Heat transfer in the attic solar pool heater can be readily observed by measuring the difference between the entering air temperature at the coil or fin side of the attic solar pool heater and the exiting air temperature at the fan side of the solar pool heater. When transferring heat, it is normal to measure a 15-30 degree F temperature drop across the attic solar pool heater airstream. Ergo (Input air temp - Output air temp = ~ 15-30 degrees F).
Heat transfer can also be observed on the solar pool heater by measuring the difference between the attic solar pool heater's output water temperature and its input water temperature. The above graph is an engineering plot of the water side temperature increase of the attic solar pool heater.
Normal temperature rise (LWT - leaving water temperature) of the attic solar pool heater is a gain of 2.0-6.7 degrees F when the water flow rate is between 15-55 GPM. Slowing down water flow will increase the output temperature, but will not increase the overall heat transfer in terms of BTUS/HR. See the "Heat Transfer Btus" link below for further information. There are additional heat transfer curves located in the first generation PCS1 attic solar pool heater technical manual, which is still available to view or download online. Note that this engineering data was not reproduced in the PCS2 or PCS3 Manuals. The heat transfer and flow rates are the same in all three attic solar pool heaters.