I've thought about this, myself: using a water heater, perhaps with a filter in the input side, to supply hot water for brewing.
I remember a friend scolding me as I went to rinse the spaghetti by running hot tap water thru the colander. He told me all about the water sitting in the tank.
I've always been on municipal water. Occasionally, the water folks would flush pipes by turning on the fire hydrant. Sometimes, there would be a repair to a main line somewhere between me and the supply. Thereafter, there would be a certain amount of yellow-orange junk in the water (Georgia red clay is real).
When I ponder how water gets to me: going thru some miles of underground pipe > water tower > back down thru underground pipes that get tapped into from time to time > to my house I often wonder how much crap ends up in my water where it enters my house simply from all the conduits en route to my house. It just seems that, in the overall scheme of things, having water sit in a small water heater (that gets flushed frequently to remove sediment) would not have any appreciable "greater" effect on the water than what's already happened to it on the way to my house.
Of course, I have no science to back any of that up, it's just my trying to "reason through" the process.
Thanks,
Keith