I have an auxiliary water heater, 110V electric, in a master bedroom that's about 50 feet from the "hot water core" of the house. It's worked perfectly for nearly two years, but now has started venting regularly from the popoff valve. (Which made a mess, initially, as I hadn't bothered to install a proper drainoff line...)
I've fixed and tried everything and it's still occurring; anyone who has a clue as to why is most cordially invited to share a clue with me. :)
1) It's a 2.5 gallon 110V electric that feeds only the sink. 2) It was recently reinstalled with tidier plumbing in a remodel - the first round was kind of hacked and slashed into the existing cabinet. 3) Both the cold tap and the heater pull from the cold water line; the existing hot supply line is capped. 4) It worked perfectly for a year before, and for a couple of months after the remodel. 5) The temp was set moderately, about 125. I've tried cycling the temp setting back and forth to free any stickiness and set it down to 110. 6) I flushed the blowoff valve several times. 7) I emptied some hard water scale and grit from it while replacing the blowoff valve. 8) I finally switched the fraggin' thing off for a couple of days.It continues to vent, about a cup at a time, several times a day. It's apparently unrelated to the heating function, since it's still doing it with the power off. I can't match the blowoff to any other event in the house (showers, other water use, etc.) And the main heater isn't doing it.
Either I got a bad valve with the new one, or there's some sort of surge in the lines exceeding the 150 psi blowoff point. Or...?