Your water heater tank has a valve at the top. This valve is usually hooked into a drainage pipe that is routed down to the ground. Water shouldn't come out of this pipe unless pressure has built up in the tank and caused the water to overflow.
Immediate Steps Following a Leak
It's never a good sign to find water flowing out of the pressure relief valve on your hot water tank. The goal is to stop the flow so you can minimize damage and prevent the tank from bursting completely. Begin by shutting down all power to the tank so that the water is no longer heating up and gaining in pressure. This is most easily done by turning off the circuit breaker that powers the heater. Most water heaters are on their own dedicated circuit.
The next step is to turn off the water flowing into the tank. Find the water inlet line's shutoff valve. This is typically located on the side of the tank near the bottom. Turn it counterclockwise until it is completely shut and no new water is flowing into the tank. Use caution when approaching the tank, as the water draining from the overflow valve may be hot enough to cause burns.
Possible Causes of Overflow Issues
Overflow occurs when there is too much pressure in the tank, usually as a result of overly high temperatures. Issues with the thermostat are the likely culprit. Either the thermostat settings were changed so they were too high and the water began to boil or the thermostat failed. In some cases, the issue could also be related to the malfunctioning of the heating elements that the thermostat powers.
Hard water scale in the tank, which results when the tank isn't flushed out annually, may also lead to overflow. As scale builds up in the tank, the capacity for water decreases. This reduced amount of water may heat too quickly, leading to overflow. Scale can also cover sensors inside the tank, which may mess up temperature readings on the thermostat and prevent it from triggering off at the proper time.
A technician will need to inspect your tank to determine whether or not the thermostat or heating elements require replacement. You may also need to have the tank flushed out to remove the scale. Pressure issues can cause damage to the tank or tank liner, so in some cases, it may be necessary to replace the hot water heater completely.
The best solution in the future is prevention. Have your tank flushed annually and schedule annual inspections so issues with the thermostat and elements can be caught before a high-pressure problem develops.
Contact an emergency plumber if your water heater overflows from the top pressure release valve.