These systems use a circulating pump to generate water flow within the system, which offers you not only a higher flow rate but a higher heat transfer as well, amounting to more efficient overall heating. They also allow you to have more control over where the panels are placed on your roof, including better flexibility with regard to pipe layout. In more technical terms the pump/sensor system is fitted with a differential SolarStat and two temperature probes. One probe is located on the panel on the roof, the other is mounted in a pocket two thirds of the way up the cylinder. When the difference in temperature between the two probes reaches 12 degrees C, the pump is activated drawing cold water from the bottom of the cylinder pumping it up through the panel on the roof, and back down to the middle of the cylinder.