Here is some info I posted on another thread about my setup in a 2' x 4' x 8' closet for flowering:
I have a 1000w Quantum dimmable ballast firing a SolarMax Dual Arc bulb inside of a cool tube reflector. I also have a six tube T5 fixture with four grow and two bloom lamps mounted to the back wall and a dual T12 fluoro fixture loaded with two Philips UVB med bulbs. The only real heat maker is the HID although the others do add a slight amount of additional heat to the closet.
My 40 lb. can filter is mounted upside down in my closet with the opening facing down. Due to noise security considerations I needed to put my fan up in the attic. I connected my filter to my cool tube reflector with a short piece of semi rigid duct making a gentle elbow to make the angle. Having to re-route the air flow up and into the attic I attached an 6" elbow to the other side of my reflector and a section of semi rigid duct up and through the ceiling and connected it to my Ostberg 630cfm inline fan. To help with the noise I took the large box my 40 lb. filter came in, glued sound absorption 3" foam to cover the inner surfaces of the box, cut a 6" hole in the bottom of the box to route the duct through and I placed the fan inside the box and I keep a flap open on the top of the box for air to escape. It reduced the noise by around half. To eliminate any vibrations coming through the ceiling I used rubber straps hung from attic rafters to elevate the box 6" and prevent any coupling with 2 x 4's in the attic...
Having said all that... Whew!
I'm amazed at the power of this fan. There is plenty of pull to keep the temps in the closet only a few degrees warmer than the ambient temps in the house. It stays between 74 and 77... perfection! I just wanted to share this in case someone else is in the same situation... You can run things this way as long as your fan is powerful enough to handle the pull and the Ostberg I've got kicks ass... I am currently running my ballast at 75% (750w) and when they really start to trigger flowering I'll boost it up to 100% and it will run my Quantum ballast at 1000w without any heat problems.
I used to run a 600w HPS in an enclosed air cooled non tube style 4" reflector and with the same inline fan I have now and no carbon filter and I couldn't keep the same closet below the lower 80's without keeping the door of the closet cracked open an inch or two. The biggest change has been going to 6" ventilation ducting with a 6" cool tube reflector. Even after putting the filter first inline this fan still has the power to keep things nice and cool.
Happy Growing!