Its 3.7 from the valve to normal standing water level at the top of the vent (as per image). When you are using water the level is from the outlet (spout, shower rose etc.)  to normal standing water level at the top of the vent. This is usually less than 3.7 I have attached an image if that shower rose is 3 meters away from normal standing water level at the top of the vent then your shower pressure will be 29.43 KPA (Gravity is 9.81m/s2 at sea level, so an easy approximation is: 1 metre of head = 10kPa)

I think Newguy is referring to a pressure reducing valve and not pressure relief valve. Usually vacuum relief valves (pressure relief valve) are only installed when a vent is not possible to short or inconvenient. If you add a 3.7 relief valve to the top of the vent then Yes you are adding another 3.7 KPA from that point on. the 3.7 relief valve will not open unless more than 3.7 head is applied to it. At that point the system at the lowest point will have over 72.59 KPA.

hi guys, Plumber says:-If you add a 3.7 relief valve to the top of the vent then Yes you are adding another 3.7 KPA from that point on. the 3.7 relief valve will not open unless more than 3.7 head is applied to it. At that point the system at the lowest point will have over 72.59 KPA.
 Has anyone tested this argument? I believe that once that a pressure relief has been installed the system is no longer under the influence of atmospheric pressure and so the pressure in the system would be whatever the pressure valve is set too and equal throughout, a pressure gauge at the top and bottom of the system would prove this theory one way or the other, has anyone got proof, might have to do it,cheers

just a quick question on the pressure relief valves, is the kpa rating calculated from releif  valve installation height and above or from just from the bottom of the hwc, eg if a 3.7m relief valve is on the hwc vent on the roof is the effective head 3.7m or the height of the vent pipe above the highest outlet in the house plus the 3.7m relief valve?

I measure from the base of the cylinder because all cylinders have a max kpa /pressure rating...if you have  a vent/stand pipe 3 metres above the cylinder then thats loosley an extra 30 kpa(10 kpa per meter) on the base including cylinder height and then if you add  a 3.6 PRV then you are not adding any extra weight to the standing pressure BUT once the element kicks in it will add an extra 36 kpa to the cylinder before any pressure is relived so that would be cylinder height + height of stand pipe + the rating of the prv installed on the top of the vent/stand pipe = the pressure the cylindre will be under when working and if its below the cylinder rating pressure all good no damage should be done...but be aware if it is an older cylinder and inform the coustmer that because of age the cylinder,it could recieve damage if the pressure is upped on the cylinder this should relieve you of any responsibility if the older cylinder ruptures after installing a prv valve on top of the stand pipe.

This is a Heineken reply so i take no responsibility for any information given lol.

hi guys, made no sense to me,cheers

replaced my grandfathers original cylinder in the family crib a few years back - it was original to the property that he built in the late 50's? maybe early 60's

it was a very nice quality premium thick guage copper jobbie - was just a bare cylinder with a blanket around it!!

and it was still operational when taken out! only taken out due to renovation - I remember working out that it had just ticked over 50yr old at the time!!