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Ventilation Math

stratlogic

Active Member
Does a formula or rule of thumb exist? I have calculated that my grow box has a volume of 16 sq/ft in the grow chamber. Does any one know of proper cfm(cubic feet/minute) to sq/ft ratio when deciding on what a fan's cfm should be? For example, if I bought a 160cfm fan or multiple fans equaling a total of 160 cfm, that would theoretically replace the air in the grow chamber 10 times a minute, or once every 6 seconds. Is this enough, too much, or not enough? Help please if you can. Thanks. My goal is to not have too many fans or too big of fans because I am trying to keep the fan noise to a minimum while maintaining proper air flow.

Heres a pic of my box to give you a better idea.
 

stratlogic

Active Member
Thanks Dank! You sure do have all the answers. So if I have this right, I could get by using a 40cfm fan? That would replace the air in a 16 sq/ft box 2.5 times a minute. cfm/(sq/ft)= number of times air is replaced/min. Does that seem too small of a fan though? Only 40cfm? Or is once every 5 minutes the bare minimum? Or is "more the merrier" in a situation like this? Thanks for your help.
 

stratlogic

Active Member
I am definitely going to put an oscilating fan in the box to get the plant nice and strong. And there will be no minimum anything during this grow, I can assure you that! haha
 

fallen

New Member
yeah for real, because i'm about to start making everything you're putting on these forums and i want to see the creators results :p
 

stratlogic

Active Member
Of course guys, you won't have to worry about that. I am in the process of getting a med card so I can go pick up some seeds locally. I'm still a bit reluctant to order them online. I need to research that some more because I may not be able to wait much longer.
 

zolar

New Member
volume of cubes and rectangular boxes is usually [lengthXwidthXheight]

multiply all three together in the same units ie [ft X ft X ft ] [in X in X in ]
this give you cubic units cubic foot/feet cubic inch

this is your static volume of your space ie 4ft X 4ft X 6ft = 96 cubic feet

fan speed 24 cubic feetper minute volume size is > fan speed

so divide space / fan speed 96/24 = 4 time to change air
in room in in and out vents are equal in minutes


a 24 CFM fan will move 96 cubic feet of air in 4 minutes so in a cabinet with equal in/out airflow a 4x4x6 foot cabinet total air change once every 4-5 minutes or 12 to 15 times an hour hope that helps on a simple level of course you could look up the EPA air report for your location and calulate the CO2 & O2 percent from athmosphereic gas percent standards and really complicate things.......
 

stratlogic

Active Member
Thanks Zolar. Thats exactly what I was looking for. Would you, or anyone happen to know roughly how much an air filter obstructs air flow if it does at all?

I am concerned how well my two 4" 'puter fans would hold up if I stuck a carbon filter on both exhaust fans. I would think it would reduce my cfm a significant amount, but not sure.
:hmmmm:
I saw a dude somewhere post plans on how to make one with heavy duty chicken wire, panty hose, activated carbon from pets mart (or even kitty litter) and a few more items. It sounds like it worked well so I wanted to make one too. Two actually. I'm thinking I might need to pick up some more fans if the two in the mail aren't up to the job. I don't know.

Does anyone who uses a carbon filter have any thoughts because I have never used one and want to be sure about this. My girls deserve it.
:laugh2:
 
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stratlogic

Active Member
I am starting to think a squirrel fan is in fact what I need. I need a fan with impeller blades and not propeller blades like those on the fans I just ordered according to some info I read on another site. Doh! Well, I guess I just have to find a way to make them work. Not sure if a squirrel fan is in the budget.
 

fallen

New Member
I just looked on ebay and they go for as low as like.. 15 - 20 dollars, so maybe you could get one there
 

stratlogic

Active Member
Thanks fallen. I found a squirrel cage fan for only $15 each so I picked up two.

Specs:
• Current: 1.20 amps (1200mA)
• Speed: 4300 rpm
• Noise: 57 dBA
• Dimensions: 97mm x 93.5mm x 33mm (3.82" x 3.68" x 1.30")
• CFM: 28
• Weight: 205.5g (7.25 oz)

They looked a little small so I decided to get another despite the 28 CFM so I should be set. All I need now is some nutes and seeds and I am all set to start a journal. Can't Wait!
 

stratlogic

Active Member
I'm thinking of using both squirrel fans for exhaust and two puter fans for intake down below and one for exhaust on the light. I'm hoping to have it finished within the week.
 

jwb87

New Member
Hey man, love your builds, very inspiring. I'm looking to start building mine this weekend and hopefully start ordering my parts this next week, but I'm having the same question you did here.

I noticed you weren't a huge fan of the sqr cages in your other thread so Id like to stick to PC fans. I'm looking to grow 4 DWC plants in a 48"(H)x30"(W)x24"(D), and looking to vent as you did with a set of upper fans passing air by my lights and a lower set of intake/out take in the lower portion passing air by the plants/res.

From your experience do you have any suggestions ON fan size? Im afraid of over heating and was thinking 2 Intake 120mm and 2 Outtake 120mm, both intakes on the right side and both out takes on the left, spiced into a similar variable speed setup you have. Thanks man, keep up the awesome work!!!


EDIT** Rechecked your other post and this: 420 Magazine ® is exactly what I'm looking to build (looks like we have the same closet haha). No upper section and no sliding back light. Im looking to build some sort of a reflector to house 8 CFLs rigged to a jackchain so i can raise and lower depending the stage/size of the plants.
 
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