Expansion vessel pre charge

[Matt0029]

I have a 35litre central heating expansion vessel that has come with a pre charge of 1.5 bar. Does this sound a little heigh? As the pressure in the system will have to rise above 1.5bar for it or absorb any expansion? Thanks

 

[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?

 

[ShaunCorbs]

Correct as it’s not designed for pressure always open to atmosphere eg like a vented cylinder

 

[John.g]

I thought a vented tap was for use with a unvented HW heater and should only leak/drip while the water is heating up and could be used in lieu of a expansion vessel but obviously not.

 

[ShaunCorbs]

Correct but it’s not unvented if it’s open to atmosphere all the time

open the tap water going into the heater and comes out of the tap at the same time so no pressure in the heater

 

[John.g]

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

Any calcs with air (or any gas) pressures involved must be done in absolute (a) units, absolute pressure is Gauge Pressure (g) + 1 (bar) you then subtract the 1 to return to gauge pressure. Atmospheric pressure is 0 Bar.g or 1 bar.a

 

[Matt0029]

I thought a vented tap was for use with a unvented HW heater and should only leak/drip while the water is heating up and could be used in lieu of a expansion vessel but obviously not.

Same, hence why this forum is great for building up knowledge.

 

[John.g]

See attachment.

 

[Matt0029]

Can I also ask what happens to the pressure when the pump kicks in as the gauge drops by 0.4bar (ish).

 

[ShaunCorbs]

Nothing other than the vessel being on the suction side of the pump

 

[Matt0029]

Nothing other than the vessel being on the suction side of the pump

Thanks so it takes awhile for the pressure to stabilise again once the pump has started? When the pump stops the pressure doesn't jump back down 0.4bar.

 

[John.g]

This is the way its supposed to work.
If the expansion vessel is on the pump suction and near the pump, the pump suction will be at the EV pressure and the pump discharge will be at the EV pressure + the pump head. If the EV is on the pump discharge and near it then the pump discharge will be at the EV pressure and the pump suction will be at the EV pressure - the pump head.
Its recommended the EV is positioned at the pump suction as the whole system will run with a higher average head.

One would think that the pressure should fall by 0.4bar (the pump head) when it stops, can you show a sketch of layout of pump, EV and pressure gauge location?.