In a closed loop drain-back system the heat-transfer fluid is distilled water. When the system is at rest (not pumping), the solar collector is empty and the distilled water is stored in a reservoir tank, usually located just above the solar storage tank.
When the pump turns on, the distilled water is circulated from the reservoir tank back through the collector where it is heated; then it flows through the heat exchanger, passing heat to the potable water in the solar tank. When the pump shuts off again, the distilled water drains back into the reservoir. Hence the name “drain-back.” The collectors must always be higher than the storage tank to allow for the water to drain back to the reservoir tank; this ensures against any freezing.
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