A passive intake (one where there is no fan) is a totally viable option, however it will create a strain on your existing fan that is not necessary (your exhaust fan is working double to push air out and also bring it in) . If you have the $$$ I would buy a small intake fan (less CFM than your current exhaust fan) and set it up. That way you will get fresh air entering your room at a faster rate, and your exhaust fan won't have to work as hard. Also, your A/C unit won't have to work as hard either if the air you are sucking in is cold this time of year (are you in the northern hemisphere?). These are all things to consider. It is cheaper in electrical terms to run a small fan than it is to run a large A/C unit to achieve the same amount of cooling.
Personally I would go with this
http://www.amazon.com/LEDwholesalers-GYO2402-6-Inch-Hydroponic-Booster/dp/B003YFADW8/ref=sr_1_1?ie=UTF8&qid=1356153634&sr=8-1&keywords=6+inch+inline+fan these guys are money, I have like 4 of em in my grow. Im in no way affiliated with this company, and in fact the fans are kind of shitty (although mine are a year old and still chuggin along), just thought id show you a cheap viable option. also some more explanation of your setup will help us determine what you need as far as intake goes
Hope that helps...you were definitely explaining it right its just hard to get a quick response sometimes on these forums if you're not showing off pictures of dank
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