WRONG. I have 4 80mm fans on top pushing the heat back down. Top of my incubator temp was IDENTICAL to the bottom. My probe was suspended in middle of the incubator not attached to the heat tape.
Picture shows Herpstat and accurite same temps top to bottom.
Also ALL fans generate heat. Whether its bearing or bushing they will generate heat. Take your temp gun to the center of your fans in computer and youll notice a 80-90F temp coming off them. Dont matter if its AC or DC