Expansion vessel pre charge

[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?