CFM math !@#$

I'm trying to find some fans to cover my MASSIVE grow space. The math for finding the CFM i need is freaking me out though. Can someone double check me?

Here are the dimensions of my massive space:

WxHxD
16"x18"x8" (inches)

So in order to get the cubic feet, i divide all my dimensions by 12 and then multiply them together?

(16/12)x(18/12)x(8/12)
i get ~1.4^3

So at a minimum I only need a fan to cover 1.5CFM? A 60mm fan @12CFM would clear the space 8 times per minute?

What am I missing? Why do i feel like i need more fans?

Your numbers are fine.

When I smoke a bunch I become untrusting of my own calcs a bit too hehe. Always double check. Over fertilized once like that...woke up at 2 am. I guess I realized it in my sleep and was like OH FUCK. Shitty taxi ride that was.

for that massive space,your calculations are right,kinda.Your looking at roughly 3 ft cubic space,you only need to exchange the air once every two minutes or so,with 5 being the norm for co2 supplementation.

My math concurs with yours. It doesn't feel like that's enough, but it is.. at least for displacing and replacing your airspace. Now, depending on what kind of lighting you use, you may need to up your cfms substantially. Or you could employ a cooltube.

Where are putting the fan? If you are only using one fan I suggest putting it on the exhaust and allowing the fresh air to enter the space passively, rather than vice versa. You want negative pressure rather than positive to keep the temps lower and to keep the stank in If you were planning on fans at both ends, then the more the merrier.

EDIT: Is this a PC grow? And I assume your using CFLs, correct?

I'm using 4, 26W CFL's.

I'll have an intake at the front near the bottom... im guessing around 60mm at whatever CFM.

My exhaust is going to be a 120mm at whatever CFM. The bottom of the fan will be at the mid-point of the grow space height. Near the lights, but not completely at the top.


What kind of negative pressure am I looking for? I know i want more going out than in, but how much more?

CaNNaBiZ CaNucK said:

Is this a PC grow? And I assume your using CFLs, correct?

Click to expand...

Yes, and Yes

dark farmer said:

I'm using 4, 26W CFL's.

I'll have an intake at the front near the bottom... im guessing around 60mm at whatever CFM.

My exhaust is going to be a 120mm at whatever CFM. The bottom of the fan will be at the mid-point of the grow space height. Near the lights, but not completely at the top.


What kind of negative pressure am I looking for? I know i want more going out than in, but how much more?

Click to expand...

Just enough so that your not causing a slow down in the airflow. If you have a higher cfm fan intake then there is a build up of pressure. More air going in than out. If your exhaust has a higher cfm than more air is exiting than what is being drawn in from the lower cfm intake. Therefore you will be drawing air in from the cracks as well,, which is better than the former.

From the looks of your setup, having the larger 120mm fan on the exhaust should do you fine, providing it has a higher cfm than your intake 60mm.

Excellent.

If i were to throw in another fan specifically for air circulation inside the case, how does that affect the negative pressure?

luvvin growin said:

Your looking at roughly 3 ft cubic space

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I know 1.5 to 3 ft isn't that big of a deal, but for my future reference, how did you come about 3 cubic feet?

dark farmer said:

Excellent.

If i were to throw in another fan specifically for air circulation inside the case, how does that affect the negative pressure?

Click to expand...

You should be ok if your circulation fan has equal or lower cfms than your intake and exhaust. If it is higher, you'll be outta whack.

dark farmer said:

I know 1.5 to 3 ft isn't that big of a deal, but for my future reference, how did you come about 3 cubic feet?

Click to expand...

Your math was correct, Dark. 1.365 or something..

CaNNaBiZ CaNucK said:

Your math was correct, Dark. 1.365 or something..

Click to expand...

haha cool, thanks Cannabiz

No worries. Hope it all comes together nicely for you

CaNNaBiZ CaNucK said:

No worries. Hope it all comes together nicely for you

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I ran the lights with an 80mm exhaust fan for 6 hours just to see what kind of temps I'm looking at. The highest I got was 82. Then my heater turned on and bumped it up to 86 degrees. Not bad. After the heater turned off it settled around 84.