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I'm looking into building a solar water heater, but I'm wondering, is it possible (and what products should I be looking for) to turn on a pump when the water in the solar heating device is hotter than the water in the water tank.

I would assume it would take a thermostat that is set by a thermometer in the solar device. So the thermostat would know the temp in the water tank, and kick on whenever the thermometer in the solar device is higher than it is. But I have no idea if it's possible to set a thermostat based on a thermometer (or if this is an overly complex solution and there's a really simple one instead :)).

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You're referring to a differential thermostat control, which I believe is fairly standard on solar water heaters:

In active systems using pumps, whenever the collector is hotter than the storage tank, the pump should be on and the system circulating. When the tank is hotter than the collector, the pump should be off. This function is performed by either a differential thermostat control system or the use of a PV-powered pump. The differential thermostat controller compares heat sensor readings from the storage tank and collectors and switches the pump accordingly.

http://homepower.com/basics/hotwater/#ControlsComponent

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