Just one thing bugging me about this thread: AFAIK scale only builds up from water once ie. if the the water has lost its carbonates and other stuff (really scientific, huh), further heating does not allow extra minerals to precipitate (cause there are none, that would). So water just heated and kept hot precipitates a certain amount of scale according to its carbonate and total hardness (another factor is temperature) and then it goes flat, it`s just lacking the minerals to create scale.
The source of my very poorly written post Jim Schulman`s water FAQ
chapter 7 is a good place to start.
However, i do find that water left sitting in brew boilers tend to get a bit skanky, thats why i change the water in boilers (also steam boiler) every once in a while. Just a note, so you`d know where i stand, im using DallaCorte Evolution 2group machines+DC Mini.
Caffeine. My drug of choice.