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Drain Back Systems use distilled water as the heat-transfer fluid (HTF) in the collector loop. A pump circulates the water through the collectors. The water drains by gravity to the storage tank and heat exchanger; there are no valves to fail and the system is completely non-pressurized. When the pumps are off, the collectors are empty, which assures freeze protection and also allows the system to turn off if the water in the storage tank becomes too hot.
Since water is used as a heat transfer fluid, it never needs to be changed like pressurized antifreeze systems. Most plumbing codes do not require double wall heat exchangers for drainback systems using distilled water.
Advantages of this System
The system has no check valves, no air vents, no pressure gauges, and no expansion tanks.
Cannot reverse thermosyphon at night.
Drainback systems may out-heat antifreeze systems by up to 8%.
Larger piping (3/4″ copper pipe) and insulation must be used.
The drainback system components cost about 15- 20% more than an active open direct loop solar water heating system for residential water heating.