Several folks who have passed the 5-year mark are noticing that their gas hot water tanks are beginning to pass water. In most cases, the mini-expansion tank rather than the pressure relief valve has failed. Some folks have this expansion tank on the inlet side (cold side) of their hot water tanks. Others have it on the outlet (hot side). I've been told the correct side is the cold side. A replacement expansion tank sells for $35-$40. If mine were on the wrong side, I'd probably leave it that way and simply replace the expansion tank. It's a cheap, relatively easy fix if you know what you're doing. Moving the location of the expansion tank on the other hand is not quite as easy, and I could see a plumber charging $200-$250 to make the switch but simply replacing an expansion tank in the same place shouldn't cost that much.
The expansion tank has a rubber bladder inside which traps about 10 psi of air. When the bladder fails the tank fills up with water. The weight of a full expansion tank begins to pull down the pipe. If you see a deflection in the pipe with the expansion tank, that's another indication that it's time to replace this expansion tank.
George
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