I built a collector for my hot water system and used .018" aluminum I think. I don't know the grade of metal. It was Amerimax(?) flashing, painted. I probably overdid the silicone, but the 36 sf collector easily produces 21,000+ BTU on a 70F day in 4-5 hours of sun. Copper tube spacing was about 5.75" if I remember. I did not do the "coil in a tub" like many/most builtitsolar projects. Used a plate exchanger. Waiting to see how it holds up while exposed to city chlorine. Seems ok after 9 months.
Thanks, I did join that user group. I'm thinking the thicker metal will just conduct more heat to the pipes. If its too thick it would act as an insulator, I would guess. 21,000 BTUs for 36sf. Not too shabby.
Gravity is a harsh mistress. But this tiny ad is pretty easy to deal with: