Negative/Positive Air Vent Pressure???

Discussion in 'Indoor Growing' started by faithless, Nov 19, 2006.

  1. faithless

    faithless Registered+


    So when you build vents, whether it's for a grow room, or a whole building, theres a whole scientific theory behind whether the intake or the exhaust should be more powerful, vacuum and all that stuff.

    I seem to remember that the exhaust should be more powerful than the intake, can anyone confirm or refute please? I've googled like mad, but I just can't find any good answers and it's time to get building a proper space...
    I'd be really greateful for any help. Thanks! :pimp:

  2. Dutch Pimp

    Dutch Pimp Up in Smoke want it about the same...good air in ..bad air close to neutral as's not a wind tunnel...LOL...
  3. faithless

    faithless Registered+


    Heh, well perhaps not a wind tunnel, but if I'm going to stick 1 000 w in a wardrobe, I suspect I need to put quite a bit of work into getting the vent right.

    I bought a restaurant 2 years ago and had it rebuilt. We fought a daily battle with the ventilation system because the air pressure was wrongly balanced, either the exhaust or the inlet was disproportionate, can't remember which, which made the whole system innefficient, not because one or the other was too weak, but because the balance was out of whack. So I suspect i should be learning from that lesson for this closet?

  4. babystarbud

    babystarbud Registered+

  5. babystarbud

    babystarbud Registered+

    a friend of mine is actually doing an HND in air-con/refrigeration surprising how much complex maths is involved in this kind of stuff....if i see him soon ill ask if he knows any easey to understand online resorces, as im sure a lot will be benificial to growers.
  6. Dutch Pimp

    Dutch Pimp Up in Smoke

    ...I used a 400HPS light in my air from the A/C..blowing in from the bottom...exhaust..going out the top....with a digtal temperature/humidity gauge...somewhere between the lights and top of the plants (not directly in the light)...I hear 1000HPS ..gets seriously hot....test everything....before you pop the seeds...
  7. harris7

    harris7 Registered+

    well if you want all the air to go through a filter you would need a negetive pressure (exaust stronger)

    If there is a positive pressure then air will leak out any holes. this will allow smell to escape. It's not a problem for me, but maybe for you.
  8. faithless

    faithless Registered+


    Thanks, yes, I suspected the exhaust needs to be stronger. The smell isn't the main concern, it's moving the hot air out that I want to concentrate on.
    I learned during our rebuilding that there is a whole complicated architectural science behind it, so that would be great babystarbud, if you meet him ;)

  9. harris7

    harris7 Registered+

    a great option is a cool tube. They work amazingly well.

    a buddy of mine has a 1000W hps. He has a 165cfm fan going though a cool tube, which removes a hell of a lot of heat (its the only exhaust). and then a small 90cfm fan as intake. and his temps are fine.

    without a cool tube your going to need to buy a bigger fan, which can cost a lot. as well, cool tubes still work well when the intake air is at a higher temp. So if the room your wardrobe is in gets warm, in summer or w/e, a normal set up wont do you much good. it'll get hot inside the grow room. (happens to me). With the cool tube it will remove practically the same amount of heat no matter what the intake temp is.

    This is because the bulb is like 1500 degrees F. so a change from 70 to 85 F dosn't make too much difference.
  10. Cornelius

    Cornelius Registered+

    i agree w/ harris 7 that a cool tube is your best bet. 1000w in that kind of space demands serious heat control, and a cool tube can give you that
  11. faithless

    faithless Registered+


    Trouble is I already own most of the lighting, almost 800w, currently.
    That may be enough anyway, and if it's not, it's dead easy for me to upgrade if I want to. Just get another 250HPS or some fluors.
    I'd love to get a cool tube otherwise. Can't be bothered with the hassle of selling it all either, in case someone suggests that. :Rasta:
    I wonder what a fan behind the lamps at bulb level and a stronger exhaust at the other end, also at bulb level, that leads out would do?

    Last edited: Nov 20, 2006
  12. masterjointsmith

    masterjointsmith Registered+

    yo ive got an idea..... details to come
  13. masterjointsmith

    masterjointsmith Registered+

    details ... consider lay all your shit out in a line long way figure out what size piece of vent (probably square in your case) you can fit your shit in ontop of a piece of tempered glass that will hold the heft of all your lights and give ya a 2-4" gap all the way around your equipment .... lay your equipment on top of the piece of duct and trace around its "footprint" on to the duct with a sharpie put all your lights back on the piece of glass cut your holes slide the glass in the vent put a reducer on one end to 8" get a nice 8" fan to put on it and some flex conduit and pump the heat else where just my high thoughts
  14. masterjointsmith

    masterjointsmith Registered+

    another thought i wouldnt cut a hole out of vent larger then 1/3 of the vent for structural reasons but maybe vents thicker where you come from poor paint brush jpg tocome
  15. masterjointsmith

    masterjointsmith Registered+

    like this ya didnt give the any of your dimensions for the lights or the cabnet there goin in so maybe this is very poorly proprotioned/scaled but thats the idea and just exaust it somewhere out side

    Attached Files:

  16. masterjointsmith

    masterjointsmith Registered+

    another afterthought i dont know what that shit will wiegh put togeather but maybe reinforce it with some earthquake strap bands (like a barrel but maybe a couple more bands then you got lights in it) then hang it from the bands that way it supported from the bottom
  17. GaGrown

    GaGrown Registered+

    If you have a 265 or larger fan,Whatever size your exhaust opening is.Then that should be the size of the intake.It will be a bit of negative pressure. But moderate. It's the same principle as your central heating and air system in your home.You can't have more cfm's than you are pulling on the return. If you were to cut a 6 inch intake and a 4 inch exhaust,you would cool the room and exchange the air at an even rate.A 6 inch hole is 100 cfm.Distribute the cfm accordingly.Like part of it hook to the light.A 4 inch hole is 50 cfm.

    Ga Grown!

    BOYZNUS Registered+

    Since smell isn't an issue for me, I have positive air pressure in my room.

    It keeps the bugs and stuff under control.

    And I use passive exhaust and hood exhausts.

    Best of luck
  19. bongerstonerd00d

    bongerstonerd00d Registered+

    Well, I read someplace on this site that you wanted your exhaust to be ~1.5 times the size of your intake. So I have a 4" Vortex bringin air in, and a 6" Vortex drawin stale air out thru carbon filter and it works great. Not sure if I have positive or negative airflow. I used a "Y" and I blow the hot air from 3 Sun Tubes out same line as my exhaust. Plants seem very happy wit the setup, too. Oh yeah, I go up to 8" duct hose just outside my grow area also. My room is 112" x 112" x 96" or roughly 650 cubic feet.

    I am going to add a duct damper and CO2 next month. The WE grow sum trees:D :D :D :D :D


    P.S. Now this is a great discussion. Beats the shit out of reading "Where can I order seeds from"........<laffin>
  20. kent tha killer

    kent tha killer Registered+

    im running a 1000 hps in not even 4x6 and i battled that for awhile... i found what works for me is more exhaust than intake, not too much more or u will get a vaccuum affect and thats BAD, i find just run your exhaust when ur lightis on and have your intake kick in when your thermostat tells it to
    Last edited: Feb 9, 2007

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