Hi,
I'm just getting going on my 2nd closet grow setup and I've made some good discoveries that have really helped me tweak in my grow space. The most important thing is to get yourself a cool tube reflector that can be ventilated with a kick ass inline fan to pull the heat off the bulb and reflector. The best design that I found was this reflector called a 6" Silver Star Cool Tube Reflector. Unlike most reflectors on cool tubes that are flimsy bat wing style reflectors, this has a cool tube built into a decent sized, heavy duty reflector. The only place I saw this was at Texas Hydroponics' website:
http://texashydroponics.com/shop/product.php?productid=2790&cat=55&page=1
You've got a nice amount of space to work with. If temps become a problem you can even put a portable A/C unit in there and vent the exhaust into the attic. My closet is much smaller... 26" x 50" x 8'. I have a 40lb activated charcoal can filter (yes it's overkill but this is one area NOT to skimp) that sits on the heavy duty closet bar (metal pipe) with the opening down. I used a short piece of semi rigid 6" duct to connect the filter to the cool tube, the other side of the reflector feeds an elbow connector that re-routes through another 4 foot piece of 6" semi rigid duct and I ran it out through the ceiling into the attic. In the attic I connected the duct to my Ostberg 6" 630cfm high velocity inline fan and I blow the exhaust into the attic. You could route it outside but since there is no odor I am displacing all the air into the attic. It was a bit noisy so I created a sound muffler... I took a large cardboard box, glued sound absorption foam to the inner walls of the box and put the fan into the box and put an 8" hole in the top of the box for the air to escape. I also have the box sitting on chunks of foam so the vibrations and noise can't couple with the ceiling and wall joists. It cuts down all the vibration and probably 60% of all fan noise and isn't noticable at all in my house. I only run the fan from just before lights on until 15 minutes after they go off so that the bulb can cool down nicely after it goes off. I also have the fan come on every few hours for 5 minutes to keep the odors from building up in the closet and may have to make these ventilation cycles more frequent as the flowers develop.
I picked up a new digital ballast as well for this grow and I love it. I bought a Quantum 1000w dimmable ballast. It can run at either 120v or 240v and fire either 1000w Metal Halide or High Pressure Sodium bulbs. I am currently running a 1000w SolarMax Dual Arc bulb which has both a 600w HPS and a 400w MH in the same bulb. The Quantum's dimmable feature is very cool and useful too. You can run the ballast at either 50%, 75% and 100% (500w, 750w or 1000w) to control the intensity of the light as well as the heat produced. Since I sprouted and vegged these kids under T5 compact fluoros, when I put them in the bloom closet I started them off at 500w for a couple days and then 750w for the past few days. Today is the first day of 12/12 and I'll be switching to 100% in a few days.
I thought I'd have to feed A/C into the closet but I'm amazed at the pulling power of this fan! It truly kicks ass... As long as the temps in the house/bedroom that this closet is in is kept cool, the ventilation temps only 2-4 degrees above where the house temps are. I keep the temp in the house around 70-72 and the closet stays 73-77 degrees. The key is having a great can style inline fan. I thought I'd have to feed A/C into the closet but I'm amazed at the pulling power of this fan! It truly kicks ass... AND I'm doing this with an elbow bend before and after the reflector due to space limitations. The straighter you can keep your duct runs to and from the fan the better the pull.
Any-hoo. Good luck with your setup and grow... I did a grow several years ago in this closet but I could never get the temps to stay down. I had a 4" air cooled reflector adapted to a 6" high velocity can fan and vented it out of my attic vent but that setup didn't have enough air pull even without a can filter inline. I used to have to keep my closet open an inch or so to keep the temps down. A 4" ventilation setup might be enough for a cool tube but I'd suggest going with at least a 6" cool tube reflector and 6" ductwork hooked up to a 6" can fan. I'm sure 8" would work even better...
Happy Growing!