Back in the 1980's this was state of the art technology. Even though it is not today it still does exactly what you need to do for a solar water heater. It detects that the solar collector is hotter than the water heater and turns on a pump to flow the water to the collectors. All you need is sensors which you can buy on eBay. Make sure that this is what you are looking for. It will ship in the box as shown in the auction unless other arrangements are made but shipping will go up. As with all my auctions there are no returns.
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