Now that I have been through 2 water heaters in 2 years, I am ready to learn how to maintain them.
I knew you were supposed to drain them every once in a while, but it is easy to assume they are working and hope for the best. If you do this, make sure you put a pan under the water heater to catch the dripping water when it does give out. I had rust tracks on the top of the old one.
The warranty is for 6 years. Ours lasted 12 years without doing any of these things. Delaying maintenance is a calculated risk.
This is from an Rheem Water Heater manual:
Replace the anode when 6" of the center wire is exposed. You should probably check it 6 months after getting the water heater and every year after that.
The anode rod is about 2.5" down through insulation on the top of the unit. There is a plastic cap on the top.
It takes a 1 1/6" socket to remove the anode rod. The plumber had to hit the top of the existing rod to loosen it. He said next time it will be easier. He put plumbers lubricant as well as teflon tape on the new rode before inserting it.
After 2 years, pretty much all of the rod was exposed.
Drain several quarts every month.
Check the pressure valve once a year (make sure it is drained to a proper place).
If you are thinking of a tankless water heater, consider the cost of wiring for it. The old water heaters used 10 gauge (10/2 on the wire) and the tankless needs 8 or 6 which is thicker. Larger tankless needs 2 sets of wires ( or maybe 2 hots and 1 neutral).