I built the window screen solar box from build it solar last fall a year ago. It ran like a 3600 watt electric heater all winter. I have made several air to air heat exchangers in the past. This is the most efficient one so far. I used a snap switch mounted in the center of the panel to turn on a 50 cfm fan when the temp reached ~120 deg F.
I took temperatures inside the box with a HF laser temp gun. The idea was to harvest some hot water also but the temp on the bottom of the box was not warm enough to get anything worthwhile out of it. I had to change the snap switch to a lower value because 3 screens were taking most of the heat out of the box!
So for my money, I'll have panels of water or air but not both.
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