Unvented Expansion Vessel Pre Charge

[Gus Potter]

6 minutes ago, Nickfromwales said:

I'm a married man, mate, I'll take whatever is on offer.

I could have worded that post better. Still pals I hope?

[Nickfromwales]

11 hours ago, Gus Potter said:

Still pals I hope?

One x would have left that as questionable, but 3 x's made me feel special . I felt bad because I never got you a card, or flowers, but there's always next year.

[steveoelliott]

@Mattg4321 Did you ever get to the bottom of this issue? Was lowering the pressure to 1.5 bar your ultimate solution? I have an ongoing issue, not yet resolved here:

The latest I've gathered is that EV pressure should be ~0.2 bar lower than dynamic pressure. But what is an accurate way to measure it. We are fortunate to have a gauge on the PRV at the stop cock. Is just opening up a tap enough to measure pressure? The thing is the pressure shown on the gauge will vary depending on the tap used / flow rate etc.

Edited Tuesday at 11:58 by steveoelliott

[Mattg4321]

I lowered the pre charge in EV to about 1.5bar and then also lowered the pressure at the PRV to about 2.5 bar. It's now acceptable.

[steveoelliott]

@Mattg4321 Thanks. Did you set the PRV for the cold main based on static or dynamic pressure? 
 

Also did you look at my post / watch the videos. Interested to see if what you saw was similar.

[steveoelliott]

I’m also interested in the theory that the PRV should be set to the incoming mains pressure or marginally below but whether this means the static or dynamic pressure.

[Mattg4321]

I’d say it sounds quite similar. I just dropped the PRV pressure slightly so that there wouldn’t be such a big difference between static and dynamic pressure which I think is probably the root of the problem. The pressure in EV/UVC rapidly changes and causes the noise

[steveoelliott]

Thanks @Mattg4321 my main concern was actually damaged or rupturing of pipe joints due to the vibration.

 

in terms of what you did with your PRV, did you open a tap and then set the dynamic pressure to 2.5?

 

In my case static is around 4 bar dropping to around 2 bar with a tap open. The more taps I open the lower dynamic goes naturally.

[steveoelliott]

This is a really good example of the problem but not my video.

 

[Mattg4321]

On 30/04/2025 at 08:11, steveoelliott said:

Thanks @Mattg4321 my main concern was actually damaged or rupturing of pipe joints due to the vibration.

 

in terms of what you did with your PRV, did you open a tap and then set the dynamic pressure to 2.5?

 

In my case static is around 4 bar dropping to around 2 bar with a tap open. The more taps I open the lower dynamic goes naturally.

I thought nearly all UVC came with a pressure reducing valve pre set at 3 bar. 4 bar seems unnecessarily high. 
 

I found setting the PRV below 2.5 bar resulted in poor shower pressure. I just played around with it for a while to see what was a good compromise between shower pressure and noise. 
 

Just follow the instructions for your PRV. Mine needed Allen key in the top of it to adjust. 

[Nickfromwales]

I bumped everything up on a house with massive showers and big baths, body shower jets and so on. I set that around 3.8 bar iirc as the dynamic flow rate through the CG was hugely different with just that slight nudge up from the factory 3 bar. I was asked to resolve retrospectively so the options were limited, plumbing was shite so I’d have done it all totally different from scratch but that house was large, expensive, finished, and “not great” for M&E. 

[steveoelliott]

@Nickfromwales and @Mattg4321 - The PRV on the megaflow is fixed at 3 Bar (no adjustment on the BAXI one). However, the one on the incoming cold main is adjustable.

 

Question is mine does not have a setting with pressures, rather you adjust it with water running and use the gauge. To find dynamic pressure, I figured this would be with one tap open but wondered what is the correct way to measure dynamic pressure.

 

I read elsewhere that this noise is caused by the sudden rush of flow when you open a hot outlet propelled by the charge of the diaphragm in the vessel. In an ideal world we'd have a valve that allows water to fill the EV unencumbered but restricts it emptying. Not sure if such a valve exists.

Edited Thursday at 16:25 by steveoelliott

[Nickfromwales]

19 minutes ago, steveoelliott said:

@Nickfromwales and @Mattg4321 - The PRV on the megaflow is fixed at 3 Bar (no adjustment on the BAXI one). However, the one on the incoming cold main is adjustable.

 

Question is mine does not have a setting with pressures, rather you adjust it with water running and use the gauge. To find dynamic pressure, I figured this would be with one tap open but wondered what is the correct way to measure dynamic pressure.

 

I read elsewhere that this noise is caused by the sudden rush of flow when you open a hot outlet propelled by the charge of the diaphragm in the vessel. In an ideal world we'd have a valve that allows water to fill the EV unencumbered but restricts it emptying. Not sure if such a valve exists.

Apologies, I was referring to upping the one on the main, the CG at the UVC was left stock, as when I put a 3bar PRedV on the mains the flow rates through the UVC CG suffered a lot (poor design and pipe size / layout) but upping the one at the main slightly made a huge difference; the mains there was crazy so needed the second PRedV fitting by myself after plumber #1 had ‘tried his best’. 

[John Carroll]

On 30/04/2025 at 08:11, steveoelliott said:

Thanks @Mattg4321 my main concern was actually damaged or rupturing of pipe joints due to the vibration.

 

in terms of what you did with your PRV, did you open a tap and then set the dynamic pressure to 2.5?

 

In my case static is around 4 bar dropping to around 2 bar with a tap open. The more taps I open the lower dynamic goes naturally.

