far from frighten me dude, this is real encouragement and exactly what i need.

firstly let me explain my vent system. as i said the grow unit wil be small, only 2'x2'x8', so for extraction i'm using an inline fan with a 4" duct from the top of the grow cab followed by a carbon filter made from a 6" 4" reducer i got the designs for on this forum.

the grow room will be completely sealed so to allow air in i'm installing a simple 1way ball valve system which works as follows. a 2" piece of plastic pipe is mounted horizontally through the side wall, it protrudes about 4" where you put on a 90degree elbow facing upwards. in the mouth of the elbow you secure a funnel thats had the bottom cut off so the opening is slightly smaller than the pipe.then into the funnel you drop a hollow plastic ball. when the fan aint runnin the ball sits perfectly in the funnel and the room is still sealed but when the extraction fan runs the slight negative pressure set up by it lifts the ball and allows the air in. simple cheap and effective i recon.