Not intending to hijack this thread but this is still about water heaters. Maybe it will help someone.
A few years ago, my ~5 year old water heater started to leak at the high pressure valve. I changed the valve (simple job) but the new one started leaking too (within a few minutes). I changed it several more times and they all kept dripping.
OK, after a few months, I replaced the water heater.
Guess what, the new one started leaking at the valve too. (WTF) Then I started reading a lot about water heaters, troubleshooting and how they really worked.
In the end, I installed a ~2 gallon expansion tank and that fixed the problem.
"Apparently" about the time this problem started, my local water supplier had significantly raised the water pressure and installed check valves in their local distribution pipes and never told anyone.