DHW Cylinder Expansion Vessel Pre-Charge

[belfastbob]

Hi guys

I have a quick question regarding the correct pre-charge pressure for an expansion vessel (24 ltr) on the cold supply of a DHW cylinder (300 ltr).

As measured after a combined 3 bar PRV + 6 bar safety relief valve assembly, the static pressure of the cold supply is 2.5 bar. The dynamic pressure is 1.7 bar with a few cold taps open.

What pressure should the expansion vessel be charged to when it is disconnected from the cylinder?

Rest assured, I have no intentions of recharging the vessel myself as I am not qualified to work on unvented cylinders, but it seems every plumber I ask has a different answer. Some say charge it to the PRV limiting pressure, others say charge it to the dynamic (working) pressure of the cold feed.

So, can some kind (competent) chap on here provide me with the genuine answer?

Thank you in advance

Bob

 

[Steelmasons]

Vessel pre-charge pressure should be 0.2 bar below static cold fill pressure.

In your case........(8 bar tpr/6 bar prv)

Volume in vessel when cold = 1.3 litres
Volume in vessel when hot = 12 litres
Pressure in cylinder/vessel when cold = 2.5 bar
Pressure in cylinder/vessel when hot = 5 bar

Above is based upon a supply temp of 10c/max HW temp 70c.

I take it the "3 bar pressure relief valve" to be a typo error? ......it not then vessel size should be in excess of 100 litres.

 

[belfastbob]

Vessel pre-charge pressure should be 0.2 bar below static cold fill pressure.

In your case........(8 bar tpr/6 bar prv)

Volume in vessel when cold = 1.3 litres
Volume in vessel when hot = 12 litres
Pressure in cylinder/vessel when cold = 2.5 bar
Pressure in cylinder/vessel when hot = 5 bar

Above is based upon a supply temp of 10c/max HW temp 70c.

I take it the "3 bar pressure relief valve" to be a typo error? ......it not then vessel size should be in excess of 100 litres.

Hi steelmasons

No typo. The tank is a Danfoss DWH-300. It comes with a 24 litre expansion vessel. The spec is (typed from the datasheet)

Max water supply pressure 10 bar
Pressure reducing valve 3 bar
Expansion valve 6 bar
Expansion vessel charge pressure 3bar
Operating pressure 5.5 bar
Temp & pressure release valve 10 bar, 90-95 deg

This spec seemed pretty standard for the tanks I looked at.

My question was more about what to do if my mains static pressure is always less than 3 bar (so the PRV ends up doing nothing) should the expansion vessel pressure be reduced to match? Or should I actually fit a lower pressure PRV and then charge the vessel to match that?

 

[Steelmasons]

So a 3 bar pressure reducing valve as opposed to a pressure relief valve.

Had you of mentioned an expansion valve in your opening thread then there would be no cause for confusion.

Dynamic pressure means diddly squat when sizing vessels.

The above precharge pressure still stands.

 

[belfastbob]

So a 3 bar pressure reducing valve as opposed to a pressure relief valve.

Had you of mentioned an expansion valve in your opening thread then there would be no cause for confusion.

Dynamic pressure means diddly squat when sizing vessels.

The above precharge pressure still stands.

Sorry for the confusion - I always thought PRV was Pressure Reducing Valve