Here is my fan philosophy: You can't have too many!
I increase airflow as the plants grow & are able go handle a little more wind stress. If you can replicate nature in your tent you will have sucess. The spots in the world where the best landrace & heirloom strains originate are usually extremely windy (steep, rugged mountainsides in Asia, coastal mountains in the US, Central and South America)
Airflow does many things like increasing bud size/density, making it hard for small bugs/mites to attach to the leaf surface, preventing "hot spots" in your grow area, preventing powder mildew, and allowing you to run your lights much closer to the plants without heat stressing them. Moderation is the key, too much wind on non "wind trained" plants can cause issues.
Young plants/clones: (1) 6" oscillating fan on low blowing across the tops. Just want to them go dance a little jig. Too much wind will remove the moisture from the air they require at this stage. 2x2 tent
Veg: (2) fixed 8" fans. One blowing across the top of the canopy, the other blowing below in the opposite direction below the canopy. Speed is increased as stalks get more strength. Plants are rotated 90°daily. 3x3 & 4x2 tents.
Flower: Same as veg but I put a 20" box fan in front of each tent door when the lights come on. The doors are left open at the bottom corner and the fan blows in from outside the tent. These blow cool air from below and really help the growth of the lowers. The fan speed is increased as the buds grow. By late flower the fans are running on speed 2/3 and it's like a hurricane blowing in there. 3x3 & 4x2 tents
I also have small 4" fans at the top of my tents that come on when the lights turn on to help cool the LED drivers and dissipate that heat & extend their life.
Vent fans are always on to remove stale air from the tents, control odor + heat. Cloudlines with the LED controller are my choice.
