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too much negative pressure in my grow room?

Discussion in 'Indoor Marijuana Growing' started by u mad?, Mar 19, 2010.

  1. i have a 850cfm exhaust fan in my 10x10ft room
    im using a 10" port with about 3 feet of ducting for my passive intake

    now the problem i have is the mylar that covers the window keeps getting sucked off the wall because theres too much negative pressure (im think).

    when the door to the rooms open, theres no problem, but as soon as i close the door it balloons up with air and slowly begins to come off the wall.

    if i was to add another intake port to my room, would it help lower the negative pressure? (and get more fresh air into the room at the same time?)

    or would getting an intake fan help the issue more efficiently? if so, what cfm?

    please please please please please help
  2. #2 strain stalker, Mar 19, 2010
    Last edited by a moderator: Mar 19, 2010

    ...you should have your exaust fan attached to a carbon filter, blowing out of your room....you don't need a fan blowing air INTO your room, you only need holes....your intake hole should be at the bottom of the room....if your having problems with neg. pressure, try to make your intake holes smaller.

    ...you don't want to blow air into your room, you want to suck freash air through your room.
    ...put small holes at the bottom of your room, then put your exaust fan w/ carbon filter at the top of your room and as far away from the intake holes as possible.
  3. Yes, add a slightly smaller intake fan.
  4. #4 WishBone, Mar 19, 2010
    Last edited by a moderator: Mar 19, 2010

    I think you really mean larger?
  5. thats what i have done

    by make the intake holes smaller, wouldn't that give me MORE negative pressure?
    how small? 250-500cfm intake?

    thanks for the replies guys

  6. ...do this if you want a bigger ballon.

  7. ...is your room caveing in or blowing up like a ballon?
    ...if it's cavein' in, you need a bigger intake hole, if your room is blowing up like a ballon, then you need smaller intake holes.
  8. but if i were to put an intake fan in my room, wouldn't it decrease my negative pressure thus stopping the mylar being sucked of the wall?

    im confused
  9. caving in i guess you could say

    imagine it like a grow tent- the walls are getting sucked in

    i think we had a misunderstanding, sorry for the confusion.

    another intake hole, coming right up!

    thanks heaps guys!!
  10. #10 strain stalker, Mar 19, 2010
    Last edited by a moderator: Mar 19, 2010
    ...okay, yeah....when you said "balloon up", I understood that as blowing up like a balloon.

    ...so, you are correct, an intake fan will help....but, I've only made bigger holes as an intake in your situation.

    ...if your mylar is still falling off, cut you some small squares of cardboard, and staple the cardboard onto the mylar. The carboard gives more of a surface area as support.
  11. Hey u mad, im confused if theres positive pressure or negative pressure. You say negative, but then you say the room "balloons" up with air when the door is closed, which is positive pressure. When you go to first open the door to your room does it "suck" open like it is trying to pull into the room.....Or is it a little hard when you first open the door? Kinda like someone is on the other side "pushing" it closed in your face.

    The "suck" is negative and the "push" is positive.

    I also had this problem and it could be a couple of things. What size is the ducting on the inline fan that is sucking or pushing air out of the room compared to the size of the inline fan? If its necked down you can get positive pressure.
  12. thanks for the tip man, sounds like a great idea
    its getting sucked in strongly for sure, soon as i twist the door nob it practically opens half a foot or so.

    the only mylar in my room is were there are possible air leaks (over my window which i attempted to seal) so i think the airs getting sucked through the gaps where it then tries to get into my room but the mylar is in the way, hence filling up like a balloon.

    i hope this makes sence to use lol

    again, thanks for the help.
  13. Sounds like too much negative pressure, which makes sense because he said when he opens the doors it stops sucking in.

    You either need to push more air in, reduce your exhaust cfm, or cut larger intake holes. Make sure those intake holes have light traps.

    I have 6inch Can inline fan pulling air through a carbon filter and out with 4inch blower fan pulling in fresh air throught a HEPA filter :) That seems to give perfect neg pressure.
  14. hey mate

    yea i also have a can fan (10" though) moves alot of air!

    yea i would turn it down, would be the easy solution but then my room would heat up even more! think i'll just cut another hole, might even lower the temps a bit (85-100!!)

    im not running any filter for my intake, is/will this be a problem?

  15. Does anyone know what ratio exactly works, cfm/intake size, I have a small room, maybe 300 cubic feet. So if I use something like a 500 cfm fan what size do I make the intake hole/holes?
  16. how exact do you need it to be to balance the room out? 
    cut a 1" hole and guage it from there. unless you need something professional, i suppose 500cfm has to be leaked in passively?? so if its sealed tight the intake needs to be that much larger. 
    i'd like to know more too, 

    As i searched with google, i found people talking about 2:1 exaust to intake ratio 
    quoted from:
    Urban Garden Magazine


    Read the rest its a big read lots of good info and diagrams
  17. No need for an intake fan, just add more passive vents. The two to one ratio works well.
  18. Thanks for the info, that's a great article.

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