A water heater must be installed with a temperature and pressure relief valve(T&P) to ensure that dangerous overpressures don't cause an explosion. If yours is leaking water occasionally it may be a sign of a problem. Some of the causes for a pressure relief valve to open and discharge water are:
- Thermostat is set to high or faulty and is causing the temperature inside the tank to rise to a dangerous level. (see this article for testing a thermostat)
- The tank was completely drained of hot water and reheated, causing an excess of pressure in the system. This will only happen if there is a backflow preventer installed on the service. This can be remedied by installing a potable water expansion tank on the cold supply to the water heater.
- There is dirt stuck in the valve allowing water to get by. This will result in a constant leak from the relief valve's drain. You can try to reseat the valve by lifting the lever on top of it for a few quick blasts to clean the seat. The water coming out will be very hot so be careful.
- These devices are very reliable but everything will fail eventually. If there is no other problem with the tank and the valve still leaks, it may be faulty and must be replaced. They cannot be repaired for safety reasons.