CFM Calculations: Not sure how to use these numbers

randybishop

randybishop

Well-Known Member
I used this online app to calculate the CFM needed for my grow box: http://cheaphydroponics.com/Merchant2/cfmcalc.htm

It said that I needed 1.7361097222200002 every 5 minutes -or- 8.6805486111 every 1 minute for best performance.

Most computer fans I see online are 40 CFM. Does 8.6805486111 every 1 minute mean that a 40 CFM would be powerful enough?
 
qwizoking

qwizoking

Well-Known Member
I don't know any cfm calculations.. but that doesn't sound strong enough for even the smallest of grows...for people in a budget, I recommend a 20" box fan. Can be found under 20 bucks and push 2000-3000 cfm depending on brand..you can then turn it down if you feel its too much
 
n0tmycupoftea

n0tmycupoftea

Well-Known Member
The airflow calculations are correct. The math is simply getting your grow areas space in cubic feet. My box is 16in by 16in by 7in. So my total in cubic feet is 1.33 x 1.33 x .583 = 1.03. Now what this simply means is that i have 1.03 cubic feet of area, so if i wanted to change out the air completely and have new fresh air in my box i would need a fan to pull about 1.03 cubic feet per min. Now ive only seen this used for like bathroom fans because the problem youre going to run into in a micro grow is dissipating all the heat that your lights are putting out and bringing in new fresh air. Thats why you see most setups with only one intake and two exhaust fans or one small intake and one huge exhaust.

A 40 cfm fan will be more than enough to pull in new air for your micro grow, the only thing you have to worry about is keeping your temps straight through properly placing your exhaust fans to be effective enough to move all the hot air out of the box.
 
randybishop

randybishop

Well-Known Member
n0tmycupoftea said:
The airflow calculations are correct. The math is simply getting your grow areas space in cubic feet. My box is 16in by 16in by 7in. So my total in cubic feet is 1.33 x 1.33 x .583 = 1.03. Now what this simply means is that i have 1.03 cubic feet of area, so if i wanted to change out the air completely and have new fresh air in my box i would need a fan to pull about 1.03 cubic feet per min. Now ive only seen this used for like bathroom fans because the problem youre going to run into in a micro grow is dissipating all the heat that your lights are putting out and bringing in new fresh air. Thats why you see most setups with only one intake and two exhaust fans or one small intake and one huge exhaust.

A 40 cfm fan will be more than enough to pull in new air for your micro grow, the only thing you have to worry about is keeping your temps straight through properly placing your exhaust fans to be effective enough to move all the hot air out of the box.
Click to expand...
Thanks for the explanation n0tmycupoftea! :)
I have a LED light, so I don't think heat should be a big problem.
 
Red1966

Red1966

Well-Known Member
Put your exhaust above the light and you should be good. That's an awful small growbox.
 
emannn2010

emannn2010

Well-Known Member
hey bishop, tea is correct, it probably would be easier to put ur exhaust fan inline with ur lamps, u will find even with passive intake ur temps are controlled, I think that we sometimes over complicate when growing. I have looked at a lot of pc grows and most seem to pull 3/4 to an ounce dry. good luck and happy growing :-)
 
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