Most leaking hot water heater problems are caused by plumbing connections, parts, or valves leaking.
If you can hear water spraying inside the tank, the tank has failed and will need to be replaced. In this case, you should shut the power/gas off and drain the tank right away.
Even with the water shut off it will still leak until the water level drops below the area where the leak is located.
This is the most likely area for leaks because the highest temperatures and pressure occur here.
Their should be a water shut off valve just above your water heater on the cold water line ( see pic.).
If you are looking for a small leak do not shut the water of until after you find the leak.
Small water heater leaks are hard to see or hear. Dry your hand and feel along all the water pipes above your water tank.
Sometimes a gate valve (like the one in the picture above) will drip water just below the handle. If you have this problem, tighten the packing nut, located below the handle.
Check all of the plumbing fittings and connections leading to the tank. Water can drip on to the top of the tank, and run down the side onto the floor.
Check for water water leaking at the top of the tank where water enters and leaves the tank.
The tank itself could leak at this point or the nipple that screws into the tank may be the problem.
If you are replacing the nipple on top of the tank, use a die electric nipple as shown in the pic.
The inside of the nipple is lined with hard plastic to prevent rust.
If you have a flexible supply line leaking at the nut,tighten it one eight of a turn at a time. If these are tighten to tight, they will leak and have to be replaced.
The pressure relief valve will be located on top of, or near the top of your water heater.
These valves should be plumbed to a drain in the floor or to the outside.
If your pressure relief valve is leaking, go to this page for more help.
The drain valve located near the bottom of the tank could have a slow leak.
Check it with your finger, to see if its wet inside.
A quick, temporary fix for a leaky drain valve is to screw a garden hose cap onto the valve.
A garden hose sprayer works well also. Don't do this if
you have a child in your home with access to the water heater.
An electric water heater can leak around the elements.
In the pic. you can see water stains below the access panel.
To check the elements for leaks, shut the power off. Standing in water and touching an electric water heater can kill you.
If you are not sure which breaker controls the unit, shut off all of the breakers or the main breaker.
Remove the top access panel and the plastic safety shield. Check to be sure electrical power is off.
If you do not know how to use a meter you should call a someone knowledgeable to help you. A water heater heater leak is not worth getting hurt or killed over.
With the power off, if you can't see a water heater leak, check the insulation below the element to see if it is wet.
Now, you can also remove the lower access panel and check the bottom element.
If you have a leak around an element, and your water heater is not to old, you could try to tighten it.
If that does not work, remove the element and replace the washer.
You can find element washers at big box stores and most hardware stores.
As you can see in the picture, this water heater has almost completely rusted out. It was located in the corner of a basement.
The homeowner had noticed the rust but had never seen water on the floor.
The drain valve on this water heater had a tiny leak where it screwed into the tank.
Gas water heaters sometimes produce condensation. Condensation is a build up of moisture to a point where water will drip,
sometimes on the floor. This problem is limided to newly installed water heaters and will right itself.
Gas water heaters have a flue through the middle of the tank in order to vent carbon dioxide outside. Its not unusual to have a leak in the flue. It will drip down onto the burner, sometimes extinguishing the pilot light. Replacing the water heater is the only option.
Its unusual to have a water leak at the gas valve, but not imposible. I would elimate everything else before checking the gas valve.