Check these sticky for detailed discussion of venting considerations. This one is good: http://boards.cannabis.com/growroom-...g-hot-air.html but this one is more appropriate in terms of assessing fan needs: Noob's guide to growroom set-up. See chapter 2 for the fan discussion about half way down the first page. It gives a formula for calculating needs. And those two stickies are excellent for all kinds of setup information. Recommended reading.