i'm no guru, but here's a couple of links that helped me figure ventilation.

this one has some math in it, but you should be able to figure out your cfm:
http://boards.cannabis.com/indoor-gr...-progress.html

and this one is all about fans:
http://boards.cannabis.com/growroom-...g-hot-air.html
fluid69 Reviewed by fluid69 on . ventilation gurus help!!! quick question,right know i have an ideal setup of 2 x 600 watt in a 5 x 10room,however i need another lamp a.s.a.p. up until this point the room gets ventilation from a seperate room and exhausts into our main room"wich gets pretty hot. im afraid by adding another lamp its going to affect my veg room.i need some input on my idea, i have 30 ft of 4 inch schedule 6 abs pipe.my plan is to run all three lamps with my current 6 inch fan then reduce to a 4 inch reducer" to accomodate the piping" and Rating: 5