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Intake Outtake Fan Air Flow Question

Discussion in 'General Marijuana Growing' started by Bizzler, Nov 29, 2008.


    Bizzler Well-Known Member

    I have a general question involving the intake and outtake fan cfms.

    I currently have 2 identical fans in my grow cabinet. 100 cfm each, 1 for intake, 1 for outtake.
    I just bought a charcoal filter for the outtake fan. Now here comes my Question. The flow of air on the outtake fan has decreased because of the filter and the intake flow of air is still the same. Is this a problem? More air coming in than out?

    PhazeDaze Active Member

    Well, i doubt it will cause a problem, actually it may provide benefit.
    Those plants love oxygen & light, i would suppose that while pushing more fresh oxygen in, and less out in a grow cab the fresh air would circulate around the cab more because of the slow out take. But all in all, i doubt it will make any major difference.

    Good luck brotha,
    .Phaze :leaf:

    Bizzler Well-Known Member

    Thanks....Any other comments?

    LocoMonkey Well-Known Member

    The intake fan will create higher air pressure in the cabinet. The pressure will cause air to escape from any crack or opening. Then you leak odors. If your cab is air tight, then you will not have a problem.
    Or you can put both fans on outtake.

    arss Well-Known Member

    It won't make much of a difference except with odor control. Since more air is coming in than going out, air will start to leak out of the cracks and not go through your carbon filter.

    You could put both as outtake fans and make some passive intake holes

    T.H.Cammo Well-Known Member

    You don't have "more air" comming in than going out, because the intake fan is causing a positive pressure in the cabinet, which blows the air out through the exhaust. And no, that's not a problem - except any air that "leaks" out will be unfiltered.

    To me, the more interesting question is - Why do people use equal fans for intake and exhaust? Let's do a little theorhetical experiment here! Let's say you have a 100cfm intake fan and "adaquate" passive exhaust (just to keep things simple, no filter) - you're pushing 100cfm with one fan. But this causes an overpressure in the cabinet that results in air leakage (smell).

    Now let's say you added another 100cfm fan at the exhaust. You're still only moving 100cfm, with each fan doing only half the work (one sucking and the other one blowing).

    Let's try one more time! This time you put both 100cfm fans as exhaust fans. With adaquate passive input, you are now sucking out 200cfm (twice the airflow). This configuration creates a negative pressure in the cabinet, which prevents any air leakage (smell) from escaping. Anyone see a benifit here?

    The point is, if you are using two fans, one for input and the other for exhaust, the cfm rating doesn't add together. One fan just passes the air along to the other one! On the other hand, if you use both fans at one end (or one fan with twice the cfm), you will be moving twice as much air. There is a difference!

    mario60185 Well-Known Member

    You would then need 2 filters though.

    tusseltussel Well-Known Member

    just use the one than with a passive intake its not like haveing the intake fan is moveing more air its still just 100 cfm well prolly 80 or 70 with a filter on it

    mannurse801 Well-Known Member

    I like that big post above. You always want to creat a negative pressure in your room. You always want to be pushing air out, in effect, sucking air in. This will not allow any air leaks as you are forcing the movement of air through the outake fans. Unless your room is completely sealed, this works best. If you are sealed, just make a few cracks in the room to allow air to be sucked in....

    mario60185 Well-Known Member

    I can think of one reason.....to cool the room down. If the passive intake doesnt bring in enough cool air you would need a fan for the intake.

    potheadsmoker Well-Known Member

    you dont need an intake fan just an intake hole bigger than the size of your exhaust fan vent/hole, the point is too cool the room and to flush out the old stagnant air and bring in new, make sure to have your exhaust towards the top of the box and the intake hole at the bottom

    potheadsmoker Well-Known Member

    idk why but people dont simply look at the grow faq almost every question about growing can be answered there, not that im not trying to help but why dont people look around?

    How do I add ventilation to my grow area?

    How do I add ventilation to my grow area?

    Contributed by: Smokey D Dope

    How do I add ventilation to my grow area?
    There are a couple of considerations to observe when planning your ventilation, they’re pretty simple concepts; but they are often overlooked.

    · First, remember that warmer air will naturally rise to the top of any container, and that cooler air will naturally settle towards the bottom.
    · Also remember that when ventilating any space, the volume (VOLUME, in cubic feet or cubic meters... [L x W x H]) of air that goes IN, also has to come OUT.
    · You can’t expect to ventilate a grow space by simply forcing air in, and not providing an exhaust vent.

    Since the object is to remove as much warm air as possible, and replace it with cooler air, it will be most efficient to place the exhaust as close to the top of the space as possible, and place the intake as close to the bottom as possible.

    Should I place the fan in the exhaust, intake or both?
    The fan should be placed in the exhaust, and the intake should be a simple hole (or light trap, if light getting out is a concern).
    This type of system is known as an Active Exhaust, Passive Intake System.
    Mounting the fan in the exhaust, sucking air out of the room accomplishes a couple of things...

    · Since the exhaust is at the top of the area, the fan will suck the hottest air out of the area first.
    · The fan is actually lowering the air pressure inside the area. Any incidental pinholes or leaky seams will simply draw air in. If the fan were blowing IN, those pinholes and leaks would allow potentially smelly air OUT.

