Is there anyone out there that can help me answer a question I have related to solar heating of water. - There is a 1" diameter x 4' long glass tube filled with water having an input temperature of 250F. - There is a 2" diameter tube surrounding it, 4' long, filled with water coming from the opposite direction at a temperature of 40F. - The Flow rate is 1 gal/min. - I have 100 gal tank. How long before the water reaches 80F? For the sake of simplicity lets assume the sun shinning 24 hours a day. Any help answering this question would be greatly appreciated.