Yet Another Ventilation Question..

Discussion in 'Grow Room Design/Setup' started by Prodigus, Aug 3, 2011.

  1. I know this topic has been probably gone over and over again for years. I ready the FAQ along with some other threads covering the subject but I still need some clarification. I guess that means I'm a little slow?

    3'D x 4'W x 6'H Custom Closet

    Desire negative pressure to help with funk

    Here's where I get stuck. If I want negative pressure wouldn't that mean that I want my exhaust to be pulling a little harder than what my inlets can provide therefore creating that negative pressure?

    typical threads say your intakes should be 1.5 to 2 times the exhaust size. That sounds like there would be a lot of flow but no negative pressure. I'm sure I'm probably wrong but I'm just trying to understand this better. I'm sure I'll also be fighting with temps with a 400W HPS in there although it will have a cool tube.

    That said, if I do need 1.5 to 2x the size of my exhaust (6" powered by a 449cfm Vortex with a speed adjuster) then is it ok to have that split between multiple intakes. I was thinking of adding Qty 6, 2" PVC intakes along the front of my cabinet with the 6' exhaust of course being upper back of the cab.

    I haven't learned how to use sketchup but I tried to illustrate what I mean. Any advice would really be welcome. Thanks!

    Attached Files:

  2. If you have a 6 inch fan pulling 449cfm's, you will have negative pressure. Your plan will work as alot of people say its better to have multiple smaller intakes than one big intake. You are on the right track.
  3. I battled with the logic behind this, as well. Bigger intake to smaller outake isn't the part that builds the negative pressure. If you were dealing with a much smaller fan, I would think you would need smaller intake and larger amounts being pulled out... but your fan is strong enough for sure.

    Ventilation Chart and Calculator

    This thread will help you figure out exactly how big and how many intakes are required.
  4. I can tell you from my own observations . I have DR90 tent(54sqft) I was using two 6" vent holes on each side of the bottom of the tent as passive intakes and one 6" hole on the top as outtake and when fan was on, walls were sucked in (neg pressure) so if intake hole(s) 2 times bigger than outtake hole , it will still creates neg pressure as long as you have powerful fan (my was exchanging the air in the tent with in ~20 seconds) .
  5. What they are referring to is the size of the passive intake. Active intake is different, with an active intake you would need more exhaust then intake to get a good smell proof negative, especially since few people build an airtight room and air will find its way in naturally (ever feel the draft through a window or door?)

    Lets say you have a 6" exhaust fan, you want a 6" or bigger hole somewhere in the room to provide plenty of intake air into the room. The fan pulls the negative, but the air to fill that negative has to come from somewhere.
    If you run ductwork for the intake, you especially want a bigger hole so the fan can overcome the restriction of the intake ductwork. 1.5 - 2x is a rule of thumb to keep smell out but keep lots of fresh air flowing in unrestricted. Without ductwork, the restriction is minimal/none, a 6" hole will generally be just fine.

    Lots of smaller intakes may help with controlling the airflow into the room, but keep in mind (3) 2" intakes does not flow as much air as one 6" intake. Max CFM is exponential with duct sizing, the same is true with piping.
  6. Intake size most definitely impacts whether or not you have negative pressure. Basically, if the amount your fan is drawing through your exhaust can easily flow through your intakes then you will have little if any negative pressure. Air will take the path of least resistance, and as long is there is ample space for air to flow in freely though the intakes then there will not be meaningful pressure to pull air in through the tiny cracks and crevices in your grow box.

    The reason you see two different recommendations about intakes is that it depends who you are asking and the premise of the answer. Take a step back from MJ growing and just fans in general, and that is where the recommendation comes from to have double the intake as exhaust. Providing ample air supply for the fan to pull from prolongs the life of the fan because it isn't working as hard. But, that doesn't create negative pressure. For that you need to make the fan strain, to pull against the intakes to their full capacity and still have sucking power on the cracks and crevices.

    Basically negative pressure is bad for your fan but good for odor control.

    Doesn't matter if your intake is one hole or multiple holes, though placement could be an issue. Also keep in mind that you cannot just multiply or divide a circle's radius or diameter to figure out its area. For example, two 3" diameter holes is only half the intake of a single 6" diameter exhaust hole. (Pi * r squared is the formula.)
  7. Thanks everyone! I'll start with the 6, 2" holes and just leave myself room for more if I need them.

    Thanks again!

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