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How Much Ventilation Do I Need in My Grow Room?


New Member
Each fan has a rating that tells you how many cubic feet of air per minute (CFM) it will move. When dealing with above average temperatures you will want your fan to exchange the grow room air 3-5 times in one minute, so for a room that is 40 cubic feet, a fan that is capable of moving 120-200 cfm (cubic feet per minute) is recommended. If you only want to replace depleted levels of co2 and are growing in a closet using fluorescents, one room change per five minutes (divide room size by 5) will be adequate.

To calculate your room size, multiply Width by Length by Height, this will give you the CFM rating required for one room change per minute.

Air that's exhausted will be replaced by fresh air, which is drawn from

Passive Intakes-
located opposite the exhaust at the floor of your grow room (Recommended for closet grows) or forced in using another blower (recommended for larger grow rooms).

For internal circulation oscillating fans are In my opinion the most efficient devices for circulating air in a room. The gentle back and forth sway of the fan is very beneficial for the developing plants. These fans tend to keep anaerobic moulds down by constantly freshening any potentially stagnant air. Home improvement centres carry a large array of various types and styles of air-moving fans, there are wall-mounted styles available and most are relatively inexpensive.

NOTE: To convert CFM to CMH use the folowing formula CFM X 0.03 X 60 = CMH

To help maintain a lower room temperature follow this link detailing how to construct a DIY- Air-cooled Reflector

Author: MedMan


New Member
My problem is I live in montana, and it is winter outside. I am using a spare bedroom, and the only intake outtake i can do is the same window with a piece of plywood to block air, and two fans set inside the thing. Now should I put two fans in, inlet on bottom and outlet on top? I am going to vent the outlet up top sideways so it isnt just sucked back inside the room. Since I am growing where it is cold, should I just have a passive inlet because that much cold air is not going to do well? Or should I stick to two fans, the room is 7.5x7.5x8. Any fan suggestions would be great too. Thanks.


New Member
Claytonj, are you in a main level house, basement or upstairs because air is different depending on location. From what you are saying I would stick with 2 fans, the more moving air the better as long as it's not blowing your stalks in half. Also you want the air coming in to be around 60 degrees if possible, if you can't your room temp still needs to stay between 72-78 degrees. What are your fans CFM ratings ?


New Member
It depends on your fan. Push fan or pull fan? Loosely speaking, call it 1-1/2 feet per inch of diameter of your pipe. So 6" pipe can be ran 9 ft., but that's prolly not accurate either because its just ventilation air. Again depends on the fan rating, dryers can go 25 ft before needing a booster. But to answer your question directly, 5 ft is what the 90 counts as. If you're worried about losing cfm, simple upsize your run.

Pop Kulture

New Member
"you will want your fan to exchange the grow room air 3-5 times in one minute"

I think this might be backwards - according to most things I have read it should be noted that the air in the room should be exchanged every 3-5 minutes not 3-5 time a minute. This is a BIG difference!


On Vacation
whats the hottest u can go in cooker with out damage heat .


New Member
i have 2 questions i haven't been able to figure out even with reading all the sections on odor control, grow room security, and ventilation. how would you keep the smell from going outside your grow box without running the fans 24/7 if you have one or more passive intakes and exhaust using a fan that runs the air through a carbon filter? also, is there an easier alternative to keeping light from escaping your passive intake besides building an S-shaped light trap? any help would be greatly appreciated and thank you in advance


New Member
don't ever turn the fans off. smell will gradually seep back out the passive inlets pretty quickly. as far as light leaks, i can't tell w/out seeing u're setup, but it sounds like u're on the right track w/the s setup. u'll need a can or 2 of 'flat black tractor paint', available at any tractor supply store or lowe's/home depot. just spray paint the inside of the pipe. it'll help kill any possible light from shinning thru. good luck.


New Member
ok thanks. wasn't sure if that would work or not and there is no pipe in my passive intake. its just a hole. but i'll just get a curved piece o' pipe and paint it black inside:thanks:


New Member
I have a question my grow area is 84 x 86 according to your formula my factor is 7476. What size blower do i need. I dont quite get it. It looks like to me i need a much bigger blower have taken a string and measure my optimum lighting to place my 3 ballast and hoods correctly. This creates a lot of heat so it is important to get the air circulating I will eventually will have 15 plants in here and maybe more if it will allow. Right now i am licensed to grow 15 plants.


On Vacation
ive been using 9,000 eff. 24/7, then when system comes on,cfm fan, then keep at 76-78 deg. cooking,soil,then i swivel fan 24/7.and small cfm fan foe exhaust,24,7.its time ,leaNring, then i fun 48-55 humidity.in cooker.im legal for 24. but to keep numbers rite and have buds size of 2 liters ,in cooker.its time consuming,but always remember KEEP IT SIMPLE!!!!LISTEN TO 420 FORUM!!!!BELIEVE ME .MY SHIT IS DANK!!!!BUT YOU HAVE TO LEARN THE HARDEST. TIME !!!!!!:420:


New Member
sorry guys but i have a valid question about the needs of the plant itself.
lets just say that for my grow consideration smell is not an issue in the slightest, im trying to work on a closed enviroment, but in the mean time im not bothered.

also my grow area is cold, typically 10-20°C so i rely on my lights to keep them warm.

so my needed answers to the question [h=2]"How Much Ventilation Do I Need in My Grow Room?"[/h]are about;

How much air movement is needed for happy plants
if the plant is in a sealed enviroment what needs to be considered about that volume of air, such as oxygen levels co2 levels are any toxins produced and need to be removed and is there anything in the air other than O and Co2 that is depreciated in some way.

i know its a bit of a left wing perspective, but im just trying to get it all right in my head.
pretty much got my understanding of lights down, and coming a long way in nutrients, but the pure science of the air considerations still elude me =/


New Member
sorry if this question has been answered if it has i obviously dont fully understand as i have looked all through these posts.

Im starting a basement grow. A room 4*7*8 feet high. i only have 4 plants in it with a 400hps. Our laws in canada have changed as of late, now anything over 6 plants is an automatic jail sentence. thats how i understand is anyway.

my question is regarding the exhaust fan. i bought a 4" inline fan 165cfm with scrubber for this purpose. I have no heat issues so im really just getting rid of smell and replenishing co2. My problem is that i have about 30 feet and one 90deg turn to the outside wall. can this fan do what i need? thank you all for you time!!!
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