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Thread: Help diagnosing TPR

  1. #1

    Default Help diagnosing TPR


    I am trying to determine the cause of my water heater pressure relief valve engaging. Details:

    50 gallon gas Braford-White (12 years old)

    House water pressure is 60 PSI after reduction by the house pressure reduction valve.

    Before the water heater starts heating the pressure in the tank is 60 psi.

    The heater has an expansion tank.

    Once heating begins the pressure in the tank creeps up to 150 psi and the tpr engages.

    About two cups of cold water is discharged.

    The temperature dial is on the Hot mark with plenty of room to increase the htemperature.

    Could this be a bad thermostat allowing the water to heat too much before it shuts the flame down? I've ben slowly dialing the setpoint back but still am having the problem.

    Any other possibilities or things I should check?

    Thank you in advance for any ideas.


  2. #2
    Moderator & Master Plumber hj's Avatar
    Join Date
    Aug 2004
    Cave Creek, Arizona

    Default T&P

    The expansion tank is not functioning, either because it has lost its air charge or a valve to it is closed, (if you have a valve on the tank).

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


    Or, the bladder in the expansion tank is shot...rap on the expansion tank. It should sound hollow on the end away from the opening to the pipe. My guess is that it is full of water and there is no place to expand as it heats. If the bladder is okay, it needs some air. You need to check the tank with the water turned off and a faucet open to relieve the pressure. You can use a tire pressure guage to see what is in the tank. My bet is that you get water out of the valve because the bladder is shot.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  4. #4


    Thanks for the suggestions. I'll check it out tomorrow and let you know what I found out.

    Thanks again!


  5. #5



    You were spot on. The termal expansion tank sounded like a rock when I tapped on it and water shot out when I put the tire gauge to it. Off to Home Depot!

    Thanks again



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