The heat source is actually behind the peg board creating a physical divider between the egg boxes but is probably a foot and a half.

The fan blows downward on the heat source side of the pegboard creating a circular effect on the airflow but I do get a 1-2 degree difference on the temps between the top and bottom shelf.

when I have multiple boxes in the incubator I leave a 1-2 inch gap between boxes but I also stack the boxes two high.