All solar pool heaters operate on the principle of heat differential. In the case of a solar pool heater using roof mounted solar panels, the heat differential is the difference in temperature between the pool water and the solar thermal panels mounted on the roof. In the case of the attic solar pool heater, it is the difference between the pool water and the temperature found inside the attic at its peak [over the attic solar pool heater and out of the solar pool heater's discharged air stream].
A 32 Degree temperature difference between the attic at its peak and the pool water will produce a 60,000 btu/hour heat transfer rate inside the attic solar pool heater. It is very easy for attics to achieve this level of heat differential for the attic solar pool heater. An example is pool water at 72 degrees F and an attic peak at 104 degrees F. As demonstrated in the heat transfer curve above, the attic solar pool heater is also sensitive to water flow rate in gallons per minute (GPM). However, as can be observed in the graph above, BTUS/HR are fairly flat between 15-60 GPM. Below 15 gallons per minute, heat transfer drops off dramatically. At 1 gallon per minute, heat transfer is only 13,895 BTUS/HR.