Depends how much you are trying to heat the air. A few degrees I would imagine would work but if the room is low 70s, I wouldn't want to bet on just the heat from an 18 inch fluorescent tube to heat the air 10f+. As for which side, it shouldn't matter. I have my light fixture in the middle of my 48x23x14 Pro-Line cage but I use a 65 watt RHP exclusively as the heat source. I don't use flexwatt as I have no problem heating the cage with the RHP alone.