On the side or top of every water heater made since 1970, there is a pressure-temperature relief valve, often called a pop-off valve. It should have a small handle attached, and an extension tube that points to the floor, ending 4-6 inches above floor level. This valve is the reason we don't hear about water heaters blowing up any more. If the water gets hotter than 170 degrees, or the pressure goes over 150 psi, the pop-off valve opens, discharging water or steam to release the extra energy. This can happen if the built-in thermostat fails, and the burner does not shut off. The relief valve itself may have become weak. I highly recommend calling a service professional to diagnose and repair your water heater.
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