Replacing a water heater drain valve is the last resort after you have tried tightening it. Another option would be to cap it if the leak is not too bad.
See the articles, 'Tightening a Water Heater Drain Valve' and 'Capping a Water Heater Drain Valve' if you have not tried this yet. Need other water heater information? See 'Troubleshooting Electric Water Heaters' and 'Gas Water Heater Troubleshooting' for a list of water heater problems.
Is the valve leaking a lot? Is it leaking where it connects to the hot water tank? You will need to replace the drain valve. The ones that come with the tank are made of plastic and are not very durable.
Purchasing a Water Heater Drain Valve
If you are going to the trouble of replacing the drain valve, you want to get a decent on. Instead of plastic, you should go with brass and hopefully eliminate future problems. Water heater drain valves are use a male 3/4" iron pipe thread and should be universal.
You can get a brass one for less than ten dollars a plastic one is about half that much. Is it worth the difference.
Shutting Down the Hot Water Tank
You will need to follow a procedure for shutting down the hot water tank. The procedure is different for a gas water heater than it is for an electric one.
Shutting Down an Electric Water Heater
It is important that the power is turned off and that no one will turn it back on while the water heater is being worked on. See the article 'Proper Shut Down Procedures for an Electric Water Heater' for more information.
Shutting down a Gas Water Heater
Both the gas valve and the gas supply line need to be shut off along with other steps for a gas water heater. See the article 'How To Shut Down a Gas Water Heater' for complete instructions.
Draining the Tank
You will need a hose and somewhere to drain the water. Hook the hose up to the drain valve and put the other end in a drain. Make sure the hose is going to stay in the drain. Open a hot water valve near the tank to allow the tank to get air. Use a screwdriver to open the water heater drain valve.
For complete instructions on draining a water heater see the article 'Draining a Hot Water Heater'. This article provides step by step information on what to do.
Removing the Old Drain Valve
Getting the old valve out may be a problem. They are plastic and break easily. Use the adjustable pliers or a small pipe wrench. You want to get a good grip on the valve. Firmly try to worry it free. When the seal breaks you should be able to unscrew it. It might be a good idea to use some penetrating oil if the fitting is metal and it is corroded. Be careful, you do not want to break the valve.
Plastic valves especially are prone to breaking. If this happens, you need to get the threaded part of the valve out of the water heater. Use a bare hack saw blade to put a slice in the plastic pipe going into the tank. Be careful not to cut into the metal threads. Use a screwdriver and a hammer to chisel out the plastic. Careful with the threads.
Installing the New Drain Valve
Put some pipe dope or Teflon tape on the threads before you put the valve into the opening. Use a pipe wrench to tighten it. Get it snug and make sure the spigot is turned down. Close the valve and turn the water back on. Let the tank fill with water and then check for leaks. No leaks? Good turn the power or gas back on. You are done replacing your water heater drain valve.
Starting the Water Heater
You want to follow the right steps to turn the water heater back on. Turning the power on too soon can result in damage to the water tank or its internal components. See the article 'How To Start Up a Water Heater' for instructions and cautions.
Under ideal circumstances it may only take five minutes to change a water heater drain valve. Unfortunately, there is a lot of preparation that goes into changing one. You have to shut down and drain the water tank before you start.