There is no 'refill' for a water heater. They are always full. As you use hot water from the tank the cold filling pressure pushes the hot water out and the incoming water at the bottom of the tank will be heated. A nice balance of yin and yang in the tank with incoming cold and outgoing hot.
Anyways, it seems that your water heater may be experiencing a thermal expansion issue, so some questions need to be answered before we go too far. Maybe reading this recent thread will familiarize you with what I'm talking about and use the Q&A parts of the thread to see if you are having similar problems.
Link for thermal expansion: http://www.terrylove.com/forums/show...ng-from-toilet