Have one in my home with well water for 12 years. Love it and have had no issues. Shower is 30 feet from unit takes about 25 seconds for shower to get hot. My well water is very soft and has very little calcium in it. I also run a water treatment unit. Not sure how one would work with high calcium and no filter system. The cold water in the pipe between the heater and the shower head needs to be purged even with a conventional tank heater so you are not going instant hot water anyway.