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Thread: Do I need an expansion tank, new PRV or both???

  1. #1

    Default Do I need an expansion tank, new PRV or both???

    Hello all:

    This is truly a wonderful forum! I have been reading it for over a day and have come across enough info on expansion tanks and PRVs to make my eyes crossed...but I haven't found an answer to my question. Maybe one of you gurus can help.

    I changed the T&P valve on my water heater yesterday. The water heater is only three years old, but the valve became stuck and I had a constan flow of about a gallon per hour coming from my tank. When I inspected the old valve, it looked like some sediment got in there and kept it from closing shut.

    When I took the valve off yesterday, I went ahead and flushed the tank to remove the sediment. Then I tried to figure out why my T&P valve was popping. (evidnece of water in the crawl space leads me to believe I've been getting discharge for a while).

    I have a PRV installed just off the main line where it comes into my crawl space. Just beyond that, I have a hose bib where I placed a pressure gage. After the hot water tank was turned back on and full, I was taking pressure readings of 100 psi. As soon as you opened a faucet, however, the reading dropped to 60 psi. Once the faucet was closed, it was back to 100. We have always noticed a burst of pressure in the house as soon as the water is turned on, but it quickly reduces to a nice, comfortable flow.

    I started tinkering with the PRV and was able to increase and decrease the flow pressure, leading me to believe that the valve is working. But as soon as the flow was sut off, I always came back to 100 psi.

    Could thermal expansion cause the pressue to reach 100 psi? I have found some information here, and at other websites, that indicates my problem could be fixed with the installation of an expansion tank. Or does it sound like a faulty PRV? Or both?

    Another thing to point out is that I hung a lightweight bucket from the new T&P valve I installed yesterday instead of hooking up the drain pipe in order to monitor discharge. I had about two cups of water in the bucket when I woke up this morning.

    Thanks in advance for the resposne.

  2. #2
    In the Trades Gary Swart's Avatar
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    Thermal expansion can reach much higher psi without an expansion tank. A T/P is supposed to hold up to 150 psi and without an expansion tank they will usually trip. I agree that you PRV is probably still good, but you need an expansion tank.

  3. #3
    Master Plumber Redwood's Avatar
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    I disagree the PRV is shot!
    It should not allow the pressure to climb to 100 once the flow is shut off.

  4. #4
    In the Trades Gary Swart's Avatar
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    I yield to the real pro! Get a new PRV and an expansion tank.

  5. #5

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    That's what I was affraid of. Even if the PRV wasn't bad, everything I've read seems to indicate that I should have already had an expansion tank installed since I'm on a closed system. Well...looks like expansion tank instalation this weekend and PRV next. I can't wait until spring.

    Thanks for the helpful advice. I really appreciate it.

  6. #6

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    Well...we installed the expansion tank on Sunday and, knock on wood, I have not had any discharge from the T&P since. Next step is to install a new PRV. Thanks for the help!

  7. #7
    In the Trades Gary Swart's Avatar
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    Glad it's coming together. Give us a final report.

  8. #8
    DIY Junior Member hidden 1's Avatar
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    happy its holding thru for you..I installed prv and expansion tank and its doing fine .i just need to readjust the air pressure in tank ..i think its 5psi under pressure coming to water heater if im correct?

  9. #9
    Retired Defense Industry Engineer jadnashua's Avatar
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    You'll get varying opinions on the proper pressure...I think the manufacturer specifies set to the incoming pressure. WIthin a couple of pounds of that and you should be okay.
    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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