Step 1: Turn the gas control to the pilot setting. This is so the water heater won’t fire while it’s being worked on.
Step 2: Close the cold water supply off to the water heater.
Step 3: Attach a hose to the tank, and drain the water heater completely. To speed up this process, open a hot faucet to allow air into the system while it’s draining.
Step 4: Apply teflon tape to the new drain valve.
Step 5: Remove the old drain valve with an adjustable wrench or pliers.
Step 6: Install the new drain valve hand tight first, then the remainder with a wrench.
Step 7: Open the cold water supply only partially to allow the tank to fill slowly. This will help reduce water hammer.
Step 8: Open a hot faucet preferably at the highest point in the residence. Leave the faucet open as the tank is filling to purge all the air from the system. Once the faucet runs steady water without any air, the faucet can be closed.
Step 9: Open the cold water supply fully, and check for leaks.
Step 10: Turn your gas control back to the on position.