The pumping needs of drainback solar water heating systems poses an interesting problem. Especially when the power input is one of your greatest constraints. Drainback systems drain water (or some other heat transfer fluid) from the collector on the roof into a storage tank in the conditioned space of the house (typically in the basement). In the morning the pump has to overcome about 30 feet of head to send the heating fluid back to the collector on the roof. The pump is run with a DC motor powered by a small PV panel. PV is expensive so we need the lowest power requirement possible by the pump. Is it possible for centrifugal pumps to produce the necessary head with a low power requirement, comparable to some positive displacement pumps? Some rotory vane positive displacement pumps can overcome 30 feet of head for less than 10 watts. ***Flow is not a major concern. We are looking a flows under 1 gpm.***
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