On top of your water heater is a pressure relief valve. This valve is typically attached to either a pipe that routes down to the ground or it is attached to a smaller pressure relief tank. If water keeps passing through the valve and leaking into the pipe or tank, then you have a pressure issue. The cause needs to be found and fixed before the water heater fails or bursts.
1. Faulty Seals
One of the most common causes of leaks from a pressure valve is a faulty seal. If only small amounts of water are leaking out at any one time, it is likely the normal condensation leaching out of the tank through a failed pressure valve seal. The pressure valve should be inspected annually, as the seal around it does age and will need to be replaced periodically. An annual inspection ensures issues are caught early so the seal can be replaced before water damage occurs.
2. Thermostat Failure
The thermostat on a hot water heater is typically set to a temperature around 120 degrees F. If it's set too high, the water will overheat and pressure will build in the tank. This pressure leaks through the pressure valve. If the water is heating too much even though the thermostat is set properly, then the issue may be that the thermostat has failed and now the tank is overheating. Cut power to the tank until a plumber can inspect and replace the thermostat.
3. Mineral Scale Buildup
Mineral scale is composed of dissolved minerals in the water supply that leach out in the tank and form a crust. As this crust builds, the capacity of the tank is reduced and the likelihood that it will overheat due to the reduced capacity increases. Overheating, of course, leads to water coming out of the pressure valve. Flush out the water heater tank annually to remove any mineral scale buildup, and you will minimize the chances of pressure valve leaks from reduced tank capacity.
4. Sensor Issues
The temperature sensor in the tank is what controls the amount of heat coming from the heating element. If the sensor fails, the thermostat won't get the message that the water is at the right temperature and the elements will keep heating the water. This can cause a major leak as the water heats to dangerous levels.
Contact a water heater repair service for more assistance if you spot water leaking from the pressure valve.