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 8 hours ago 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’.