To expound on what Zem just said; You want to exchange the whole volume of air within the growroom or rooms, typically, depending on the size/volume of the rooms from 2x per minute to 1x every 2-3minutes for larger volumes. To determine what you need, you measure your room(s) then You calculate that by multiplying the length x width x height of the room(s) and that will be your total cubic footage. Your fans are rated in cfm= cubic feet per minute. If your total volume is 800cf and you have a fan that moves 430cfm then it will exchange all of the volume of your room 1x every 2min. If you have a room that is only 200cf then the same fan will exchange the entire volume of air 2x every minute or every 30 seconds.