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1st indoor grow - exhaust fan

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    1st indoor grow - exhaust fan

    hello my friends, my plant just broke the soil! so wanted to know how high do you run your exhaust fan. full speed, half speed? my fan is a 190cfm, the tent space is 3x3x6 = 54cf, so what i've read, seems like you want to double your cf, so i would need to turn over 108cfm, so should I run my at 60%. do you thing 2 to 1 is the right ratio? Also, i have my light on an 18-6 schedule, but should the fan run 24/7?

    Hi John, I have a small fan in my tent running 24/7. I use my exhaust fan only to suck the humidity out of my tent after I watered my plants. I keep my tent open during vegging and during the 12 hours of light when I'm flowering and I use a oscillating fan to blow fresh air in my tent. Don't get too hung up on numbers. Keep it simple.
    Last edited by kingfish; 11-02-2019, 01:44 PM.


      I run the fan 24/7. I keep the vent windows on the side open during the day and close them at night. The back vent i leave open and lay cardboard to cover the hole so light cant get to it. Technically they say the calculation is the fan should replace the volume of your tent every 3 min, though i think that is a little low. If you are venting outside the room the tent is in then that could be right, but since im venting into the same room my tent is in i think a higher cfm is required (and clever positioning of exhaust ducts). I keep my fan on low although its 291 cfm on low (probably overkill, but if i upgrade i already have a fan) and if im having humidity issues i turn it up to get higher air exchange. When the lights are on you may want to have it at 50-60% (they say ad 10% to cfm for every 1000 watts of lighting) but during dark period you can turn it down, but i would leave it on personally.


        I run my 4” Hurricane exhaust fan into my attic 24/7 to keep fresh air from the room coming into my tent. Mines a 4’x4’x6.5’ and I run the fan at full speed. Through two grows I’ve been pleased.
        4’x4’x6 1/2’ tent, Nextlight Mega, Roots Organics 707 soil in 5 gallon square plastic planters, GH Flora Series Performance Pack nutes alternated with Fish Sh!t in RO water


        • YYCannabis
          YYCannabis commented
          Editing a comment
          Aren’t you worried about mold in the attic? I know I would be.

        • sgiacomo
          sgiacomo commented
          Editing a comment
          Nah no worries about mold there isn’t enough moisture coming out of my little 4x4 tent to hurt anything (we have a rather large well ventilated attic that would be great for a grow but haven’t convinced my wife haha). Problem I have this time of year is too low humidity due to central heat

        johnb1963 get yourself an Inkbird humidity controller from Amazon. It monitors humidity and will turn on the fan when needed to exhaust the humidity at whatever Rh level you set it to. Have a look at the pic. Each one of those valleys represents when my fan comes on and lowers the humidity. The rise is when the fan has turned off. Each full cycle is approx 6-7 minutes (my tent is quite full of maturing plants so humidity is abundant).

        My Inkbird is set at a target relative humidity (tRh) of 50%. I have set it to turn the fan on when the limit relative humidity (lRh) reaches 55%. But you can choose whatever variables meet you needs like :
        - what type of climate region are you in? Dry/humid hot/cold
        - ambient climate of the room your tent is in.
        - how many plants/containers
        - how far do you need to exhaust the air

        Im using a 4” diameter 110cfm fan from Vivosun (Amazon) for a 4x4x6.5ft tent. My fan pulls air through a carbon filter then exhausts the air to a vent approx 30 feet away. No speed controller...only on or off.

        Attached Files
        There are 3 things you must check everyday when growing in hydro:
        1) Water level (How much h2o is she drinking in 24hrs)
        2) pH level (change over 24hrs)
        3) ppm/EC level (How much food is she eating in 24hrs)


          thanks guys, one more question, my plant just broke the soil, and with-in the first 18 hour light session, got it's first set of real leaves! my question is, when should i start running the oscillating fan inside my tent?


          • Eliot Pryor
            Eliot Pryor commented
            Editing a comment
            I run mine immediately . Its makes the stalk strong.

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