The original valve (~5yrs old) started leaking a few months ago, so I tested that I could open it manually and close it to stop the flow. But after the test it kept leaking (at that point pressure should certainly not have been the issue). I replaced it with a part from a hardware store (same length), and the replacement part tests properly: I can open it manually and close it to stop the flow and it doesn't keep leaking. But whenever the gas turns on, it will start dripping steadily. Based on a comment from "temperature pressure valve leaking", I checked whether there is an excess of hot water pressure when I turn on the hot water after the heater has turned off. There isn't. The water pressure is constant.