Expansion vessel pre charge

[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?.