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If you plan on having a closed environment (e.g., grow room, grow tent, etc...) then you need to properly ventilate your growing environment to ensure your plants perform at their best. Proper ventilation helps regulate humidity, exhaust the warm air that your grow lights and electronic equipment are producing, and provide relatively carbon-rich air for your plants to breathe.
Many websites give a blanket rule-of-thumb for CFM calculation that fails to take into account the uniqueness of each growing room.
Below we will show you a way to calculate CFM taking different aspects of your growing environment into account. We recommend that the figure this calculation generates be considered the minimum CFM for your growing environment.
Growing Environment Volume -- Calculate the volume of your room by multiplying the length x width x height. For example, if you have a grow tent that is 8' x 8' x 7', then the volume would be (8x8x7) = 448 cu./ft.
Volume To CFM -- Your exhaust fan should be able to exhaust the equivalent of your growing environment's volume in two minutes, or one minute if you are having issues with high temperatures. Assuming we are not having issues with heat, the growing environment volume from our example is 448 cu./ft., which means we need to exhaust 448cu.ft. / 2 minutes = 224 CFM. From this I know that I need a 224 CFM fan at minimum.
Other Variables To Account For -- We're not done yet. Now to account for other variables that can require you to have more ventilation:
1. Lights -- For air-cooled lights add 5%, for non air-cooled light add 10%-15%.
2. CO2 -- Add 5% for rooms with CO2 enrichment.
3. Filters -- Add 20% if you're using a carbon filter.
4. Ambient Temperature -- For hot climates (Southern California, Arizona, etc...) add 25%. For humid and hot climates (Florida, Georgia, etc...) add 40%.
This article was written by Nate Lipton, co-owner of Growers House (GrowersHouse.com)