After sanitizing as above, close the valves that enter and exit the water heater. Remove the drain plug and drain the heater. Put the drain plug back in. Remove the pressure relief valve and fill the heater with vinegar. Put the pressure relief valve back in. Run it through a few heat and cool down cycles, then pull the drain plug. Get a water heater flush stick and give it a good flushing. After flushing, open the inlet and outlet valves, fill with water, turn it on and enjoy the fresh smelling water.