Hwc valve low preesure valve vented

Hi there
I am a apprentice in my final year of training
there has been a bit of an arguement of late at my work in regard to valve vented low pressure hwc's.
now in pic 1 is a open vented hwc.
in pic 2 is a valve vented low press hwc
the arguement is that the valve vented hwc relief valve must be a lower pressure than the reducing valve
(e.g 76 kPa PR feed valve and a 32 kPa relief valve) so the pressure at the bottom of the hwc is 76 kPa but that would make the pressure at the top near the relief valve 32 kPa? or will the whole system be set at 36 kPa due to the relief valve setting?
i have been told that you must put a 76 kpa relief valve so that the whole system will be set at that pressure, but others say no especially if the relief valve is on the roof like in the drawing (quite common in my area but i am sure this is illegal to do so) that the height of the vent pipe will add more presuure, but i know head is truely only in effect if it is open to atmosphere.
i just want clarification of this question as it has been the major subject to debate at smoko for a long time and i would like to clairfy this for my own practice as i dont want to be a bad plumber and blow someones hwc up but not following the basic principals of plumbing.

cheers guys for the help

Ps why is there no apprentice section in this forum?

pps sorry if my post is a bit messy

Hi Benson, if you want exact clarification to your question you should contact Apex Valves and talk to the people that make the systems. My spin would be that most low pressure cylinders are rated to: 25ft/7.6mtr head max pressure so the only danger to these cylinders is if you were to fit a 12.2mtr medium pressure relief. You are right about relief valves fitted to roof exhaust pipes not being totally legal but as long as you cut the pipe down to just above roof level and insulate with waterproof lagging you should be right. The pressure reducing valve will always be at a lower pressure to the relief valve or the relief would run continuously, then when the pressure builds up because of expansion, water would be relieved. In any case you should always be fitting a cold water relief valve in conjunction with your new system that way the hot water relief will almost never get to relieve as the cold one will go off first. Hope this info is off some use to you, cheers.

Hi Benson,I am also at the end of my apprenticeship and was also confused by this problem you have and from what I can gather the Pressure Reducing Valve(pr) and Vacuum Pressure relief valve (vpr)should be matched
Eg : 37kpa prv with 37 kpa vpr AT CORRECT INSTALLATION HEIGHT which is at least 150mm below the top of the HWC Now here is where i think people get caught out If you have an exhaust pipe that is already 4.0m high then in efect by putting a vpr on top you are adding another 37kpa at the top of  4m Because we know that 37kpa is approximately 3.6m of head so is would be the same as adding that much more pipe .

Now if the valves were rated at 76kpa (or higher)in the above situation  you you could be in for hwc damage especially in older cylinders   .

Also under rating the PR valve and winding the screw right in will cause premature washer ware .They make  three basic models for a reason  Cheers ali

p.s my post is messy too

p.p.s Cheers Plumber for making an NZ plumbers forum