How do you set a Pressure Reducing Valve ? Static or Dynamic Pressure ?

4 Jan 2008
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United Kingdom

I was wondering if some one could help me ?

My house has an OSO Super S Unvented Cylinder 300L which is designed to operate at 3.5 Bar.

The mains cold feed to the tank is fitted with a Pressure Reducing Valve. When all the taps in the house are closed the pressure gauge reads 2 Bar. I understand that this is the static pressure reading. This pressure drops when a tap is opened.

I would like to increase the pressure of the water going to the unvented tank.

What I am confused about is whether I set the PRV to have a static pressure of 3.5 Bar or a dynamic pressure ( taps open ) of 3.5 Bar ? I understand that if the dynamic pressure was set at 3.5 Bar then the static pressure may well be higher.

Can any one please advise ? Thanks in advance.
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I would like to increase the pressure of the water going to the unvented tank.
If the pressure of the incoming water supply from the street is 2 bar then that is the maximum you will get in the house. It would require a pump to increase the pressure.
Hi Yes I understand however I know that the mains pressure is much higher - the kitchen cold water tap ( for some reason ) is T'd off before the PRV.
I appreciate that what you wish to do is a relatively straighforward adjustment but unfortunately an unvented cylinder has several safety features, the PRV being one of them. Due to that fact that the ramifications of interfering with these safety features could be explosive, can I recommend that you get a qualified UVHW (G3) qualified plumber in to alter them?

That being said, if it is the Super S 300L then the settings is in the MI. That and the PRV should be factory set. May be an idea to check the strainer too.

Have you taken a pressure reading @ the kitchen tap? How do you know that pressure is higher?

Bernard, FYI, if the PRV gauge is on the valve then that would normally show the modified outlet pressure, not the mains inlet pressure.
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If adjusting the pressure reducing valve it is best to do it at a very small flow rate from a tap of about 3-5 litres per minute.

To work properly they need a headroom of an extra 1/2 bar pressure at the inlet.

Ideally when adjusting/checking the inlet pressure should be monitored as well.


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