Answer: -Simple: You just need to drain a quart or so of water out of it because the required air pocket has gone away. Without the air pocket, the heated/expanded water has no place to go except out of the relief valve. -1. Pressure Relief Valve is defective . Replace with exact pressure rating that the manufacturer calls for. 2. The thermostat has stuck closed and overheating of the water heater is taking place which increases the pressure inside the tank causing the Pressure Relief Valve to open. 3. If the thermostat is an adjustable kind, it could be set too high causing the same anomoly as in #2. 4. Someone messed with the Pressure Relief Valve and and left its lever in the upright (parallel to the valve) position which thereby opens the Relief Valve.