We have two tank gas water heater each with expansion tanks. Recently we noticed that the T/P was leaking on our porch. Thinking I had isolated it to one tank, I replaced the valve with one from Lowe's. I then noticed that the other one was still leaking, so I replaced the other. Still leaked.
Did some research and found that whole house water pressure could be the cause and remembered hearing that my neighborhood had very high pressure though I do have a PRV. Had the utility come out and they measured 150 at the meater and 80 at the house with low volume. They concluded the PRV was bad or needed adjustment.
Had a plumber out today and they tested house pressure at an outdoor spigot and it was at 150. So they dug up and replaced the PRV and left saying the house was at 75. I also had them check the pressure in the hot water heater expansion tanks and they said they did and added some air.
As they left we noticed the T/P still leaking. He exercised the valves and it was still leaking. He took apart the T/P outlet pipe at the tanks and we saw no dripping at the valve. He then blew the residual water out of the pipe by mouth. They left hoping that all was good.
Well, they have been gone for a couple of hours and the hot water T/P is still leaking. I don't know what else this could be! Any thoughts are appreciated.
FWIW, a T&P valve leaks for one of three reasons, two are safety, one is that it is defective (or gummed up, effectively defective). The two safety factors are excessive temperature OR pressure. If I had to guess, I'd guess one of your expansion tanks is shot and your pressure is still spiking after hot water use then during the recovery. If you knock on them, each should be nearly empty and sort of ring rather than thud. Also, the only way to check the pressure on these is if you've shut the water supply off, then opened a hot faucet until the water stops running - then, while the faucet is still open, you can check and adjust the air pressure. That part is easy - a tire pressure gauge and a bicycle tire pump. If you get water out of the air valve, the tank is shot. The pressure should be the same as what your PRV is set to. The shaft and or seals on a T&P valve can get debris caught on or in them, and that can cause them to leak. At that point, you need to replace them.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013