I would imagine that it depends on your tap water, and the amount of dissolved “stuff” in it. Water quality varies a lot across the country. If you have very hard water, I suppose its possible that mineral deposits could develop and potentially clog the small conduits in a radiator, but it depends on if the chemistry of coolant, and whatever else… the metal its in contact with, would allow for that to happen. Any water chemists out there want to elaborate?