AnswerID: 145397 Submitted: Saturday, Dec 24, 2005 at 11:21
I tend to keep the black side down, so the water warmed by the black surface then rises, creating a better thermal circulation. If the black side is at the top, then the hot water collects at the top, reducing circulation.
I also try to place some insulating medium between the sac and the ground to reduce heat transfer away from the sac.
But in practice, I've found that at the end of the day, you get a hot shower
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