Expansion vessel unvented cylinder

[Matt887]

If you go to a prv dripping and cold water expansion vessel has no charge. Do most people recharge? We have to depressorise and remove to do that. So it it easyier to just fit a new vessel ?

 

[Madrab]

Nope - dead quick and easy to re-pressurise with the correct type of pump, though if it is a remote EV (not an in cylinder baffle type) then it should really be done at service time by a qualified Unvented chap.

 

[Matt887]

Thanks yes I have my G3. Have a pump etc. is a remote vessel yes. It needs removing ideally to repressure it

 

[Madrab]

It needs removing ideally to repressure it

As long as you can get the connector from the pump to the top of it then it should be able to be done in situ, I have a stirrup type pump that has a 3m hose on it for exactly that kind of thing. if not then it's a drain down and remove.

Don't forget to balance out the pre-charge to the mains pressure and expansion etc.

 

[Matt887]

Would be 3 bar, as 3 bar pressure relief

 

[Johntheo5]

A stainless steel UV cylinder with its pressure reducing valve set to 3.0bar (with a recommended precharge of 2.8bar) and a EV sized to 10% of the UV cylinder's capacity will have a final pressure of just over 4.0bar if fully heated from 5C to 65C, its expansion relief valve might be typically set to 6.0bar and its T&PRV set to 8.0bar, example, below, of final pressure.

 

[Matt887]

As long as you can get the connector from the pump to the top of it then it should be able to be done in situ, I have a stirrup type pump that has a 3m hose on it for exactly that kind of thing. if not then it's a drain down and remove.

Don't forget to balance out the pre-charge to the mains pressure and expansion etc.

Do you mean if the mains pressure is actually lower then 3 bar you'd charge to around 2 bar instead of 3?

 

[Johntheo5]

Do you mean if the mains pressure is actually lower then 3 bar you'd charge to around 2 bar instead of 3?

Can't obviously speak for @Madrab but IMO the EV diaphragm should never bottom out, this is why a precharge pressure 0.2bar less than the PRV (pressure reducing valve) set pressure, usually, 3.0bar, is recommended, some say it also helps to prevent water hammer as the EV acts like a spring to even out any pressure fluctuations. The dynamic pressure might well drop to ~ 2.0bar at very high flow rates especially during periods of high usage, at certain times of the day. If you set the precharge to 2.0bar and leave the PRV at 3.0bar, then the "worst case" scenario is that say during the night while reheating the cylinder when the PRV pressure might well be 3.0bar is that the final pressure after a full cylinder reheat will be 4.58bar instead of the above 4.05bar. If you know that the static pressure will never be higher than 2.0bar and you set the precharge to 1.8bar and the PRV slightly higher than the static pressure, to say 2.2bar, then the final pressure after a full cylinder reheat will only be 2.81bar. Of course, in all cases, the cylinder pressure will fall to the pressure after the PRV once you run off the expansion volume, almost 5L, if a 250L UV cylinder is installed and pro rata for other UVC capacities.

 

[Madrab]

Totally agree @Johntheo5 - and one of the most common mistakes is the EV vessel either being left at it's factory pre-charge on install or re-filled to the level that's marked on the side without though to the actual system pressure.

That can then sometimes create issues with over-pressurisation and that can create water hammer (as suggested) the infamous resonance horn noise and issues with the Pressure Reducing Valve

 

[Johntheo5]

Unfortunately I cannot post a excel file on here but anyone interested can build their own spreadsheet from the screenshot, then just change the values in yellow to suit their own requirements.

 

[Johntheo5]

Was able to send it as a zipped file, so just open it up