I just bought an Hydrofarm 95cfm fan for my 6'x2' closet with a 400w HPS. With it, I can do one of the two options.

1) Use it as an intake of fresh air from a window into my closet. I would place the intake at the bottom so the air would flow up through the leaves. The fresh air would replace the old air which would passively exit out a hole at the top.

2) Use it as an exhaust of old stagnant air in the closet. I would place the exhaust at the top of the closet which would suck out the air from below. Fresh air (CO2) from outside the closet would passively enter through a hole at the bottom.

These two options seem logically correct. So should I use this fan for option 1 (intake) or option 2 (exhaust). Ideally I would want two fans doing both processes but I'm on a budget so I can only choose one.

personally i'd say use it as an exhaust fan. Stale air is one of your worst enemies, and if youre sucking out all the old air, new air has to flow in, you can look at it as kind of an osmosis effect accept for air... And since your in such a small space the 400 watt light is gonna make things very hot. I'm using the exact same thing; 400 watts in a small closet and the biggest problem i had is heat... What i'd suggest if hook the tubing up right next to your light so that it sucks off the heat from the light as well as the stale air. Because believe me you'll have some big heat problems later. And new air will be sucked in from the bottom.

exhaust.

if you make it in intake, not only will it heat the air coming in, but it will also force the air out of every crack in the grow area and not just the exhaust.

as an exhaust, it will pull the hot air out of the top of the grow area and bring the fresh air in from the intake.

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wait, let me get this straight... if you fry an egg you can get fucked up?
dissent is the highest form of patriotism

I agree, it would be better served as an exhaust

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There has never yet been a man in our history who led a life of ease whose name is worth remembering. -Theodore Roosevelt

Thanks fellas. I will go with the exhaust option and throw in this 6" clip-on fan into the mix as well =D

Air King 6" 2-Speed Clip-on Fan