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

 

[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?.

 

[John.g]

 

[Matt0029]

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?.

Could the expansion stop it falling by 0.4bar when the pump stops? I will do a sketch.

 

[John.g]

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

This would indicate that the EV is on the pump discharge and the PG on the pump suction since it drops in pressure but IMO it should rise again by the same 0.4bar once the pump stops irrespective of the static pressure, hot or cold.

 

[Matt0029]

This would indicate that the EV is on the pump discharge and the PG on the pump suction since it drops in pressure but IMO it should rise again by the same 0.4bar once the pump stops irrespective of the static pressure, hot or cold.

It doesn't rise again after the pump stops. To does rise after the 0.4bar drop relatively quickly. Just conserned with it dropping on start up that if the system lost abit of pressure. Cold cut the boiler out on start up due to the pressure drop when the pump starts. Thanks.

 

[John.g]

Boilers cut out pressures are normally 0.5/0.6 bar so depending on where the EV and the boiler and pump are located and if the cold filling pressure is 1.0 bar or lower then it possibly could with a cold system, EVs are generally pre pressurized to 1 bar and filled to 1.5 bar so well away from this.
If the boiler has its own pressure gauge watch this on pump start up and see what the actual pressure is. I would assume that if any boiler trips on low pressure that it will annunciate same and have to be reset to enable boiler restart after system top up.