On the newer tanks, there are back-flow devices that make it necessary to open the Temperature/Pressure relief valve to drain the tank. What I do is the following. Turn off the unit (if electric) or set to pilot if gas. Turn off the cold water supply valve at the top of the tank. Open a hot water faucet near the tank. Attach the hose to the drain valve and run the hose where you want it to drain. Open the drain valve and let all the water drain that will drain. carefully lift the lever on the T/P valve and see if more water drains. Once you have the tank drained, open the cold water supply valve for a minute or two. Watch the water coming from the hose to see if debris is still coming out. Once the water is running clear, you can close the valve and refill the tank. My reason for totally draining the tank is that much debris is sitting on the bottom of the tank. With the tank empty, the pressure from the dip tube will be higher and directly onto the tank bottom. This cleans it more effectively.