A leaky water heater drain valve can cause a multitude of problems, from electrical fire to flooded floors and mold and mildew build-up. The function of the drain valve is to allow the tank to be emptied during replacement or repair, so water is only supposed to come out of it when the valve is intentionally opened, generally after attaching a hose. The good news is that a leaky valve is usually easy to replace.
Before beginning any maintenance, completely disconnect all power to the tank, shut off the gas feed, and turn off the water supply. Now wait for the tank to cool off before touching any parts. Safety first. While it is cooling, collect any tools you might need, including a wrench or pliers, towels to mop spills, and a flashlight if the area is not well lit.
The first thing to check of course is whether the valve is simply not screwed in as tight as it should be. Vibration occurs when the heater kicks on and off, and over time this could potentially work a valve loose. A loose valve is usually quite simple to fix with a wrench or pair of pliers. Right-is-tight, and left-is-loose, so turn the valve right or clockwise to tighten it, just like when screwing in a screw. Take care, however, not to over tighten the valve. This could cause stripping of the pipe grooves, and then there will be a really big leak.
Damaged Fittings and Adhesives
Sometimes parts just wear out over time. The water heater may be old, and therefore so are the parts. If the valve has been used several times (tank draining for other repairs perhaps), it may not be sealing correctly anymore. Sometimes sealing compounds connecting the valve to the unit will break down. If the valve and pipe was originally fitted together with a tape or putty, it may have decomposed over time from the heat, moisture and pressure. If this is the case, the part needs to be replaced, and this is a fairly simple procedure.
The drain valve sits at the bottom of the water heater tank where anything other than water in the tank will settle and collect, including rust and minerals. If this sediment builds up or attaches itself to the valve, it could cause corrosion and damage the valve surface so that it does not seal shut completely anymore, thus allowing water to seep out. In addition to replacing the valve, the tank should be thoroughly flushed of the gunk, and a chemical treatment may be needed to remove all corrosion and prevent future similar problems.
No matter the cause of the leak, to change the valve the tank will have to be completely drained and all part surfaces made clear of debris and thoroughly dry. Check with a hardware store for the proper replacement parts for the specific model, and get recommendations for the best adhesives to use for sealing the valve fittings.