That is the solar controller I have and it's great. I just set it and forget it and my pool stays the right temperature. As Titanium described, there is a temperature sensor near the panels on the roof and one in the plumbing right after the main pump. If the water temperature is below the thermostat's setting and the roof is at least a few degrees warmer than the water, it opens the valve to the solar. As soon as either the heat is no longer needed or the roof is too cool, it bypasses the solar.