All a fan does is blow around the air, no matter what temp it is. If the air is hot the fan will blow around hot air. Everyone uses remote ballasts in enclosed areas just for that reason! Most of the heat involved comes from the ballast so it only makes sense you'll want to put it outside of the closet. Or go with a large bank of flouro's. You'll still have some heat to deal with but nothing like a 250w in a closet with no ventilation beyond a fan. The flouro growers I've seen use 4-500w in each section and end up with very similar flowers as done with smaller hid's but don't have near the heat issues, but still they are there and must be vented! She cut a nice and neat hole high up in the wall near the ceiling but where a finished duct grill can be easily installed to cover the hole. She then installed Dayton a 130 fan and exhausts most of the heat into the wall and attic.