The water heater pressure relief valve is leaking when it's doing its job of letting some water to escape to lower the pressure in the system. You must have a backflow preventer on your house cold water intake line and so as water heats and expands and all the faucets are off, it has nowhere to go but out through the pressure relief valve. I think the first and easiest thing to do would be to lower the temperature setting for the water heater. See if you can find the setting that gives you a satisfactory amount of hot water yet does not pop the valve. If you can't find one, you'll need to install an expansion tank between the backflow preventer valve on the cold water line and the water heater. It is also possible that the pressure relief valve itself is bad and needs to be replaced. It was probably holding before you tested it only because the plunger got suck or calcium deposit "helped" to get it stuck. You've moved it and it's now free to, well, leak. It's just a conjecture though, you'll just have to start from lowering the temperature, since it's the easiest thing to do, and then go over all those possible remedies to know for sure.
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11 years ago