Plants do not have lungs, and they require air or wind to pass across the leaf where the plant can absorb and dissipate Co2 & O2. Plants growing inside obviously do not have unlimited fresh air and need it to be supplied to them. The effective evaporation of water and air from the leaves empowers the roots to efficiently suck up nutrient and water, thus giving you a larger plant. If the air around the plant is stale, then plant growth rates will drop dramatically.
Minimum Airflow requirements for a growroom are calculated as follows: Length x width x height x 90 (eg 2.4 x 1.2 x 2 x 90= 518m3). So in this example, you would use an 8” RVK extractor which would give you 720m3/ hour which would be more than adequate. Of course you may want to turn the speed down with a fan controller or also use a carbon filter for odour removal which might then make it more effective to use a 10” in that particular example.
Why use a fan speed controller to reduce fan speed? Answer: To reduce noise- in a large shopping centre or supermarket they use massive extractors to keep the air fresh, but you don’t notice any noise because the huge fans are turning relatively slowly. A small fan running at full capacity will always be much noisier than a larger fan running slower- a small reduction in fan speed will make a greater reduction in decibel level.
By using a larger fan than you need, turned down most of the time, will give you the option to turn it up during very hot weather and avoid disaster. When temperatures outside are higher than 24 degrees C, and a temperature inside under 28 degrees is required while running HPS lighting, the only way this can be achieved is by creating a wind chill factor in your grow-room. Oscillating fans can also help with this. The wind chill is similar to that on a beach or high up a mountain, where you don’t feel the heat but you do absorb the suns rays.
Furthermore, for noise reduction, always use acoustic ducting and avoid sharp bends (or any bends at all if possible), and keep the lengths of ducting as short as possible- a 10m length of ducting will reduce fan capacity by up to 15%.
And finally, remember, more fresh air leads to bigger growth. After good plant-lighting, adequate extraction of old air and supply of fresh air are the next most important items for a grower to provide.