The crud is near the bottom of the tank. Opening the valve at the bottom
of the tank will help to flush the crud out. Running the hot water will not
help to clean the tank. But, as Joe pointed out, be ready to replace the
drain valve since it may not close fully after you flush the tank. Commonly
you hook up a garden hose, open the valve, watch the run off, and when no
longer full of crud close the valve. Be careful with the hot water. Check
the valve for drips after you have closed it. If it is dripping they are
usually cheap and easy to replace.