Originally Posted by
YukonBikerGuy
Circulating the air inside the closet will not be sufficient, neither to remove heat, nor provide fresh air for the plants. In that space, I would think that a 400W lamp set would make way too much heat to be contained that way, even if the balast is located outside the closet. If you can get some hot air exhausting up at/near the top of the space (preferably above the light), and some fresh air comming in at/near the bottom, your grow space will be much happier for the airflow, and it will be much easier to maintain a decent temp.
Exceptionally tall closets can sometimes use bigger lamp sets without ventilation, by utilizing a big dead air space at the top as a place for hot air to gather, up away from the plants. This obviously has its limits.
If your closet has 1 or more outside walls (the outdoors is on the other side) and you live in a cool or cold climate (say Canada in the winter) then you may get the benifit of some free cooling action that will enable a bigger lamp set in the space. But it may also mean that you need heating in the dark period so the plants dont become chilled, or that in warmer seasons exposure to sunlight directly on the outside wall may raise temps during the light cycle to unacceptable levels in the space.
IMO, adequate ventilation/air exchange is key to a healthy grow, and the easiest way to deal with heat issues. Add it if you can.
YBG