I built mine without a fan, and I don't appear to need one. Mine is small, for 8x10 and 11x14 prints, with 8 closely spaced 18" BL bulbs. Even after an hour exposure (OK so my negs are bullet proof) the box is only warm, not hot. I painted the entire inside of the box with aluminum paint, so it's highly reflective, maybe that keeps some of the heat down. Perhaps if I made the big box with 12 24" bulbs I might consider a fan, as well as moving the ballasts further away from the bulbs, with a reflective barrier in between.