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Thread: How Much Ventilation Do I Need in My Grow Room?

  1. #1
    420 Member SmokeyMacPot's Avatar
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    How Much Ventilation Do I Need in My Grow Room?

    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

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    420 Member Slowpuffer's Avatar
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    Re: How Much Ventilation Do I Need in My Grow Room?

    Thanks smokey,
    I'm thinking of buying this and would like your advise. ( GrowBright 6" Inline Fan & Charcoal Filter Combo) Will this both ventilate and do odor control, with no other ventilation, except for an internal fan. Box is 4x6x6.

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    Re: How Much Ventilation Do I Need in My Grow Room?

    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.

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    420 Member Zodiakgrower7's Avatar
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    Re: How Much Ventilation Do I Need in My Grow Room?

    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 ?
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    Re: How Much Ventilation Do I Need in My Grow Room?

    I have a question regarding 90° and length of run...exctally how much cfm do you lose?

    Thank you.

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    Re: How Much Ventilation Do I Need in My Grow Room?

    I have a question regarding 90° and length of run...exctally how much cfm do you lose?

    Thank you.

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    Re: How Much Ventilation Do I Need in My Grow Room?

    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.

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    Re: How Much Ventilation Do I Need in My Grow Room?

    Thanks for the answer 2hit, that is what I ended up doing...went with 6"HO canfan (440cfm) then also got a speed controller

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    420 Member Pop Kulture's Avatar
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    Re: How Much Ventilation Do I Need in My Grow Room?

    "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!

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    420 Member What is sober's Avatar
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    Re: How Much Ventilation Do I Need in My Grow Room?

    thanks for the cfm and chm formulas
    Its so easy to make things hard

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    On Vacation DeerHunter33's Avatar
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    Re: How Much Ventilation Do I Need in My Grow Room?

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

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    420 Member treezzz's Avatar
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    Re: How Much Ventilation Do I Need in My Grow Room?

    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
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    Re: How Much Ventilation Do I Need in My Grow Room?

    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.
    Dirty9mm likes this.

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    420 Member treezzz's Avatar
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    Re: How Much Ventilation Do I Need in My Grow Room?

    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
    Check out my first grow!!!
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    420 Member Twinkie's Avatar
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    Re: How Much Ventilation Do I Need in My Grow Room?

    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.

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