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Thread: Thermal expansion tank / pressure surge

  1. #1
    DIY Junior Member purduephotog's Avatar
    Join Date
    Dec 2012
    rochester, ny

    Default Thermal expansion tank / pressure surge

    Slowly bringing the home we bought up to code. I'm replacing the wh and installing a thermal expansion tank as per code. I've monitored the water pressure and seen, on average, 60psi. Unfortunately the tattle sometimes is seen at 160 psi. I have no prv and, as far as I can tell, am in an open system. When the water authority comes out to swap the meter I've been told the new one will include a check valve.

    That said, and I'm absolutely terrified of a blown thermal tank (thanks for the horror flood stories) what can I do to minimize the risk. I truly doubt my water pressure is hitting 160psi as the tank isn't weeping. I do not have a continuous logger running.

    The tank is 10 lf or 20 piped feet from the sump.

    I have to install bvent as well, thus cutting down further any space around the heater.


  2. #2
    In the Trades Gary Swart's Avatar
    Join Date
    Sep 2004
    Yakima WA


    As far as I know, having a meter with a check valve and a PRV with a check valve won't cause any problems.

  3. #3
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    New England


    When the expansion tank 'dies', it usually doesn't leak. The flexible bladder inside fails and the tank itself fills up with water. When that happens, in a closed system, the expansion will usually cause the T&P valve on the WH to release some. Depending on how much hot water you used, it may only take a few ounces to reduce the pressure below the trip point of the T&P. Normally, those are set to 150#. If yours didn't release then, it might be defective. You should open it manually to see if it 'sticks' and be prepared to replace it if it does. It is possible you have some other valve(s) in the house that leak when the pressure rises - some toilet fill valves will do this, and the water they release just goes down the drain. Stressing the hoses and fittings in the house is not good - there's a reason code limits the internal pressure in a house. You want a safety margin. On the WH, if you install it with a pan, you can run the drain anywhere you want to help contain the water. you could look at www.wagsvalve.com and install one of these - if the tank leaks, it will shut the water supply to it off. It must be installed with a pan or a dam to retain enough water to trigger the thing.
    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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