Expansion Vessel Pre Charge

[steveoelliott]

1 hour ago, John Carroll said:

Is the outside tap teed off before (upstream of) the PRV or after it (downstream)??. What pressure reading do you get from that screw on pressure gauge when the PRV pressure gauge is showing 2.0bar or whatever at a high UVC flowrate?.

 

The Caleffi PRV should have its model  labelled somewhere on it, like a 535H or similar, also the size should be stamped on it, ie, DN20 or such.

Outside tap is downstream. Problem is I believe the outside tap has an NRV so you never see a pressure drop. I’ll try and find model of PRV.

[John Carroll]

Even though downstream doesn't show the dynamic mains pressure you should get a fairly accurate reading by allowing a very small leakage from where the PG is screwed onto the tap, this will allow the NRV to reopen when the pressure drops?.

Does the UVC noise only happen after a reheat when a  hot tap is opened or does it also happen at other times?. If you just open a hot tap very slightly then turn the PRV setting screw slowly anticlockwise until the pressure falls from ~ 4 bar to say 2.5bar ish, then the only flow in/out of the EV will occur during a reheat (in) and opening (out) a hot tap, you can allways reset it to 3.5/4.0 bar when test complete.

Edited Sunday at 07:40 by John Carroll

[steveoelliott]

It happens after a reheat if it’s going to happen, but then doesn’t always.

 

here is a picture of the PRV. I am concerned there is water in a gauge which I’ve just noticed.

 

Good advice re setting the PRV. I will try that.

 

 

Edited Sunday at 14:09 by steveoelliott

[steveoelliott]

And of course replace the gauge next week @John Carroll. I know previously you’ve said about installing gauges at multiple locations but the risk of this happening to them and potentially leaking is a concern for me.

[John Carroll]

You can allways stick a plug in that PG port once you have taken any required readings.

Without the upstream pressure though you don't really know where the pressure drop is occuring, you could either install a PG (with isolating valve) just upstream of the PRV or tee in the outside tap upstream of the PRV, you can then test the pressure with the screw on pressure gauge.

What is the ~ distance from the main ("street") stopcock to the PRV and the pipe type and diameter(s)?.

[steveoelliott]

Thanks. About 10 -12 metres to street. It’s 15mm pipe incoming. When I measured the static pressure at the outside tap, which is the other side of the house that was matching what was on this gauge though. 
 

I’ve asked my plumber to replace / cap off that gauge this week. In the interim, I am checking it regularly as I am concerned about it peeing our water everywhere but this is a new and separate issue.

[John Carroll]

You can then use that outside tap screw on PG (just leave it slightly leaking) instead of the PRV PG to set up the PRV (static) pressure to the 2.5bar or so.

 

[steveoelliott]

Thank you. I probably will avoid using gauges permanently for this reason. The outside tap one is a great idea and I hadn’t considered them having a check valve on them.

[Alan Ambrose]

If it helps: I’ve not seen a small weep in a PG develop into a full leak. Useful to put them on isolating valves IMO to allow for easy swap out out though.

[John Carroll]

2 hours ago, Alan Ambrose said:

If it helps: I’ve not seen a small weep in a PG develop into a full leak. Useful to put them on isolating valves IMO to allow for easy swap out out though.

All sealed system boilers have PGs, cant recall reading of any weeps/leaks.

But I would install a PG just before that PRV above with a isolation valve, the PRV PG port can allways be plugged after any commissioning/adjustment.

I do find it extraordinary though that PGs are not installed as standard on all UVCs.

[steveoelliott]

@Alan Ambrose @John Carroll Thanks for the feedback. The valve in this case is just part of the PRV but it can be easily capped off. I noticed this morning that the water has dribbled out the front of the gauge but there doesn't appear to be any new water. Something must have triggered this weep or it could even be condensation. My plumber is supposed to be coming to cap it or swap it.

[steveoelliott]

Interestingly I got a response from the manufacturer stating that the gauge on the PRV should only be connected when setting the PRV and then capped. They said that constant exposure will inevitably lead to corrosion of the bourdon tube and lead to leaks.

[steveoelliott]

The gauge was removed today by my plumber and capped off. As you can see there was some corrosion internally and I think eventually this may have caused some issues.

[John Carroll]

Have you reduced the static pressure?

[Nickfromwales]

Looks to have passed the O-ring and not to have breached the hydraulic body as such. Prob because string / rope was used not PTFE tape....  

[steveoelliott]

@John Carroll I didn't play with it whilst the gauge was compromised, I will need to do this with a gauge on the outside tap again.

 

@Nickfromwales Not sure if the gauge was pre-installed or my plumber put this on the PRV. I know the new cap thread was put on using liquid PTFE. That said, the leak was definitely internal to the gauge. That green staining behind the O ring was never wet and I suspect it may have been the result of condensation as I've seen it on these pipes before when cold water comes in.

[steveoelliott]

Coming back to my original issue, I even got the noise (previously) with the cold inlet to the Megaflo shut off after a reheat. Now I guess there would be pressure in the small piece of pipe between the cold inlet closed but I would be surprised if this caused the issue.

 

I measured the water temperature at the tap which is some distance from the tank and it measured 57.5 with the tank thermostat set to whisker above 3. I debated dialing the temperature down a little to see if that stops it but one has to be careful of legionnaires.

 

I haven't lowered the EV pressure to 1.8 bar yet, it's something I intend to do when back from a business trip. Playing with it just before I go and leave my wife here isn't something I'm keen on doing as at least I can keep an eye if here. Mind you it was set at 2.8 bar back in January so I suspect it will be decreasing slowly anyway

I had wondered if the layout of the pipework from EV may cause issues like this to occur or be worse.