For those familiar with Appendix L of the UPC 2015 code, the section that addresses hard water in the 2018 update will be instantly recognizable, albeit renumbered L.505 from L.605.
L 505.1 Softening and Treatment: Where water has a hardness equal to or exceeding 10 grains/gal., measured as total calcium carbonate equivalents, the water supply line to water-heating equipment and the circuit of boilers shall be softened or treated to prevent accumulation of limescale and consequent reduction in energy efficiency.