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Thread: DHW dilemma

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    DIY Junior Member nap's Avatar
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    Post DHW dilemma

    PRV on my hotwater tank is opening regularely and the pressure on the hotwater side of my taps is very low at times any comments
    Last edited by nap; 01-27-2008 at 02:16 PM. Reason: spelling error

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    DIY Junior Member nap's Avatar
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    PRV opening all the time on my hotwater tank and water pressure on hot water side at taps is very low at times

    nap

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    Retired Defense Industry Engineer jadnashua's Avatar
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    The T&P can open if it gets old, but it is designed to open if the pressure is excessive or the temp gets too high. You can measure both fairly easily. Run the hot until it is as hot as it gets into a cup or glass with a thermometer in it and see what it is. You can buy a screw on pressure gauge for about $10 - you can put in on a hose bib, washing machine supply, utility sink, drain line from the WH, or anywhere with adapters. Some have a high pressure hold.

    If you have an expansion tank, it could be shot and needs replacing. This will allow the pressure to rise as water in the WH expands. The cold water is denser, when it goes into the tank and then gets heated, it expands with no place to go creating quite high pressures. You could have this situation if you have a check valve in your incoming water lines or have a PRV.

    Now, if it is the PRV that is leaking, best to replace it.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Junior Member nap's Avatar
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    What do you mean by T&P when I am referring to PRV I am saying pressure reliefvalve are we referring to the same thing?

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    Retired Defense Industry Engineer jadnashua's Avatar
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    PRV = pressure reduction valve
    T&P = temperature and pressure relief (safety) valve
    A WH has a T&P. You may or may not have a PRV installed in your house. If you do, and your expansion tank is bad (you must have one, if you don't, get one installed if you have a prv), then the house pressure will rise when you use hot water after you shut off the water. The cold that replaces it expands and has nowhere to go except through the weakest point, which may be the T&P valve.

    PM's don't get you answers quicker! They may not get answers at all. PM's are for private questions - exchange of info not suitable for public consumption, not requests for answers on the board.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    In the Trades Gary Swart's Avatar
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    No, these are two separate things. The PRV keeps the whole house pressure from being too high. However, what many Big Box Stores don't tell you is that a PRV creates a closed system and requires an expansion tank. When the water heater heat water, the water expands. That's basic high school physics. This water has to go somewhere or the pressure in the water heater will rise dramatically unless there is an expansion tank to capture the expanded water. If you didn't have a PRV, the expansion would be absorbed by the city water main with no problem. Now if you have a PRV and no expansion tank or one that is malfunctioning, the pressure in the tank rises high enough to trip the T/P valves. This is that gismoe on top of the tank that has a pipe running to the floor. It's purpose is to prevent the pressure or temp exceed the safe limits. I don't know what you mean about the PRV opening. A PRV is located in the main water supply line, usually right after entering the house. If what you are referring to as a PRV is the TP valve, and you have no PRV, then replace the TP. They cost about $15 at any hardware store or plumbing supply.

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