JUST PLACE ONE (1) PLASMA TV IN CLOSET AND WATCH IT THERE. 400-1000 watts of heat, no risk of shock, no building nothing. And more fun, too.
Same effect. It's just proximity to any high-wattage electric device in an enclosed space. I don't have a\c and trust me when I say that my 50" plasma-as-monitor + PC + 200 watt amp for audio out rig heats up my living room like crazy in the summer, even though the room is huge (the wattage is higher than your closet rig, though).