Calculating Fan Size for Your Grow
If you're growing plants in tents or enclosed spaces, it's important that you continue to replace the air in the environment every 1-3 minutes. This is because plants require carbon dioxide (CO2) to perform photosynthesis and grow. As they use up this CO2, they expel oxygen (O2) as a biproduct. In order to keep a fresh supply of CO2, ventilation is necessary. Additionally, if you have lights over your plants generating heat, you'll need to swap warm air from inside the space with cool air from outside. This is most commonly accomplished by using intake and exhaust fans placed in-line with ducting. Fresh, cool, CO2-rich air is forced into the space and stale, warm, O2-rich air is expelled from the space.
Seems easy enough, but intake and exhaust fans come in a range of sizes and levels of power. How do you know the size of fans you need to exchange the air every 1-3 minutes? Luckily, it can be determined through a mathematical calculation. Don't let that scare you, though. We've created a spreadsheet for you to download where all you have to do is plug in the details of your space and it will tell you exactly the minimum fan sizes necessary to accomplish your goals. For anyone interested in how that calculation is derived, we've included details below.
Download: Fan Size Calculator
Note: If you use Microsoft Excel, you'll need to click on the "Enable Editing" button in the yellow bar at the top of the screen when you open the file.
Dimensions of Your Growing Area
Let's start with the size of your growing area. This will tell you how many cubic feet of air you need to move to fully replace what's in the space. The formula for this is: length (ft.) x width (ft.) x height (ft.).
For example: An 8 ft. x 4 ft. x 6 ft. grow tent would have a volume of 192 cubic feet.
Desired Air Exchange Frequency
Next, you'll need to decide how frequently you wish to replace the air in your space. We recommend fully exchanging the air every 1-3 minutes. Intake and exhaust fans are typically rated in cubic feet per minute (CFM), telling you how quickly they can move air into or out of a space.
By dividing the number of cubic feet from the previous step by the frequency (in minutes) of the air replacement, you can determine the minimum size of fan (in CFM) necessary to move air through your tent.
For example: Replacing 192 cubic feet of air every 3 minutes would require a fan operating at 64 CFM (192 cu. ft. \ 3 minutes = 64 CFM).
Filters are frequently used in conjunction with an intake fan to prevent unwanted material from entering the space and/or with an exhaust fan to prevent odors from exiting the space. When adding these components, the airflow into and out of your fans will be impacted, typically by a factor of 25%. Therefore, you need to increase the CFM of your fan so that it's 25% higher than would be necessary without the filters.
For example: If you needed 64 CFM to replace the air in your space every 3 minutes and added carbon filters, you would now need an 80 CFM fan to accomplish the same task (64 CFM + 1.25 = 80 CFM).
Ducting and Bends
Just as with filters, every inch of ducting and every bend in that ducting slows down airflow. The air from your fan has a greater distance to reach its destination which requires more force from the fan to get it there. For every foot of ducting, increase your fan's CFM by 1% and for every bend of 90 degrees, increase it by another 20%.
For example: If you needed an 80 CFM fan to begin with and ended up using 4 ft. of ducting between both your intake and exhaust fans, you'd need an 83.2 CFM fan to overcome the additional distance (80 CFM + 80 CFM x (4 ft. x .01) = 83.2 CFM).
If you then had two 90 degree bends, you'd need a 115 CFM fan to overcome the slowdown that occurs when there is an angle in ductwork (83.2 CFM + 80 CFM x (2 bends x 0.20) = 115.2 CFM).
Negative or Positive Pressure
The last decision to make is whether you want negative or positive pressure in your grow space. So what's the difference?
Negative pressure occurs when you are exhausting air from the grow space faster than you're bringing it in. A grow tent will look like the walls are being sucked in when this occurs. The benefit to negative pressure is that it helps to control odors from escaping your environment. It forces all of them out a single opening (your exhaust fan), which, when combined with a carbon filter, will eliminate odors from leaving the tent almost completely.
Positive pressure occurs when you are forcing air into the grow space faster than it's being exhausted. A grow tent will look like the walls are ballooned out when this occurs. The benefit to positive pressure is that it helps prevent any unwanted pests, pathogens or particles from entering the environment. It forces all air to enter through a single opening (your intake fan), which, when combined with a carbon filter, will scrub the air before it reaches your plants.
In either scenario, we'd recommend having one fan run at a CFM of at least 15% greater than the other fan. You can also use a speed controller for your fans to turn one of them down slightly, which would accomplish the same thing.
For example: If you needed 115 CFM fans to replace the air in your space every three minutes, but wanted negative pressure, you'd want to have an exhaust fan that was at least 132 CFM to accompany your 115 CFM intake fan (115 CFM + (115 CFM x 0.15) = 132.25 CFM). If you wanted positive pressure, you'd just swap those numbers so that your intake fan was 132 CFM and your exhaust fan was 115 CFM.