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Water heater pressure relief valve

Coqui98 | Posted in General Discussion on December 28, 2009 02:07am

To reduce my gas bill I lowered the temperature on my water heater but shortly after the pressure relief valve began to release water. I took out the old valve and found a fair amount of deposits so I replaced the valve. 20 minutes later the new valve is also releasing water. Does anyone have an idea of why a lower temperature setting should cause the valve to release pressure?

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  1. rdesigns | Dec 28, 2009 03:25pm | #1

    First, the simplest Q: are you sure you lowered the temperature?

    Some of the new thermostats are confusing because they say "Hot" and "Hotter", so I've known at least one case where the owner saw "Hot", and moved it the opposite way, thinking that would lower the temp.

    Of course, you would probably have noticed if the water was abnormally hot, but it never hurts to start eliminating possiblities.

    If that checks out,then maybe the change to a lower temp. has resulted in your nearly emptying the tank at times because you're mixing less cold water at the faucet. This means that the tank must now recover a full tank up to temperature from dead cold. When that happens, there is a greater amount of expansion than previously, and if your water heater has no expansion tank (that's a small, bucket-sized tank that would be mounted near the water heater itself), then the expanded water has no place to go, thereby creating excess pressure that is relieved by the relief valve.

    If that's happening, you need an expansion tank, because relief valves are not designed to function as an operating device--they are for emergency relief. Letting it relieve on a regular basis will cause hard water deposits to build on the seat of the valve and possibly seize it shut--very dangerous.

    Lastly, though unlikely, it's not impossible to have two bad relief valves in a row.

    OMT: if the tank is heating from dead cold, a gas water heater will condense water from the flue gases when they contact the cold surface of the tank--to eliminate that possibility, place a shallow bowl under the relief drain pipe to see if the water is actually coming from the relief valve.

    1. User avater
      rjw | Dec 28, 2009 05:54pm | #2

      >>This means that the tank must now recover a full tank up to temperature from dead cold. When that happens, there is a greater amount of expansion than previously, and if your water heater has no expansion tank (that's a small, bucket-sized tank that would be mounted near the water heater itself), then the expanded water has no place to go, thereby creating excess pressure that is relieved by the relief valve.

      Only if there is a check valve between the city supply and the system (or if it's a private system)

      If that's happening, you need an expansion tank, because relief valves are not designed to function as an operating device--they are for emergency relief. Letting it relieve on a regular basis will cause hard water deposits to build on the seat of the valve and possibly seize it shut--very dangerous.

      >>OMT: if the tank is heating from dead cold, a gas water heater will condense water from the flue gases when they contact the cold surface of the tank--to eliminate that possibility, place a shallow bowl under the relief drain pipe to see if the water is actually coming from the relief valve.

      Or just listen for the sizzling sound as the condensate in the internal flue drips down on the hot burner....

    2. DanH | Dec 28, 2009 07:28pm | #3

      It's also possible that the
      It's also possible that the thermostat has gone bad.

    3. Coqui98 | Dec 30, 2009 04:03pm | #5

      Thanks guys this is all very helpful. I do have an expansion tank and I verified the targeted lower temperature with a thermometer. Now, I'll raise the temperature in stages to see what happens. If it works I'll let you know.

      1. PatchogPhil | Jan 02, 2010 01:31pm | #6

        The expansion tank *could* have coincidentally become water logged and no longer buffer the pressure build up.

  2. User avater
    plumber_bob | Dec 28, 2009 09:34pm | #4

    I'd say that if the vessel
    I'd say that if the vessel itself is not leaking, then it's repairable.

    But,... if you live anywhere close to Louisville KY., then it's probably shot, and I'll come and take it off your hands. And you should replace it with a new one. hehehe

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