Fans - CFM Explained
Fans - CFM Explained
Fans - CFM Explained
Indoor gardeners should plan for ventilation when designing their grow rooms and use inline (vent) fans to pull stale air out and intake fans to pump fresh air in. Plants transpire water into the air and if nothing is done to replace this moist air with fresh, dry air, the stomata can't do their job effectively causing growth to stop.
When growing hydroponically, the plants grow very fast. All day and night, they're releasing moisture into the air. If the air isn't sucked out, the humidity will reach 100% . In addition to this causing the stomata problem, it also creates an environment ripe for pests and diseases.
In order to get rid of the wet, stale air, you need an inline fan pulling air out of the grow room. You may or may not need to pump fresh air in. This depends on how your grow room is designed.
For example, grow tents have vents on the bottom. As warm air rises and is pulled out via the inline fan, cool air enters the bottom through the holes. There's a bunch of these holes but they're small and mesh covered to keep pests out. Also, there's a flap that covers them to prevent light leaks. This type of system is called a passive intake.
On a side note, the tent manufacturers recommend pumping fresh air into the tents even though they're made with the passive intakes. They have holes designed for fans at the bottom so you're pushing fresh air in and your out-take fan is pulling it out. This is an active intake.
Despite the tent manufacturer's suggestion, I only used the passive vents and ducted hot air outside via an inline Vortex fan. This, combined with an air conditioner created a perfect environment and I had great success.
Also, by using just one fan and pulling air out, you'll create negative pressure in the tent which is much more effective than pumping air in at the same time. The truth is, only the biggest grow rooms require active intakes.
Now that you know you need an inline fan, you need to understand CFM, which stands for cubic feet per minute. CFM is a measurement of how much air the fan is capable of moving. When deciding on what size fan to buy, you'll need to know the total volume of your grow room.
To figure this out, simply multiply the length x width x height of your grow room. You need to choose a fan that can suck all of the air out of the room in less than five minutes so after figuring out the volume, divide the number by four (minutes). Round this number up, and choose a fan rated to move at least that many CFM.
For instance, if your grow is 5 ft 5 x ft x 7 ft = 175 (total volume) / 4 = 43.75 rounded up to 44 means you need a fan rated for at least 44 CFM. The Vortex 4 inch is rated at 172 CFM and is what I used in a 4 x 4 tent. The high CFM rating of the fan created a negative pressure and resulted in the tent always having fresh air. Again, the results were awesome.
I've written several articles explaining the formula for figuring out total volume. Here's the thing; I think any kind of math makes some people's brains switch off. Even though it's an easy thing to figure out, they see the numbers and go, "Screw it, there's math."
As a result, I keep including it in different articles hoping you'll see it so many times you'll finally realize it's easy. Knowing the total volume of your grow space is an important number that will keep coming up for different reasons.
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