Running Negative or Positive Pressure

Discussion in 'Advanced Marijuana Cultivation' started by BigHornBuds, Mar 6, 2018.


    BigHornBuds Well-Known Member

    I’m wondering if there is any advantage to one vs the other?
    I have a sealed grow room, and i have cut a hole into it and plumbed in a 4” fan to better control the CO2 during lights out.
    I’m currently pumping air into the room during lights out (positive pressure)
    Would there be any advantages to turning the fan around and running it on negative pressure?
    redi jedi

    redi jedi Well-Known Member

    Theres no need to control co2 during lights out.

    A sealed room has no ventilation. You just add co2

    You could exhaust the room during lights out to help dehumidify and cool.

    If odor is a concern, positive pressure will cause it to leak out.
    chiqifella and BigHornBuds like this.

    BigHornBuds Well-Known Member

    Smell is zero concern
    I was having CO2 dropping to around 20-30ppm durning lights out,
    I run the room at 1200ppm from an hour after lights on , for about 8hours , then let the plants bring it back down.
    Now CO2 comes on 60min after the lights and run till , lights out, then the 4” comes on and brings the room and holds it around 400ppm . (Which was the purpose of the fan) I want normal CO2 ppm durning lights out , not 20-30ppm .

    Got a Dehumitifer n mini split , not worried about cooling n humidity. Just want to see if I will see any increase or decrease in yeild by monitoring and maintaining a normal CO2 during lights out vs letting the plants fill the room with O2 .

    If it doesn’t work, won’t be hard to patch the hole and add it to the notes of that didn’t work
    Last edited: Mar 6, 2018
    redi jedi

    redi jedi Well-Known Member

    Id check or calibrate your co2 monitor...I highly doubt its going that low. Even so, no light, no photosynthesis.
    chiqifella likes this.

    BigHornBuds Well-Known Member

    I did re calibrated the sensor .
    Because I thought that was crazy low,
    Just an experiment, if no one tried something new, we be pretty far behind.

    Jaybodankly Well-Known Member

    Ambient atmosphere CO2 is around 400-500ppm with out adding any.

    BigHornBuds Well-Known Member

    Thanks facts
    What’s that have to do with positive or negative pressure?

    I posted in the advanced because I thought I might not have to spell everything out .

    This isn’t my 1st dog n pony show

    I know what the current ppm of CO2 in the atmosphere,
    I know what’s happening in my sealed room
    Because I monitor everything,
    I see my CO2 dropping and 02 climbing at lights out.
    Yes I understand photosynthesis, and knows how it works .
    But there’s a lot that we don’t know about plants n growth and hormones etc .
    I just wanted to do an experiment to see if bring the levels to normal air at night would change anything? And if it would matter if it was neg or pos.

    On Monday , I’m switching to negative And run it that way for 8weeks .

    SchmoeJoe Well-Known Member

    Do you run co2 right up to lights out or stop supplementing a bit before? The standard is that plants don't use co2 during their night period so I'm wondering what would explain you're rooms level dropping so low at night.

    BigHornBuds Well-Known Member

    I have it set to shut off about 1 hour before lights off
    I have no idea why my CO2 is dropping so low .
    I have ran though the calibration on the unit a couple times
    Had the sensor apart yesterday .

    Yesterday I decided to take a min n turn around the fan to see what happens . Negative pressure is a bad idea . If you have a film up etc , was lifting the floor cover too

    I haven’t seen any down side from pumping air from the adjacent room so far . If anything they seem extra happy at the start of the light cycle .

    SchmoeJoe Well-Known Member

    Negative pressure is the standard for bringing fresh air into a space. Plants don't use co2 in their night period so your low night levels aren't an issue. The likely cause is the hour of no supplementation. Odds are you're also depriving your plants during the day period if your levels are getting that low.

    I'd set the co2 to stop at somewhere around ten to twenty minutes before lights out and see where that puts it. Or you could just set it stop at lights out and not stress over what's really a non issue.
    DemonTrich and redi jedi like this.

    SchmoeJoe Well-Known Member

    Also, positive air pressure will raise rh. Negative pressure will lower it.

    BigHornBuds Well-Known Member

    I run about 1200ppm during the day cycle
    I checked it 30min before lights off , it was 850...
    I have only seen a drop in RH , I live in a dry climate, and the room the fan is in stays about 20-30% during the winter.

    SchmoeJoe Well-Known Member

    The dry intake air definitely takes care of any potential rh issues. As far as your co2 levels I'd just keep adjusting your off time until you see the numbers you want.

    BigHornBuds Well-Known Member

    Lights off in 3min , 600ppm

    DirtyEyeball696 Well-Known Member

    You sure sound like you already know everything already so why are you asking since this isn’t your first dog and pony show.
    Keep your room sealed completely don’t worry plants don’t need any fresh air. Maybe you’ll hang out in there long enough to loose some more brain cells lol

    Sent from my iPhone using Tapatalk

    BigHornBuds Well-Known Member


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