The attic solar pool heater has a heat exchanger equipped with 5/8 inch pipe that is nine rows high and three rows deep. Overall thickness of the attic solar pool heater heat exchanger is five inches. About 120 feet of pipe is located within the attic solar pool heater heat exchanger and each piece of horizontal pipe is referred to as a "tube". Heat transfer within the attic solar pool heater is a function of tube water flow velocity, which is measured in feet per second.

Optimum heat transfer, within the solar pool heater, occurs when tube velocity is above 3 feet per minute (FPM) and below 7 feet per minute (FPM). At lower tube velocities the heat exchanger in the solar pool heater becomes less inefficient due to laminar water flow. At very small tube velocity, no heat transfer will occur. This lower tube velocity is reflected in the graph of Heat Transfer Vs. Water Flow when input water in GPM to the solar pool heater becomes lower than 15 gallons per minute. This is the point in the solar pool heater where heat transfer rapidly declines. At higher tube velocities there is accelerated wear on the tube metal used in the solar pool heater. Keeping water flow rates to the attic solar pool heater between 45-55 GPM creates the optimal tube velocity, the optimal heat transfer and the optimal life expectancy.