I have been having problems with my gas hot water heater. The 40gal tank keeps setting off the safety valve (gals. of water on the floor). I have an expansion tank on the line and I tested it to make sure that there is not water coming out the air valve (none). I also tested the pressure in the tank when the water line has not pressure from the street or house and there is 15psi. The water heater is not set very high in fact my wife asked if it could go higher because there are times that she can run straight hot water and it is not very hot at all. Is there something else that I need to test? Something that I am forgetting ! Is there a simple test to verify whether or not the valve is good or bad? It seems to operate fine. I do not know if the spring weakens after a time. FYI the tank is around 6yrs old.