A friend of mine called me a couple of weeks ago with a slow intermittent leak at the TPRV discharge pipe. I checked the water heater temperature and pressure, both were normal. There is no PRV or check valve on the main line, and there is no expansion tank at the water heater. I installed a new TPRV for her. Next day, still leaking. I went back and checked, and it didn’t leak when I was there. It does not leak when the unit is heating up. Also, she says that the water heater only supplies water for one short shower before it turns cold. It is a 50 gallon tank, electric, about 5 years old. Any thoughts?
Take a look at this article.