COMMERCIAL GREENHOUSE : The size of the duct, 16" may seem a bit large, but It's designed to move a lot of heated air under the plant bed. Jugs of water or some large stones placed in the path of the air would help to extract more heat. I don't think that placing jugs of water inside the plant bed would help much. Wet soil acts as a heat sink by itself.
This is the kind of duct fan that you would
probably want to use