 

Are there two PRVs between the stopcock and the UVC cold inlet?, one, with the pressure gauge, the other, at the UVC cold inlet?.

 

Also, I think you said somewhere that Megaflo offered you a new UVC? 

[steveoelliott]

7 hours ago, John Carroll said:

 

Are there two PRVs between the stopcock and the UVC cold inlet?, one, with the pressure gauge, the other, at the UVC cold inlet?.

 

Also, I think you said somewhere that Megaflo offered you a new UVC? 

One at the main and one on UVC. It’s this that gives the balanced cold. They offered a UVC but I took the option of a boiler instead as I planned not to use a megaflo going forward to opt for another cylinder. The thing is I want to be certain it is the cylinder before swapping.

 

Are you able to advise how I set the dynamic pressure on the PRV off the stop cock. Is it just open one outlet and set? The static is around 4 bar and between 1.8 and 2 bar when running.

 

thanks again.

[John Carroll]

The balanced cold should really be taken off downstream of the UVC valve set but it is what it is.

You said the UVC is set non adjustable at 3.0bar, if the stopcock PRV is set to 3.0bar and assuming the dynamic pressure falls to 2.0bar then, assuming, EV precharge set to 2.8bar and  no heating on, 0.9L will flow in/out of the EV on opening/closing  a hot tap. After a full reheat, 3.4L will flow into the EV to give a final pressure of 3.99bar, a total of 4.3L will flow back out of the EV when a hot tap is opened and 0.9L will flow back in on closing the hot tap.

If the stopcock PRV is set to 2.0bar then no water will flow in/out of the EV with no heating on as the pressure downstream of the stopcock PRV is now almost constant. After a full reheat, 3.4L of water will flow into the EV to give a final pressure of 3.99bar. So, the only movement of water in/out of the EV is a very slow one during heat up and a fairly rapid one on opening a hot tap when the pressure falls from 3.99bar to 2.8bar.

 

I'm assuming that the stopcock PRV then also feeds to the UVC PRV inlet??. 

 

Edited 23 hours ago by John Carroll

[steveoelliott]

Yes the stop cock PRV is on the main and prior to the UVC PRV. It seems to be the rapid discharge from the EV that triggers this. 

Edited 23 hours ago by steveoelliott

[John Carroll]

If you ensure the stopcock PRV is set to a max of say 2.0/2.5bar then (and assuming the EV precharge is still set to 2.8/3.0bar) then the only possible movement of water discharge from the EV is after a reheat, at all other times, opening/closing a hot tap should not cause the EV to trigger anything?

[steveoelliott]

8 hours ago, John Carroll said:

If you ensure the stopcock PRV is set to a max of say 2.0/2.5bar then (and assuming the EV precharge is still set to 2.8/3.0bar) then the only possible movement of water discharge from the EV is after a reheat, at all other times, opening/closing a hot tap should not cause the EV to trigger anything?

Thanks. What’s the correct way to set the PRV though. With a tap open? And should I aim for static pressure at 2 bar? I think the issue is the static pressure at 4 bar despite it dropping to 2 bar static when tap open.

[John Carroll]

Suggest setting it to say 2.2/2.3bar with a very small flowrate, this means that with no flow the downstream pressure should still be ~ 2.5bar (static) but should still ensure that the PRV goes fully open at higher flowrates. When set, and with no flowrate, either hot or cold, keep an eye on it eo ensure it doesn't rise above ~ 2.5 bar over a short period and doesn't reach the full static pressure of 4.0bar or whatever which would mean that its not drop tight.

What pressure is it reading now with no demand immediately after water shut off and say 15/20 minutes later still with no demand?.

[Mattg4321]

If it was me I’d drop the pre charge pressure in EV slightly to similar to dynamic pressure. 
 

Then set the PRV static pressure to 2.5 bar, as less than this will likely affect shower pressure in my experience. Forget about setting dynamic pressure. You can’t adjust this, only static pressure (which in turn may affect dynamic pressure). 
 

Turn off main stopcock. Open a tap to release pressure in system. Close tap.  Adjust screw on PRV slightly. Open main stopcock. If pressure is too high still go back to first step. If pressure is too low adjust screw on PRV. 
 

https://www.caleffi.com/sites/default/files/media/external-file/38538_EN.pdf

[steveoelliott]

12 hours ago, John Carroll said:

Suggest setting it to say 2.2/2.3bar with a very small flowrate, this means that with no flow the downstream pressure should still be ~ 2.5bar (static) but should still ensure that the PRV goes fully open at higher flowrates. When set, and with no flowrate, either hot or cold, keep an eye on it eo ensure it doesn't rise above ~ 2.5 bar over a short period and doesn't reach the full static pressure of 4.0bar or whatever which would mean that its not drop tight.

What pressure is it reading now with no demand immediately after water shut off and say 15/20 minutes later still with no demand?.

It reads 4 bar almost immediately after shutting tap off and stays there. Might be a bit more or less depending on time of day etc. it never really creeps up. It is a large 5 bed house with lots of outlets / 4 bathrooms so a big system to flow around.

Edited 1 hour ago by steveoelliott

[John Carroll]

What is the make/model/size of this PRV?.

You said the dynamic pressure falls to ~ 2.0bar, can't remember the flowrate, can you take the PRV upstream pressure anywhere like from a outside tap which will give the pressure loss in the piping up to the PRV.

All PRVs fall off in pressure with increasing flowrate even with a constant upstream pressure, but nowhere near the 2.0bar you are getting but you should try and establish that upstream pressure if not allready done so.