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Thread: Active intake Vs. exhaust

  1. #1
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    Question Active intake Vs. exhaust

    Hi folks this is my first grow room. I cant figure out whats best in term of ventilation for making
    negative pressure in my flowering room. I need someone to give me some advice.

    My room is sealed, its 4.5 by 4.5 foot, with 5.5 foot high.
    I bought a 245 cfm fan and a 50 cfm fan the lady at the indoor store suggested according to
    my room mesurements.(Ones 5 times bigger) The biggest being the intake has she said it should be.

    ..(here is what stucks me,...) On internet i found many people saying that the air
    intake should be 5 times bigger than the outtake, but only if the intake is passive..
    ( no fan..just a hole.)... and this should build the wanted'' negative pressure''.

    Q; How true is that? And what about If i want my intake active? Does it still needs to be 5 times
    bigger than exhaust or will that demolishe negative air pression? Whats the gauge? How to mesure?


    Im running a 400w, my 50 cfm fan is connected to the flexi foil that conduct to my homemade
    charcoil filter wich makes a few turn here and there and is about 30 foot long leading outside.
    Ive tested my exhaust and its working,air fairly goes trough even dough my filter and flexi tube
    lenght causes a diminution of air flowing out.( but im aint flowering or using the room yet so i have
    no clue about the smell.. therefore dont know how good it works.

    NOw, since my intake wont be passive, but active ( i already bought the fan and an active intake sounds best for it
    will keep airflow in my room;...Q: Would it be a good idea to switch fans, and use my
    245 cfm as exhaust, and put the 50 cfm as intake, since i have another intake ( wich is at the same
    time the outake of my vegetative room) ?
    ..As i mentioned already; my flowering room outake (50 cfm) is having a
    little harder time pushing out that it should ,..
    This second intake is a computeur 80 mm fan 0.19 A ( 30 cfm approx).

    So swithching the fans would give me: flowering room; (intake #1: 50 cfm) + (intake #2: 30 cfm) = 80 cfm intake active vs.
    a 245cfm outtake ...this one considerably reduce in capacity ;...(charcoal filter, 30 feet long flexi tube
    in the way yadiyadiya...). Is it a big deal if the exhaust is bigger than intake afterall?

    Does that make any sense anyone? what other option would be best? Keep it as it is would mean a 50 cfm exhaust
    with diminished capacitiy blowing out vs. 2 full capacity active intake 30 cfm + 245 cfm = total 275 cfm intake active.
    ( ...im afraid too much air will be sucked in, not enough sucked out and therefore odor will leak out cracks.) What to do?

    Also in my vegetative room i have this intake computeur fan (70 mm 0.40 A... approx.30 cfm or little more),
    wich blow a little harder than the exhaust (80 mm 0.19 A.. 30 cfm approx or little less.)
    This room is 2 by 4 foot with 5 foot high. Should i let the intake run? or shoul i take the fan out and
    just leave a hole 5 time bigger ..like a passive intake? or is it good enough with both running... in and out
    fairly equal?? HELP Tks

  2. #2
    420 Member MisterPeabody's Avatar
    Join Date
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    Re: Active intake Vs. exhaust

    If you go passive, you need 4-5 times passive intake vs supply. Here's a link you might find of interest. Grow Cab Ventilation Redesign-All About Heat

  3. #3
    420 Member
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    Re: Active intake Vs. exhaust

    As Peabody said For passive systems you want 4-5 times the air "passing" through the intake then exhausting through fan Move that 256 to the exhaust side and check temps...in the past adding a smaller fan to intake (not atleast 75% of exhaust cfm) just impedes on air flow.. fans more productive "pulling" than pushing. good luck

  4. #4
    On Vacation
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    Re: Active intake Vs. exhaust

    Quote Originally Posted by Beefree View Post
    Hi folks this is my first grow room. I cant figure out whats best in term of ventilation for making
    negative pressure in my flowering room. I need someone to give me some advice.

    My room is sealed, its 4.5 by 4.5 foot, with 5.5 foot high.
    I bought a 245 cfm fan and a 50 cfm fan the lady at the indoor store suggested according to
    my room mesurements.(Ones 5 times bigger) The biggest being the intake has she said it should be.

    ..(here is what stucks me,...) On internet i found many people saying that the air
    intake should be 5 times bigger than the outtake, but only if the intake is passive..
    ( no fan..just a hole.)... and this should build the wanted'' negative pressure''.

    Q; How true is that? And what about If i want my intake active? Does it still needs to be 5 times
    bigger than exhaust or will that demolishe negative air pression? Whats the gauge? How to mesure?


    Im running a 400w, my 50 cfm fan is connected to the flexi foil that conduct to my homemade
    charcoil filter wich makes a few turn here and there and is about 30 foot long leading outside.
    Ive tested my exhaust and its working,air fairly goes trough even dough my filter and flexi tube
    lenght causes a diminution of air flowing out.( but im aint flowering or using the room yet so i have
    no clue about the smell.. therefore dont know how good it works.

    NOw, since my intake wont be passive, but active ( i already bought the fan and an active intake sounds best for it
    will keep airflow in my room;...Q: Would it be a good idea to switch fans, and use my
    245 cfm as exhaust, and put the 50 cfm as intake, since i have another intake ( wich is at the same
    time the outake of my vegetative room) ?
    ..As i mentioned already; my flowering room outake (50 cfm) is having a
    little harder time pushing out that it should ,..
    This second intake is a computeur 80 mm fan 0.19 A ( 30 cfm approx).

    So swithching the fans would give me: flowering room; (intake #1: 50 cfm) + (intake #2: 30 cfm) = 80 cfm intake active vs.
    a 245cfm outtake ...this one considerably reduce in capacity ;...(charcoal filter, 30 feet long flexi tube
    in the way yadiyadiya...). Is it a big deal if the exhaust is bigger than intake afterall?

    Does that make any sense anyone? what other option would be best? Keep it as it is would mean a 50 cfm exhaust
    with diminished capacitiy blowing out vs. 2 full capacity active intake 30 cfm + 245 cfm = total 275 cfm intake active.
    ( ...im afraid too much air will be sucked in, not enough sucked out and therefore odor will leak out cracks.) What to do?

    Also in my vegetative room i have this intake computeur fan (70 mm 0.40 A... approx.30 cfm or little more),
    wich blow a little harder than the exhaust (80 mm 0.19 A.. 30 cfm approx or little less.)
    This room is 2 by 4 foot with 5 foot high. Should i let the intake run? or shoul i take the fan out and
    just leave a hole 5 time bigger ..like a passive intake? or is it good enough with both running... in and out
    fairly equal?? HELP Tks
    Well what I use us a intake fan that is 1/3rd less powerfull as my exhaust fan I have a 450cfm exhaust and a 250cfm intake the room is 520 cf so emptying the room 1 time per minute and filling it 1 time every 2 minutes gives me excellent negative pressure as my room is fairly air tight the best way I have found of testing negative pressure is to use a match or lighter to test the draw if it sucks the flame inwards with fair strength then your laughing personally I found that every place where air can be drawn in along the edges of doors and the like the negative pressure is strong enough to extinguish a lighter.

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