Mineral scale buildup visible on heating elements or inside the heat exchanger; reduced performance.

Safety: Turn OFF power and close water supply. Allow the unit to cool before descaling.

Descaling procedure (general):

• Isolate the heater using service valves on the inlet and outlet if installed. If not, install them for future maintenance.
• Connect hoses from a descaling pump and bucket to the inlet and outlet service ports.
• Fill the bucket with an approved descaling solution (such as food-grade phosphoric or citric acid) per manufacturer instructions.
• Circulate the solution through the heater for the recommended time (often 30–60 minutes) until scale is dissolved.
• Flush the unit thoroughly with clean water to remove all chemical residue.
• Inspect elements and internal surfaces; if elements are heavily pitted or damaged, replace the heating elements.

Consider installing a water softener or scale reduction system to reduce future buildup.