I'm not sure this is the "approved" way to flush a tank but it works well for me. I turn the inlet water off, then hook a hose to the drain. I open the drain and at the same time I open the pressure relief valve. Then I have a fitting that fits at the end of the pipe that comes out of the relief valve that allows me to inject air from a compressor into it with an air gun. I inject air into the tank through the open pressure relief valve until the water runs pretty strong out of the hose. Then I stop and let it drain for 30 seconds or so and then do it again. I do this until all the water is out of the tank. Then sometimes I'll refill the tank and do it a second time. This seems to push a lot of crud through the hose. I know there are other ways to flush a tank but this is how I prefer to do it.