Air flow in your greenhouse, grow room, or grow tent is as critical an element of success as the lighting, water, or nutrients you provide for them. Healthy plants rely on a steady supply of fresh air. In order to determine what you need for your airflow system, you will need to calculate your Cubic Feet per Minute, or CFM. A CFM value tells you what volume of air needs to be moved in the period of one minute. Another key value in this calculation is ACH.
ACH represents Air Changes per Hour. This calculation indicates how many times per hour the entire air volume within a space is replaced with recirculated or supplied air. It is sometimes referred to as the air exchange rate.
The ACH that you want to achieve is based on several factors. For growing crops, it should be a minimum of once every 5 minutes, which represents an ACH of 12. To determine this, divide the number of minutes in an hour (60) by the frequency in minutes that you want the air exchanged. Once every 3 minutes is an ACH of 20, once every minute is an ACH of 60, and so on. Every crop may have a different ACH can be found with a quick online search.
There are other factors to consider in real world environments that will affect the CFM value. There is the resistance of ducts to overcome as well as filters. Extra-long ducting or bends of 30 degrees or more will add resistance as well. We can account for these by increasing our CFM number as you will see below.
Calculate Your CFM
The airflow in your growing space needs to be strong enough to accommodate the required ACH. To find out our CFM, we need to know the volume of the room and the ACH. The calculation looks like this:
Grow room cubic area (Length x Width x Height) x ACH/ minutes in an hour (60).
As an example, to determine the CFM calculation for a 20’ x 10’ grow room with an 8-foot ceiling with a crop that needs to have its air changed twice per hour would look like this:
(20 x 10 x 8) x 2 = 3200
3200 / 60 = 60
CFM for this room is 60
Once you have your CFM, that is not the end of the story. Your CFM should be considered your minimum requirement. You should add another 25% to compensate for duct resistance. If you are using carbon filters, have extra-long ducts, or ducts with sharp bends, add an additional 25%.
In our example above where the CFM was 60, let's assume we have duct resistance and are using a carbon filter. Our calculation would be:
(CFM x 1.25) x 1.25 = revised CFM
60 x 1.25 = 75
75 x 1.25 = 93.75
Our new CFM factoring in resistance, would be 93.75.
If you already know your CFM, and need to size your ductwork accordingly, you can follow these general guidelines:
CFM up to but not above 50 = 4 inch duct
CFM 50-79 = 5 inch duct
CFM 80-119 = 6 inch duct
CFM 120- 169 = 7 inch duct
CFM 170-229 = 8 inch duct
CFM 230-299 = 9 inch duct
CFM 300-499 = 10 inch duct
CFM 500- 739 = 12 inch duct
CFM 740- 1049 = 14 inch duct
CFM 1050- 1399 = 16 inch duct
CFM 1400 – 1874 = 18 inch duct
CFM 1875 and above = minimum 20 inch duct
Other Airflow Units
CMF is not the only unit out there. This is the one most commonly used in the US, but in much of the rest of the world, this value is determined by calculating the liters per minute (l/min) or cubic meter per hour (m3/h). To convert CFM to these other values, plug in these numbers:
-1 CFM = 28.317 l/minute
- 1 CFM = 1.699 m3/h
Conversely, the approximate values are:
-1 l/minute = .035 CFM
-1 M3/H = .589 CFM