I have questions about the pressure test. May I assume you have a pressure regulator valve on the water supply line, and if so, it is either malfunctioning, improperly set, or your gauge is broken. The pressure after the PRV should be somewhere between 40 and 60 PSI, give or take a bit. The air pressure in the expansion tank should be set to match the PRV. It is this balance of pressures that make the expansion tank work. First thing to do is to make sure your gauge is working, then adjust the PRV to a workable pressure...say 50 psi. Then adjust the air pressure in the expansion tank to 50 psi. It all components are functioning properly, your home free. If the relief valve continues to release, put the gauge on a hot water faucet and test the pressure before and during the time the heater is heating water. If the pressure skyrockets during the heating cycle, I'd replace the expansion tank. Go to a neighbor and check the gauge first so you know that it is recording pressure properly. 15 psi won't operate a home water system.