    How big should the passive intake be?
    It should be slightly larger than the exhaust. Remember, the volume of air being blown out, will be replaced through the intake. Using a bigger intake hole allows the incoming air to be at a lower velocity (speed), which minimizes mixing up of the air in the area. It will also allow the fan to operate more efficiently.

    How big should the fan be?
    Fans are rated in either cubic feet (CFM) or cubic meters per minute in North America. In Eurpose, metric fans are rated in m3/hr - cubic metres per hour (m3/hr).

    That means a 70CFM fan will move 70 cubic feet of air in one minute.
    Your fan should be big enough to move the volume of your area 2 to 3 times every minute. A 70 CFM fan would be adequate for a 35 cubic foot area, and would be optimal for roughly a 23 cubic foot area.

    · To figure your area’s cubic volume, multiply (in feet) the length by the width by the height.

    What if I have more than one fan? Should I use one to blow air in and one to suck air out?
    Not if the object is to provide as much ventilation and cooling as possible.

    · If you have two 3-inch diameter fans, and you mount one in the intake, and one in the exhaust, you have a total intake area of one 3-inch hole and a total exhaust area of one 3-inch hole.
    · If you use both fans as exhausts, you have TWO 3-inch exhausts and two 3-inch intakes (actually, two 3.3 inch intakes. They should be bigger than the exhausts, remember?).
    · Twice as many holes, twice as much airflow.

    Enhanced Blower Mods

    Timer Options

    If you find that the "Lights off" temps are lower than you'd prefer, you can simply run the fan from the same timer as the light by using a multi outlet power strip connected to the timer. Plug the lights and the fan into the power strip, and the fan will turn on/off with the lights. If you're using more than one fan, you could connect some or all of them, remembering that the more fans you have running, the lower the temps will be.
    * SAFETY NOTE: The timer must be able to handle the additional electrical load, or an additional timer must be used. SAFTEY FIRST.

    Sound Suppression
    Making the ventilation system quieter can be an important consideration, and it's important to remember that the air moving through the intakes and exhausts make noise, as well as the fan itself. Some of the fan noise from vibration can be overcome by mounting the fan in a non-rigid manner. The fans can me mounted using rubber grommets to help dampen the vibration. Self-adhesive foam rubber window insulation can also be used. In some installations, it can be mounted by threading a bungee cord through each mounting hole, then attaching the other ends of the bungee cord to the exhaust hole.

    · Generally, air moving through ductwork or tubing can become noisy, particularly if the air has to move at a higher velocity. More, larger diameter intakes and vent tubes will generally be quieter than fewer, smaller diameter intakes and vents. The fans also don't have to work quite as hard.
    · Finally, although popular and easy to use, flexible "Accordion" type hose, commonly used to connect clothes dryers to external vents are not always the best choice, as they cause a great deal of drag, (making the fans work harder) and generally air flowing through them is noisier than smoother ductwork.

    FilteringOdor Control
    Connecting a Carbon scrubber is a good method of controlling the odor that can be a dead giveaway to an otherwise stealthy installation. A carbon scrubber is simply an expansion chamber (box) into which the smelly air from a flower chamber is pumped. The chamber has a large exhaust vent, which is covered by an activated carbon air filter. The chamber must be big enough to provide a damping effect of the incoming air. If too small a scrubber is used, the fan will not be capable of pushing air through the filter. Here, you actually want the exhaust vent to be considerably bigger than the intake.

    · There are also several DIY Odor killers available, which work to varying degrees to provide an "Odor Cover-up."
    · Remember, the term "Low Odor Strain" is relative.
    · Even the low odor strains generally still stink pretty badly near the end of flowering.

    Now go grow something!
    Added on: Tuesday, May 1, 2007 Viewed: 2190 times

    AliceBTokeless Active Member

    That kinda reminds me of all those guys back in the day (when Radio Shack started selling pc cooling fans) that uesd a bank of fans to BLOW cool air on their stereo equipment. Nice idea but all wrong, same as your set up ... like the man said ... Intake is ALWAYS useless unless !!! you want to increase pressure to overcome the resistance from filters ... THE WAY is always use exhaust fan or fans to control flow pulling air in cab. Use Fans as exhaust and go to home Depot or w/e an get an adjustable Louver (Spelling) as your passive control, can achieve best results if you have a slight neg pressure in cab. What is almost more important is WHERE your exhaust is actually vented to. Are you trying to 100% scrub the air to return to your house or venting outdoors? If you are recirc'in to your house, that 1% unscrubed eventually turns into 100% unscrubed air, pretty much defeating the purpose..... Yes, exhuast is additive over time ... Stick a Fork in me ...

    LastOneLeft Well-Known Member

    Very useful infomation there Potheadsmoker!!!!

    Just what I needed to clarify with my tent ventilation situation. (Thumbs Up)

    Thanks manbongsmilie

    kanz Member

    Only reason I can think for an intake fan , is stealth. Easy to put a small vent over the hole with a nice small 5" fan pulling air in.
    Where as trying to hide a big passive instake can be a problem.

    I may be wrong but ive heard your passive hole should be 4 times the size of your exaust fan.
    So a 8" exaust fan would require 4 x 8" holes, or one small simple 6" vent hole with a 6" fan.

    Well how I see it but im no expert.

    spandy Well-Known Member

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