hotwater system issue

Hi     I have a Hardie Dux HDE250 electric hotwater system, 250L. installed 1993. I have had no problems with it except the the Pressure reducing valve was leaking water so I replaced it. The hot water system accepted max of 680kpa input pressure, the old valve was 0.53l/s at 700kpa, the new valve is 0.50l/s at 500kpa. A plumber who sold me the pressure reducing valve told me the old one was too high(700kpa), and said 500kpa was better for my hotwater inlet. Fine, I replaced it, no more leaking water, but now I have a few more problems since I did this. First night I took a shower the hot water ran out after 2-5mins and took a long time to recover. I then tried again the next day, then it was fine, no problems for 2-3weeks. Till last night, the Temperature Pressure Relief Valve (TPR valve) that goes from heater to drain was making a constant hissing noise all night and the Discharge Tube was hot. I got up the next day to take a shower and the hot water only lasted for 2-5mins and took a long time to recover.
Is he TPR valve stuffed? Are they expensive? The Pressure reducing valve cost me $65 (AU). The Discharge Tube is "always very hot" even when the heater was working fine , should this be the case? If the TPR valve is stuffed, how do I replace it with out burring my self, do I turn off the water mains and hot water power and drain the water from the heater by opening the TPR valve till the pressure in the heater tank goes down? Could this be due to the change of the Pressure reducing valve? I don't see anything on the heater were I can adjust the temperature of the water.
thanks caven
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
caven wrote:

(Almost everyone on here is US, but we'll take a stab.)
250L is a rather large storage tank heater. As the water heats, it expands and attempts to increase the pressure in the tank. Some reducing valves have an internal "bypass" feature which may relieve the excess pressue. Your new one may not or it may be ineffective for one reason or another. So tank pressure builds up until the TPR relieves it. So the TPR is not at fault.
In the US, we use an expansion tank like the XT model here: http://www.zurn.com/pages/catalog.asp?ProductGroupID &OperationID
Codes here mandate an expansion tank anytime there is a reducing valve in the system or check valve or city mains pressure is high.
(I'm guessing that thermal expansion is causing your problem, but the symptoms sound like it.)
Jim
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.