Discovering a puddle of water near your hot water heater is never a good sign. Hot water heater leaks often start off as small problems that quickly become huge if you don't address them immediately. Knowing what to do when you discover a leak is vital, for both the safety of your home and for your own safety.
Cut Power to the Unit
Your first step should be to cut power to the unit. The last thing you want is hot water leaking into your home. Depending on the nature of the leak, there could be other malfunctions in the unit that could be leading to electrical shorts or a danger of overheating. Electric hot water heaters typically have their own breaker in the circuit box, which makes it easy to turn them off. A gas heater will likely have a shut off switch or valve on or directly adjacent to the unit. Once the power is cut, allow the unit to cool down before attempting to fix a problem on your own.
Find the Water Shutoff Valve
Once the power is cut you can slow the leak until the unit cools down sufficiently for safe troubleshooting. Find the water main line that feeds into the unit and turn it off. This unit-specific shutoff valve is usually located on or near the hot water tank. If you can't find the shutoff valve right away, perhaps because it isn't yet safe to approach the heater due to the hot water coming out, then turn off the main water shutoff valve to the house. You will have to go without water until you can shut down just the heater, but this is just a temporary inconvenience.
Troubleshoot With Caution
Once the heater is safe to approach you can try to troubleshoot the problem on your own. If you can locate where the leak was coming from, then you may find the problem. Often, the problem is with a worn valve that requires replacement. In others, it could be a failed thermostat that is allowing the water to overheat. Overheated water leads to pressure buildup, which may be released through the overflow valve.
Call In a Professional
You will need to call in a water repair technician if you can't troubleshoot on your own or if you aren't comfortable fixing the problem yourself. You should also call in a pro if the water heater was steaming or rumbling before it was shut down. These symptoms indicate major pressure buildup that could lead to an explosion if the heater isn't properly serviced or replaced.
Contact a plumbing repair service in your area for more help.