As the water temperature increases, the pressure also increases. The purpose of the relief valve is to prevent the tank from going through your roof. With a thermometer check the temperature of the hot water at the nearest faucet to the water tank after the tank has sat for a few hours with nobody using the hot water. If the temperature of the water is excessive then lower the thermostat setting. If the water temperature does not go down when you check again, then the thermostat is probably bad. If the water temperature was normal when you first checked
it, then the relief valve is probably bad and needs replaced. GOOD LUCK!!!