Minerals build up inside tankless heating coils. We descale (remove) those minerals.

Manufacturers' comments on scale buildup:

Rinnai	Over time, a tankless water heater may accumulate mineral build up which can erode the walls inside your tank's heating chamber. To properly maintain and clean your tankless water heater, it’s important to flush those mineral deposits at least once a year.
Navien	Removing excess mineral deposits is important, because they can erode and clog the walls inside your heat exchanger, reducing efficiency, reducing flow, lowering the overall performance, and potentially causing premature failures.
A. O. Smiths	Water hardness leads to scale formation and may affect/damage the water heater.

Takagi	Hard water is a severe problem for the copper coils inside heat exchangers. Heat exchanger failure due to scale buildup from hard water conditions is NOT covered by warranty.
Rinnai	• Improper maintenance (such as but not limited to scale build-up, freeze damage, or vent blockage)
Rheem/Ruud	j) Heat exchanger failure (leaks) caused by operating the water heater in a corrosive or contaminated atmosphere or damages, malfunctions or failures caused by lime, scale, or mineral build-up.
Noritz	Problems resulting from scale formation are not covered by the warranty.
Navien	Any damage caused by local adverse conditions including but not limited to hard water deposits, lime or mineral build-up, operating in corrosive atmospheric elements.