Safety Devices, the TPR Valve

All water heaters need a safety device know as a temperature pressure relief valve. This valve will allow water to escape if the pressure or temperature gets too high. The TPR valve should be located on the top of the water heater or within 6" from the top with an extension or discharge that extends to within 6" from the floor. The end of the extension must not be threaded to prevent people from capping the extension.

You can read about water heater maintenance at http://www.thisoldhouse.com/toh/how-to/step/0,,20351985,00.html