# Air exchange per cubic foot ratio?



## gchristo (Sep 29, 2011)

Fellow growers, 
I'm in the process of updating my ventilation equipment.  
How many times per minute or hour should air in grow cabinets be exchanged?  Have 2 cabinets, 1 veg - T-5's  (35 cubic feet), 1 flower - 600 hps w/ cool tube hood (90 cubic feet), for a total of 125 cubic feet.  Planning to vent both w/ same fan.  Will a Vortex in-line 4" 175 cfm be enough?  Or should I move to a 6" w/ a cfm of 450? 
Cheers.


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## 4u2sm0ke (Sep 29, 2011)

I exchange air every 2 minutes..

:48:


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## gchristo (Sep 29, 2011)

4u2sm0ke, 
Thanks.  That makes it a load easier.  My current ducting is 4" and moving to 6" would be a real pain.  
Cheers.


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## Stoneage (Sep 29, 2011)

How does the math work again? #x#x#=csf?


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## gchristo (Sep 29, 2011)

Stoneage.  Example:  width x length x height = cubic. ie.  52" x 31" x 83" = 133796 cubic inches.  1 cubic ft = 12 x 12 x 12 = 1728.  133796 / 1728 = 77.4 cubic ft. Vortex fan 4" = 172 cubic ft per minute.  Therefore Vortex 4" would flow at a rate of 172 / 77.4 = 2.22 times per minute. In other words it would empty and replace the air 2.22 times every minute or once every 27 secs.  

PuffinNugs. My dimensions are a bit different, 125 cubic ft, at a flow rate of 172 / 125 = 1.376 per minute, or once every 43.6 seconds. For the extra work of enlarging current cabinet holes to 6" from 4" vs a bolt in replacement, I am bolting.  Thanks for the input though.  

Cheers.


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## The Hemp Goddess (Sep 29, 2011)

Get a larger fan than you believe you need.  If you put a carbon filter in, you are going to want the extra power of the 6" as a filter will cut your cfms.  You may find it hard to vent both your rooms with the same fan.


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## gchristo (Sep 29, 2011)

THG.  Origional question was: How many times per minute or hour should air in grow cabinets be exchanged?

4u2sm0ke responded: I exchange air every 2 minutes. Working back I derived:  rate / vol = cfm / vol = cfm / cf = m.  m in 4u2sm0ke's case is 2 or 2 minutes. 

PuffinNugs suggested: "youll want to atleast double the cfm on your fan compared to your room size" or 2(cfm) / cf  = 2m or twice (time).  Still not really answering, but obviously I could have worded the question more clearly.   

THG.  What is your recommended rate of cfm / vol.  or  time?  

THG.  Your suggestion re filter does bear some thought though. 

Thanks for answering.
Cheers.


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## The Hemp Goddess (Sep 29, 2011)

I personally like to exchange the air in my space 3-4 times a minute.  Many think this is overkill, but it is what I learned many years ago.  I can keep my space cool in 100+ temps exchanging the air this many times per minute.  I do not, however, use a filter.

This was not part of your question, but it is pertinent.  How exactly do you plan on running the 2 rooms together?


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## gchristo (Sep 29, 2011)

THG, 
Really more like two cabinets side by side w/ common wall.  One veg, one flower.  Run duct through common wall into a tee.  Extend one arm of tee through top to exhaust. Inline fan will draw air up through single riser connecting to both cabinets via tee. Each arm of the tee must have an elbow attached to eliminate light leakage from veg into flower cabinet.  Since I don't usually grow skunky mj, odor is not usually a problem.  A homemade charcoal scrubber can be attached up-stream from fan to negate odor if necessary, but exhausting to attic of garage should eliminate this step as vented air will be allowed to escape garage attic via ridge vents. Riser from top of cabinets to attic is only 3 ft, so run is not long. 
Thanks for input. 
So now I have rates from 3 to 4 times a minute (The Hemp Goddess) to once every 2 minutes (4u2sm0ke).  Since both of you grow marvelous specimens, and I would be pleased to consistently grow what you two routinely offer up, somewhere between the two should suffice.   I found a section in Jeorge Cervantes' book that says once every 4 or 5 minutes, but that seemed too low a turnover rate. 
Thanks. Cheers.


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## Lemon Jack (Sep 30, 2011)

Yes I believe that is way too low. I try to change my air at least once a min its closer to twice i believe.(I have a slanted roof so Its really hard to figure it exactly lol.


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## The Hemp Goddess (Sep 30, 2011)

If you are doing 2 duct runs to a tee that will be connected to your fan, you are going to want to try and make your duct runs and your offsets equal on each side or one side will pull way more air than the other.  I always run my venting in parallel rather than series when connecting 2 lights to one fan.


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## docfishwrinkle (Sep 30, 2011)

gchristo, try to not use tee's & go for wye's. the least amount of bends & angles the more cfm's. my veg cab is 75cuft & i have a 350 cfm fam on it & thinking of swapping it w/ a 583 cfm just sitting here, but its a pain to access. so i exchange veg at rate of ~4.5x's a min & my bloom room is 403 cuft & gets exchanged 2.5. both are just fine

also try to use rigid duct as much as poss cause flex duct causes alot of turbulence. i think the in series w/ lights will be better than a parallel setup due to the fact that there is more velocity "sucking" through the one duct than 2 side by side. plus much easier & cheaper to install as takes up less space. jus my 2 cents


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