Expansion vessel pre charge

[ShaunCorbs]

Normally 0.2 bar below operating pressure matt

 

[John.g]

Can I also ask what would happen if the system was filled to 1 bar cold. And the expansion vessel was left at the pre-charge of 1.5bar? Guessing it wouldn't start using the expansion till it gets to 1.5 bar. So the pressure would rise quiet alot?

No, since water is virtually non compressible it would almost immediately rise to 1.5 bar (after heating up a few degrees) and would behave exactly the same as if the pre charge and filling pressures are the same ie 1.5 bar.

If the system capacity/precharge/filling are 400ltrs/1.5bar/1.5 bar the final pressure will be ~ 2.16 bar with a 35 litre EV.

If the system capacity/precharge/filling are 400ltrs/1.5bar/1.75 bar the final pressure will be ~ 2.57 bar with a 35 litre EV.

If the system capacity/precharge/filling are 400ltrs/0.75bar/1.0 bar the final pressure will be ~ 1.63 bar with a 35 litre EV.

 

[Matt0029]

No, since water is virtually non compressible it would almost immediately rise to 1.5 bar (after heating up a few degrees) and would behave exactly the same as if the pre charge and filling pressures are the same ie 1.5 bar.

If the system capacity/precharge/filling are 400ltrs/1.5bar/1.5 bar the final pressure will be ~ 2.16 bar with a 35 litre EV.

If the system capacity/precharge/filling are 400ltrs/1.5bar/1.75 bar the final pressure will be ~ 2.57 bar with a 35 litre EV.

If the system capacity/precharge/filling are 400ltrs/0.75bar/1.0 bar the final pressure will be ~ 1.63 bar with a 35 litre EV.

Thanks I was going to go with one bar system pressure cold. And a pre charge of 1 bar. This should go up to around 1.75bar did you say?

 

[Matt0029]

Normally 0.2 bar below operating pressure matt

Ah so I should drop the pre charge to around 0.8. But then the expansion vessel will use some of its storage even when the systems cold? Interesting thread. Didn't no the ins and out of pre charge pressures etc. Just thought was abit high at 1.5 bar on the vessel bought.

 

[Matt0029]

Yes, the advantage is that you have a reserve volume of water to make up for any air venting, micro bubbles collapsing or any tiny leaks without the system pressure falling below your minimum requirement of 1.0 bar before top up is required, you can also monitor for any leakage as the pressure will fall from 1.5 bar cold to 1.0 bar before exhausting this reserve.
If you take a fairly standard CH system of 75 litres with a 12 litre EV pre charged to 1.0 bar and filled to 1.5 bar then you will have a reserve volume of 2.4 litres and a final hot pressure of 1.92 bar, if both pre charge and filling pressure are 1 bar then no reserve and if any slight leakage/venting etc the cold pressure will start falling immediately, but yes, the reserve does reduce the amount of expansion available as the final pressure with no reserve will be 1.26bar (vs 1.92bar), any pressure up to 2.25/2.5 bar is not a problem for any system, one can always install a slightly bigger EV if any worries re final pressure.

I think that answers my above question.

 

[John.g]

Ah so I should drop the pre charge to around 0.8. But then the expansion vessel will use some of its storage even when the systems cold? Interesting thread. Didn't no the ins and out of pre charge pressures etc. Just thought was abit high at 1.5 bar on the vessel bought.

If the system capacity/precharge/filling are 400ltrs/1.0bar/1.0 bar the final pressure will be ~ 1.53 bar with a 35 litre EV......no reserve

If the system capacity/precharge/filling are 400ltrs/0.8bar/1.0 bar the final pressure will be ~ 1.61 bar with a 35 litre EV......3.5litre reserve

If the system capacity/precharge/filling are 400ltrs/0.8bar/1.5 bar the final pressure will be ~ 2.52 bar with a 35 litre EV......9.84litre reserve

As you say, you can see above that the reserve of 9.84 litres (filling pressure of 1.5bar) eats up more of the expansion which results in a final pressure of 2.52bar vs a filling pressure of 1.0bar which results in a reserve of 3.5litres and a final pressure of 1.61bar.

You can do your own calculations, below. No, you can't, cannot attach a zipped file.

 

[king of pipes]

0.8 to I bar will be ok you should over size the expansion vessel by 25,% for safety but hey ho crack on . Kop

 

[Matt0029]

I have also got a little 2 litre vessel to fit for a 7litre under sink heater. That has come. With a pre charge of 1.9bar. Surely this should be 3/3.5 bar? As mains pressure after the reducing valve to the under sink heater will be around 3 bar.

 

[ShaunCorbs]

Could be a 2 bar prv

 

[John.g]

If the pre charge is left at 1.9bar then even with a PRV pressure of 3.5 bar the final pressure will be 4.0 bar, if the precharge is increased to 3.5 bar the final would be 3.81 bar with PRV of 3.5 bar.
You will also require a safety (PRV) valve of 6.0 bar.

Around here AFAIK a E.vessel is not required for water heaters up to 10 litres, the expansion is taken up by the mains, but the nearest cold water draw off must be a minimum distance from the heater. (can't remember what this is).
As you have a PRV (pressure reducing) installed you probably still need the E.vessel as the PRV will act as a NRV.