What's the general rule of thumb on intake and exhaust?

How big should a passive intake be to accommodate a 6" exhaust fan? The room is 392 cubic feet, a 6" fan is rated at 440 cfm, would it hurt to use a 8" fan? Also is it true that the bigger the filter the longer it lasts?

The thumb rule for passive intake area is twice the area of exhaust. So if you have a 6-inch exhaust, you will need at least two 6-inch intake holes.

HTH

more suck then blow, to hold a negative pressure

I am planning on sucking air from another room, should i use a bigger exhaust fan then i really need? not so much for air flow but more so to keep the smell under control? i am running a 4" @170 cfm only while the lights are on, but am thinking about a upgrade to a 6" or a 8" fan, the room is 392 cubic feet and a 6" exhaust fan is rated @440 cfm, and 8" is rated @740 cfm. i guess i am going to start running the fan 24/7 because when it's turned off the smell becomes noticeable in other parts of the house, mainly when coming in from the outdoors you can smell it as soon as you come through the door. (after the exhaust fan has shutoff) also when you enter the grow room you can smell it pretty strong, even after the 4" has been running for a couple of hours. maybe i should just stay with the 4" referencing something i read on RIU that the exhaust fan should be able to clear the room in 5 minutes, that would mean the 4" moves 850 cfm in 5 minutes.

I'm only going to talk to your filter question. Increasing the carbon filter size needs to be done in conjunction with the extraction fan. A four inch fan for example will have to little static in order to move the air through a 8" carbon filter. Sure it will work , but not without sacrificing noticeable air flow.
Too strong of a fan and you will compromise the carbon filter since the air is moving too fast (feet per minute/ fpm) though the filter. So going bigger on the fan is ok as long as you have a fan speed controller to throttle it down with.

Good luck mate,
Unity

Your exhaust should run 24/7, because the plants always need fresh air. Your fan sounds fine for your room, and running it constantly will help with any odor problems.

HTH

No reason to exhaust a room with air conditioning is there? I thought I would get the kool tube a light for my 1000 watter to keep that furnace cool and run a portable ac to keep room at 80 degrees...doesn't sound logical that anyone would exhaust ac??????????????????

SmokenGirl said:

No reason to exhaust a room with air conditioning is there? I thought I would get the kool tube a light for my 1000 watter to keep that furnace cool and run a portable ac to keep room at 80 degrees...doesn't sound logical that anyone would exhaust ac??????????????????

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As long as you add co2, but do not forget about ventilation during lights out, since you will not be running co2 then

Unity