Hard water can contribute to mineral scale in parts of a solar water-heating system that carry potable water or operate at elevated temperatures. The effect depends heavily on the system design.
In a direct system, household water may circulate through collectors or related components, so hardness can matter directly. In an indirect or closed-loop system, a separate heat-transfer fluid may circulate through the collector, while hardness concerns apply mainly to the potable-water side and heat exchanger. Generic advice should not treat the two designs as the same.
Scale can form on heat-transfer surfaces, in narrow water passages and in storage components. Reduced flow or heat-transfer performance can have several causes, however, so those symptoms do not by themselves prove a hardness problem.
Do not use a universal 6- or 12-month descaling schedule or generic vinegar procedure. Solar systems vary in valves, pumps, heat-transfer fluids, freeze protection and collector design. Follow the manufacturer's service instructions or use a technician familiar with that system.
A softener reduces calcium and magnesium hardness. A sediment filter removes suspended particles. Scale-control devices use other approaches and should be evaluated against the manufacturer's requirements and the actual water chemistry. A TDS reading alone is not a hardness measurement.
If the system has leaks, damaged components, abnormal pressure, unknown heat-transfer fluid, or an unclear service history, avoid improvising a maintenance procedure from a general article.