[jrref]

Concerning flushing the tank, most people don't flush or even know what it is or that it shuld be done periodically. They just don't do any maintenance and wait for the tank to fail then replace it.

But If you have a sediment filter system such as a Nova or Express Water three stage system, you should never have to flush your hot water heater because the amount of sediment will be minimal.

What most don't realize or understand is the Anode rod installed in the tank and its effect on the tanks longevity. The anode rod is a sacrificial rod used in water heaters. It helps protect the lining of the water heater from corrosion and generally lengthens its life.
When metal and water combine you get galvanic corrosion. Galvanic corrosion is defined as an electrochemical process in which one metal corrodes preferentially to another when both metals are in contact. So, your piping, which is one kind of metal, and your tank - which is another kind of metal - and the water together set the stage for some fantastic galvanic corrosion. This is not good.
To prevent the tank from rusting or your heater element from corroding, a sacrificial rod is installed in the water heater tank. The idea is the anode rod will corrode first, leaving the metal of the tank (and element if you have an electric water heater) alone - saving you from dealing with a rusty behemoth that randomly springs leaks.

Here with the water we have in the Villages, the Anode rod will be "used up" by the time the water heater warranty is finished. For example, if your water heater has a six year warranty, after the sixth year, it's almost certain your Anode rod is mostly gone. At this point, the inside of the tank begins to rust at an accelerated rate and it's a gamble when it will start leaking. Hot water tanks with a longer warranty such as a 12 year, will have a thicker Anode rod. In some tanks, the manufacturer uses two anode rods to extend the life of the tank. As far as the Anode rod being used up, several of my neighbors and myself pulled out the anode rod on the water heaters we replaced and confirmed what I described. Depending on your water, the Anode rod may last longer or fail in a shorter time but for us here in the Villages, this is a good guideline.

All this said, this is why it's recommended to start planning changing your hot water tank after the warrenty period expires because no one spends around $300 to replace the Anode rod every couple of years.

Hope this helps.