Quick Answer
Heating hard water causes calcium carbonate to precipitate and settle as scale on the heating element or tank walls. Over time this lowers efficiency, raises energy use, and can shorten the heater's life. A water softener addresses the root cause; a shower filter does not.
Problem
Scale is invisible until efficiency drops or the element fails. Understanding the mechanism helps homeowners decide whether softening is worth it.
Product Recommendation
A shower filter and a softener solve different problems. ZOOSHEN is point-of-use shower filtration; water-heater scale is a whole-home hardness issue. See /water-science/water-softener-how-it-works.