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Establishing proper ventilation in your grow room or growing area is an integral part of designing your room. Sufficient ventilation is critical for growing nearly any plants for a multitude of reasons. For one, temperature and humidity are very important conditions to control and proper ventilation will go hand in hand with those and other important environmental conditions. Other issues that proper ventilation help with are disease and odor control, as well as providing what is known amongst indoor growers as fresh air exchange. Simply put, proper ventilation is very important if you take your harvests seriously. Figuring out how much ventilation you need is not rocket science, but it is not impossible. Using a few formulas, we’ll walk you through the process of determining how much air you really need to move.

What Size Exhaust Fan For Your Grow Room?

A quick Google search will likely reveal many different calculations for grow room exhaust fan size. The problem is that these formulas and calculations are entirely too theoretical and do not take into account the other practical factors that come into play. We’ll get into that later, but first lets look at calculating the bare minimum CFM needed to ventilate a grow room of known dimensions.

Step 1 – Determine The Volume Of Your Grow Room

Volume is simply the size of your room accounted for in three-dimensional space. So, to determine the volume of your room, simply multiply its Length, Width and Height. So a room that is 5 feet long by 7 feet wide and 10 feet tall, would have a volume of 5*7*10, or 350 cubic feet. The term “cubic” simply indicates that the number represents the volume of a three dimensional space.

Step 2 – Determine CFM Requirements – How Much Air Exchange is Necessary?

Exhaust fans are usually rated in CFM, or the amount of air moved, in cubic feet per minute. Ideally, you will want a ventilation system that can exchange the air in your grow room AT LEAST once every three minutes. To determine what your CFM requirements are, simply divide the volume of your room by 3 minutes. Sticking with our example of a 350 cubic foot grow area, we would divide 350 Cubic Feet / 3 minutes to come up with a number of roughly 166.67 CFM. Thus, the BARE MINIMUM expected of an exhaust fan in this room would be 166.67 CFM.

Step 3 – Other Considerations & Variables

We’ve just helped you to calculate the MINIMUM CFM needed to ventilate your grow room. These calculations fail to take into account other factors that will affect how much ventilation is necessary. You will almost certainly need more ventilation than what you found in step 2, so don’t forget to take these things into consideration!

  • HID Lights (high intensity discharge) –  Grow lights that run “hot” such as HPS lights and Metal Halide lights increase the amount of ventilation needed. Once you’ve calculated your CFM needs with the method listed above, add 5% of that number for each HID light that is air cooled. For non-air cooled lights, you may need to add 10-20% for each light.
  • Filters – If a carbon air filter is to be used with your exhaust system, you must account for the friction this hardware will add to your system. Add 20% of your total CFM calculated in the three steps above if you are using a carbon air filter.
  • Temperatures – If your grow room runs hot you may need to add up to 25% of the CFM you calculated earlier. Hot and Humid climates may require up to 40% more than the CFM calculated above.

Example 

In our 5x7x10 room, let’s say we are running two 600-watt HPS lights, which are air-cooled. We also would like to use a carbon air filter so that the smells from our indoor garden don’t leak into the rest of our building. The ambient temperature outside of our grow room is often 100 degrees, but we will be maintaining our optimal temperature and humidity with separate environmental equipment and controls.

So using our steps outlined above, we can calculate a much better approximation of the CFM needs of our grow room, taking into consideration not just size of the room but other factors such as the amount of HID lights, carbon filters and other equipment.

  • Step 1 – 5′ x 7′ x 10′ = 350 cubic feet, which represents the volume of our grow room.
  • Step 2 – 350 Cubic Feet / 3 minutes = 116.67 CFM minimum needed, not taking into account variables such as HID lighting, carbon air filters, etc. It can be useful to round this up to the nearest round number. We will use 120 CFM instead of 116.67 for easier math in the following steps.
  • Step 3 – This step will simply take the number we found in step 2 and add percentages to itself based on our variables and the recommended percentages from step 3:
    • (CFM as calculated in step 2) + (CFM x 10% for our two air cooled HID lights) + (CFM x 5% for CO2 usage) + CFM x 20% for our carbon air filter)

At this stage of the calculations, start plugging in your CFM value. For our example, step 2 resulted in a CFM of 116.67, which we will round up to 120 for the sake of easier math.

(120) + (120 x10%) + (120 x 5%) + (120 x 20%)

Simplified, this becomes:

(120) + (12) + (6) + (20) = 158 CFM.

Now, we have a number of 158 CFM, which factors our added percentages for variables such as HID lighting, carbon air filters and the presence of added CO2.

Keep It Simple

Remember that the our calculations above are based on wanting to exchange the air every three minutes in your grow room. If you’d like to have fresh air exchanged once every two minutes instead of three, simply divide the number by two instead of three in Step 2.

Air Exhaust Without Air Intake Port

One often overlooked concept when it comes to installing a ventilation system or exhaust fan in your grow room is that there must be a place for air to enter your grow room. In other words, running an exhaust fan without any place for new air to enter your grow room will be largely ineffective. At best, your fan will struggle and overwork itself to try and push air out that simply isn’t there. At worst, your fan will not truly be able to bring new air into the room and will simply be circulating air around with minimal air exchange occurring. Your harvest and yields will suffer, and you will be more prone to plant disease, insects, etc.

To make sure there is plenty of incoming air to work with your exhaust fan, an intake port must be installed in your grow room. Anything from a small hole or gap under a door should be enough to let in ample fresh air. Some people drill a hole to create an intake port in the grow room. Many people believe the best place for an intake port is diagonally a crossed from your fan. The reason for this diagonal placement is to make sure that any new air that enters the room has to pass through the entire room before being expelled by your exhaust fan.

Also be aware that if you are not sure where the air that feeds your intake port is coming from, you may need to add a screen to prevent insects, animals, or dust and pollens from entering your room.

Many other people also recommend placing a circulating fan near the intake port to help “pull” in the fresh air, which would be considered an “active” intake system.

Relying only on your exhaust fan to pull in new air from your intake port would be referred to as a “passive” intake, which is not quite as effective. Many people end up purchasing an equally sized exhaust fan setup to run as intake fan from their intake port. If you can only have one or the other, the exhaust fan is much more important, but adding an intake fan can be very beneficial, especially in rooms with limited natural air flow.