Domestic water that is high in mineral content (or "hard water") may cause the buildup or scaling of mineral (calcium) deposits on heat transfer surfaces. Scale buildup reduces system performance in a number of ways. If your system uses water as the heat-transfer fluid, scaling can occur in the collector, distribution. Most well-designed solar systems experience minimal corrosion. When they do, it is usually galvanic corrosion, an electrolytic process. Solar water heating systems, which use liquids as heat-transfer fluids, need protection from freezing in climates where temperatures fall below 42ºF (6ºC). Don't rely on a collector's and. Solar water heating systems that use only water as a heat-transfer fluid are the most vulnerable to freeze damage. "Draindown" or "drainback" systems typically use a controller to drain the. Overheating occurs when there is little hot water use in the home but the sun continues to heat the water. The controller will turn the pump off when the solar storage tank hits an upper.