Another intake question

My grow room is 8' x 9' x 7' if my calculations are correct that would be 504 cubic feet, i am running a single 4" exhaust fan that is rated at 170 cfm, I believe that be sufficient, would it hurt to run 2 4" exhaust fans?

Assuming one 4" exhaust fan is sufficient how big of a intake should i be using? I don't understand the concept of negative pressure as apposed to positive pressure, i also don't understand how you figure out if the airflow is creating a negative or positive pressure?

I am thinking a powered intake would help keep the smell from creeping out of a passive intake, does that make sense or is passive the way to go?

Ideally you want to replace all the air in the room at a rate that lets you keep ideal temperatures of 70-80F. Your fan is replacing the air about 20 times an hour, which is more than sufficient for odor and plant, but may need more to cool the area enough. Only you will know if it is enough.

Your intake is generally going to be smaller than the exhaust, this helps to create the negative pressure that makes odor control easier. You know its negative pressure if you have a smoking punk near the intake and the smoke gets drawn into the room from any opening. Basically if the smoke gets sucked into it that is a good sign.

A passive intake is one which has no device to bring air in, it is simply a hole, the neg pressure from the exhaust fan will draw the air in. Intake on bottom where cool air is drawn in and exhaust out the top where the hot air rises. An active (Powered) setup requires that the air being blown in does not overpower the exhaust and send odors out any unseen cracks. Generally it works better to have passive intake if its a smaller grow area and powered for larger, hotter setups.

If using a carbon scrubber setup, it is more efficient to suck air through the filter than it is to blow air through it.

Hope that helps some.