Expansion vessel pre charge

[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.

 

[Matt0029]

Thanks but would the vessel use all its expansion before its needed when the water heats up? If left at 1.9bar?

 

[John.g]

You can see the various air space and water volumes below in a fairly basic spreadsheet.
If you wish, I can show you how I do these calcs.

System or HWC capacity 7.00 Litres Vessel Capacity 2.00 Litres Pre Charge Pressure 1.90 Bar.G Pre Charge Pressure 2.90 Bar.A Filling/supply Pressure 3.50 Bar.G Filling Pressure 4.50 Bar.A Air Space Vol 1.29 Litres Reserve Water Vol 0.71 Litres Expansion % 1.83 % Expansion 0.13 Litres Water Space Vol 0.84 Litres Air Space Vol 1.16 Litres Final Pressure 5.00 Bar.A Final Pressure 4.00 Bar.G

 

[John.g]

Its recommended that the pre charge pressure is set 0.2bar lower then the cold water pressure at the cylinder, shown below, as it keeps the diaphragm away from the cylinder end and can help to prevent water hammer. In your case because the EV volume is huge (almost 30%) in comparison with the HW volume then the difference in final pressures between 1.9/3.3bar precharge pressures is only 0.2bar.

System or HWC capacity	7.00	Litres
Vessel Capacity	2.00	Litres
Pre Charge Pressure	3.30	Bar.G
Pre Charge Pressure	4.30	Bar.A
Filling/supply Pressure	3.50	Bar.G
Filling Pressure	4.50	Bar.A
Air Space Vol	1.91	Litres
Reserve Water Vol	0.09	Litres
Expansion %	1.83	%
Expansion	0.13	Litres
Water Space Vol	0.22	Litres
Air Space Vol	1.78	Litres
Final Pressure	4.82	Bar.A
Final Pressure	3.82	Bar.G

 

[Matt0029]

So with the 2 litre expansion vessel having a charge of 1.9bar it won't really effect its performance in this case? The ones usually get are silver with a pre-charge of 3.5 bar I believe. The standing pressure of the system is 4 bar. No pressure reducer is currently fitted. I'm adding controls to a little under sink heater that at the moment has a vented tap that is broke. And a replacement is around £100. So cheaper to add a 6 bar pressure relief below the sink and expansion. Cold water draw off too close to allow expansion in to the mains

 

[Matt0029]

Can I ask what is ment by bar A/ bar G pre-charge?

 

[ShaunCorbs]

It’s a drip tap so you can’t install expansion vessel as the unit isn’t rated for pressure it’s open vented

what’s broken on the tap ?

 

[Matt0029]

It's a little 7 litre unvented cylinder. Its fed of the mains. It makes a racket when used have stripped down and re-grease to no luck. Or no luck till some one turns it too tight again.

 

[Matt0029]

That's what's on it

 

[ShaunCorbs]

That’s the thing it’s not unvented it’s gravity / equal pressure

the heater isn’t under pressure it’s always open to atmosphere hence when it’s heating the tap will drip most people try and tighten it down more which just breaks them

 

[Matt0029]

That’s the thing it’s not unvented it’s gravity / equal pressure

the heater isn’t under pressure it’s always open to atmosphere hence when it’s heating the tap will drip most people try and tighten it down more which just breaks them

Thats what's happened yes. So a pressure relief and expansion valve under the sink won't work on this heater?