i have a 6 in outline fan sucking aroma n hot air from hoods to outside, and have a 4 in inline bringing in fresh (Co2)... i think there may be to much negative vacuum effect going on.. or is there even a such thing as to much neg. vacuum? any idea on how to regulate vacuum effect, maybe switch out the 6 in for the 4in and have the 4 in do the outline and 6 in inline?