Now you must consider air circulation and work out a way to get hot, stagnant air out of your space. The solution may be as simple as running a squirrel-cage fan with flexible tubing out a convenient window or into an exhaust vent. Remember to always filter outgoing air through a charcoal air-scrubber (can filter) to remove odors. Use at least one smaller rotating fan inside the space to keep leaves moving and to circulate CO2-rich air.

Lighting a grow closet requires being realistic about your given parameters and your ability to vent heat. Running a 1,000-watt high-pressure sodium (HPS) grow lamp in a 2' x 3' space is completely unrealistic and quite literally overkill. Even a 400- or 600-watt bulb might fry your tops in a poorly cooled environment. I recommend a 250-watt system or smaller, unless you have more than 9 square feet of floor space (3' x 3'). Always keep high-intensity discharge (HID) lighting, such as HPS and metal-halide (MH) lamps, at least 12 inches away from plant tops during all periods of growth. Lower and raise them accordingly.

As an alternative to HID systems, many of the new horticultural fluorescent lights are perfect for closets; they emit little heat and use less electricity, yet provide enough lumens for a nifty ScrOG (Screen of Green) garden.
The ScrOG method involves using chicken wire and selective pruning to provide a level canopy of buds that benefit from even light distribution. Shoots are tucked under the wire as they grow, to make a horizontal "screen" of plant tops and significantly increase harvests in small spaces. This technique can be used with HID lighting as well, as long as you set up the screen farther away from the bulbs.