I saw some other answers. I figure the relief valve needs replacing, ok no problem, but exactly how much pressure are we talking about here? what is the correct procedure to shut this thing down to replace the valve?
First if you follow the correct procedure the pressure is no problem. 1 Kill the power or gas to the heater-very important for electirc elements will pop if not done before water is drained. 2 Hook up hose to drain on the bottom and open it up to flow water outside of home. 3.Open hot water valve on tub and sink to allow water to drain after you shut off the main water supply. 4.Shut off the main water supply and let tank drain for an hour. 5. Use a pipe wrench to remove old pressure relief valve. 6 Pipe dope new pressure relief valve and install. 7Open main water vlve and close and remove water hose on drain. 8Let water flow out of tub and sink for 5 minutes after air has bled out. 9Turn system back on and be sure check for leaks.
Same thing happened to me, but with an oil boiler. Shut off power to water heater. Find the drain valve on bottom of unit, attach hose if it has a fitting. If you have an open sump pump in you basement, just drain into there, or you can fill buckets and keep emptying them. Use a pipe wrench to take of pressure relief valve. Take the valve to a good hardware store, or plumbing supply place so they can match it. Put teflon tape on threads of new valve (thats all i used, no leaks) and install then refill tank, you may have to manually hold down the water feed line. turn on power, and open all faucets to bleed out the air.
You have already done what you are supposed to do. My only thought is that the thermostat on the heater is bad. I have installed hundreds of State heaters and the only problems I have ever had was a bad thermostat. Good luck. :) But be sure that you have an expansion tank on the heater. I forgot to mention that earlier. 70 PSI is higher than normal, but perfectly acceptable. I always have my house at 70 psi and no problems (also a State heater). I have a 50 gal heater.....I am not as rich as you are....LOL
Shut of gas or electric and water feed valve to tank. Open hot water faucet to relieve pressure then close faucet. Connect hose to drain at bottom of heater and open drain. Put teflon tape on new relief valve. Loosen relief valve to introduce air so water drains out. It is possible to do this on the fly by removing old valve and installing new one immediately. If your not that confident, you can just loosen old valve by hand to the point that you have just a couple of threads holding it in place. You will hear air sucking in. The tank only needs to be drained to below the level of the relief valve.
