Fans are not just for cooling your high powered grow lights and ballasts. I use fans in every grow space to create wind. Wind will help to move the air around in your room so it does not stagnate and get used up. Plant leaves like oxygen and breathe like we do. They need a constant supply of fresh oxygen to be used at any given time. If your air never moves, your plants are losing all the extra oxygen they could be pulling from fresh air. An exhaust duct with a strong inline fan will move some air from negative pressure, but it is no where near what the plants are used to from Mother Nature.
Take a look out your window. If there are trees around you, see if their branches are blowing in the wind. Think about how brutally bent you’ve seen some trees and plants after a storm. If the plant is not uprooted or snapped, it seems to recover from the storm. You do not want to create a wind tunnel that has your plants curving to the ground, but a nice breeze is a good idea. The best thing about indoor gardening is you become Mother Nature. That means you choose the time and strength of the wind.
I use desk fans of various sizes to control air circulation in my indoor garden. I look for fans with various speeds that can be easily hung in the space I need. Letting the fans sit on the ground causes loud vibrations that get annoying after a few hours. I suggest hanging inline fans as well for the same reason. High power inline fans can get extremely loud if not hung.You can decide the location of the fans inside your grow room. I like to position at least one towards the center of the light so the hot air is pushed where the exhaust fan can pull it out. Another good place for a circulation fan is close to the intake fan. The circulation fan will move the colder fresh air where it can be used by all your plants. Check your plants needs to make sure this won’t make your medium too dry or too cold. Sometimes having a warm ballast in the room is needed for heat depending on your complete cooling system.
Wind will make your plants tough and force them to thicken their stems. Thicker stems can be beneficial for supporting a plant’s weight and ensuring the proper amount of nutrients and water to each stem. If your plants never experience wind they may be limp or droopy. If you do not introduce a plant to wind early, it may develop too thin and weak. A plant that is weak will suffer stress when it is first introduced to heavy winds. It should eventually adjust and strengthen its stem, but you may be diminishing your yield from a lack of planning. Starting seeds inside that will be transplanted outside need to be hardened off. You must gradually expose them to more light and wind until they can handle the harsh outdoors. You may have two fans of different strengths placed in two different spaces in your grow room on two different time settings. Having this much variation would vary the speed, and direction of the wind throughout the day. That type of air circulation is closer to the real experience for the plants.