How big does my fan / filter need to be?

Discussion in 'Grow Room Design/Setup' started by Irie67, Nov 14, 2011.

  1. I have a 4 x 4 x 6.5 tent that Im gonna flower in. How big does my fan / filter need to be? At the moment I have a rather small set -up. The fan is 190 cfm. The filter im not sure what it is. Bought it gently used on craigslist and there's no writing on it. Im worried that its not powerful enough. Im in veg now and don't have much smell yet. but im planning for the future. It does not suck the walls of my tent inward like I've seen some people comment. Here's a pic
  2. Helli Irie67,

    So the basic formula for figuring out how much air you need room LxWxH / 5. This is the total room area, in your case 4x4x6.5 = 104 cu/ft. Divide that by 5 because you want all of the air removed within 5 minutes or less and you end up with a mere 20.8cfm that you need moved. So your 190cfm fan should be sufficient... however...

    Your filter may be low quality and 'suffocate' your fan. If the filter isnt well made you wont be able to pass a lot of air through it which can cause problems with not only smell, but air flow. Stagnant, or old, air is a bad thing. Plants use the giving CO2 in the air rapidly so a fresh air source is required.

    This exhaust fan will act as whats called a 'passive' intake which will suck more air out of the room, and by nature suck more new air 'in.' Passive intake general rule is 4:1 (Exhaust:intake).

    So back to your questions, the CFM rating on your fan SHOULD be fine for the area, but if you are worried about fresh air, and removing heat... it may not be a bad idea to go with a more powerful fan, or get a high quality carbon filter.
  3. I have a 4 1/2 by 5 room i use a max fan three speed fan 300 cfm attached to a thirty pound canfan filter..i can grow anything i want with no smell move ever..i no longer live in fear..before it you could smell ganja thirty feet from my front god what did my mailman think? Thank god hes cool
  4. #4 nu11va1ue, Dec 1, 2011
    Last edited by a moderator: Dec 1, 2011
    I also have a new grow room. Before starting this run, my main concern was odor, therefore, it was important to get the correct carbon scrubber/fan combo for security. My thought process was this so you can apply the same formula for your room. Just substitute your room measurements.

    My room is 5' wide X 3.5' length X 8' tall
    5*3.5*8 = 140 feet cubed or 140 cubic feet

    I want to completely exchange the air in the room at minimum 3 times and ideally 5 times per minute. This means move the volume of all the air of the room at minimum 3 times and ideally 5 times in one minute.

    The rating on your fan is measured CFM, which means "cubic feet per minute".
    So to move the 140 cubic feet of my room 3 times will take a fan that is 420 CFM to move 140 cubic feet 5 times is 700 CFM.

    Therefore I need to get a fan that is between 420 CFM and 700 CFM.

    Tips or things to think about:

    Your filter should have a bigger or equal CFM rating as your fan.
    Generally, more powerful the fan the louder it will be. I considered purchasing fan reviewed as more silent than others, but it cost too much. Also, I just used insulated ducting and maintained an awareness of vibration and just adjusted to minimize that.
    When in doubt, go bigger and get more than you think you need. Better than buying twice and think about security.
    CFM is just a benchmark, there is also resistance. Depending on your set up, there will be air resistance from pulling through the fan or pulling through a fan + reflector or the length of your exhaust duct and bends/turns from the duct.

    I got a 8" fan and an 8" 720 CFM filter (off Ebay for under $200. I shopped around and I negotiate).
    I worried about the noise (would it sound like a Boeing taking off in my closet?!) I thought about buying a variac (a voltage regulator that will control or slow down the fan. Turns out, didn't need it.
    Insulated ducting at Lowe's, $25, worked fine
    For max efficiency and to minimize noise, my set up is completely vertical, no bends. The filter, which is on a crate, is by the door and low to the ground where the plants are, then that big ass 8" fan, then insulated duct which is exhausting to my attic.

    I don't exhaust my reflector. I bought a cool tube so I could control that separately. I felt that it might affect the efficiency of my security. Odor control, for this grow, is non-negiotiable factor for me as I have neighbors this time.

    Negative pressure works like a dream!! I have to *tug* the door open. No smell escapes the closet, much less the room. I don't need intake since a rush of fresh air comes in under the door. The fan doesn't even need a speed controller.

    Hope my tips help you with your decision.

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