When it comes to growing I find it works best for me to have a room run under negative pressure. I dunno thats just me. I dont use co2 all the time just when the lights first come on. I would suggest more info because if he ties his exhaust from hoods or ballast, et. he will just bring it back in through the registers. If he does that and runs his a/c, he better be sure the a/c is big enough to handle the load durring peak summer time. Waht kind of system is it a package unit or split system?