This thread, and another, mention sediment build-up. You no doubt know that we've got some pretty hard water here. Phoenix-specific facts:

Definitions of hard water vary from: Hard = 10 to 20gpg; Very hard = 20+gpg. to Hard = 7.1 to 10.5gpg; Extremely hard water = 10.6+gpg. The water in the greater Phoenix area ranges from 15 to 40gpg. Whichever definition, we've got hard water.

On maintenance: Twice a year (or every other month if you live in hard water area), drain a bucket of water from your hot water heater storage tank. Seems like this routine would be even more prudent if you were planning to use the tank(s) as a water source in an emergency.

Dan