Just watched a rerun of "Mythbusters" where they blew a house to bits with a hot-water tank, prompting me to check my Thermal Relief Valve... Pulled the lever, and NOTHING! not a drop. I figured i should just replace the valve, but if it's plugged, what is the rest of the tank going to look like? I also can't see the expiry date of the tank. Should I just replace the whole heater? My house was built in 1992, and as far as I can tell the heater is original.