Round here Richard, as you know the first time you boil the kettle is the last time you see the bottom! Rain water is clean, but still has some residue in it from atmospheric pollutants, and the ions. The very pure water distilled on nuclear submarines has the odd side effect of removing calcium from bones of the crew. This is then passed in a solid form. It plays merry hell with the toilet systems apparently. The macerators used to chop and diperse the waste are jammed by the stones. I was also advised years ago by an old mechanic to add K Bar conditioner to the system when I cagnged anti freeze, I've done that ever since. Touch wood never lost one to freezing.