How many cfm do i need??

How many cfm should my exhaust/inline fan be to ventilate a 2x4x5 grow tent with mylar inside and a 400w hps with cheap reflector and no cooltube??

430cfm minimum

thanks man but do i need intake and exhaust fans,or just 1 intake? also, would 2 fans with 240 cfm be the same as 480 cfm? cuz amazon is selling 240 cfm duct inline fans for like 36$. Just wondering if i should really spend a hundred on this fan.

I don't have experience with those so I can't say, all I know is what works for me. I use a single 6" inline fan that's rated at 430cfm, you should be able to find HydroFarm 6" fans @ 400cfm for around $125 online

430 is overkill for such a small space. You are venting 40 cubic feet, and that needs to be done at least once every 1-3 minutes. Without a cool tube, I would say the 240 should work just fine.

What about a space thats 1' L x 2'6" W x 3'2" H? Im using 4 35cfm each pc fans now and its getting roughly 95 up in veg and 80 flower. flower has alot more lights but its lower. 2 Intakes and 2 Exhaust. I just purchased 2 pc fans that have 75CFM. One for intake and Top Veg exhaust. So total CFM is going from 140cfm to 220CFM Think that will work?

HomeGrown&Smoked said:

430 is overkill for such a small space. You are venting 40 cubic feet, and that needs to be done at least once every 1-3 minutes. Without a cool tube, I would say the 240 should work just fine.

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I run 430cfm in my DR120W (2x4x5) and it is not overkill. It sucks the heat right out of there, especially here in sunny San Diego

yea someones tellin me for my space all i need is 120 or a little higher, maybe 2 somethin??

Honestly it is hard to answer this question since there are so many variables. I would say 400 cfm and get a speed controller to tune it down a bit if you need to. Remember it is better to have too much exhaust rather too little and have heat issues.

Go with 200 and when you're in the market for a 400 or more you'll remember this thread

I agree, more is better and is NOT overkill

How many of these threads did you make?? I see 2 in this forum and at least one other in another forum.

yea if i could afford the 430 u reccommended i would definitlly do that, but its christmas and gota buy every1 presents, so for now i think ill settle with a nice 250 and upgrade when i can. Thanks every1

Heres a cheap fan in the cfm range your looking
http://www.businesslights.com/valueline-inch-435-cfm-centrifugal-inline-fan-p-2045.html

bajafox said:

Go with 200 and when you're in the market for a 400 or more you'll remember this thread

I agree, more is better and is NOT overkill

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When you look at cost vs. results, it is overkill. Running the air 4 times more than it needs to (for cooling purposes) will not result in an equivalent amount of bud, but will cost you a lot more to purchase of front. It's not like it will kill the plants or anything, it is just pointless to spend that much money for something that doesn't need it. If he was running a 600 or a 1000 without a cool tube I could see using the 430, but since he is using a 400 it would just be a waste of money.


Good luck with the grow bangthang

Each fan has a rating that tells you how many cubic feet of air per minute (CFM) it will move. When dealing with above average temperatures you will want your fan to exchange the grow room air 3-5 times in one minute, so for a room that is 40 cubic feet, a fan that is capable of moving 120-200 cfm (cubic feet per minute) is recommended. If you only want to replace depleted levels of co2 and are growing in a closet using fluorescents, one room change per five minutes (divide room size by 5) will be adequate.
To calculate your room size, multiply Width by Length by Height, this will give you the CFM rating required for one room change per minute.
Air that’s exhausted will be replaced by fresh air, which is drawn from
Passive Intakes- located opposite the exhaust at the floor of your grow room (Recommended for closet grows) or forced in using another blower (recommended for larger grow rooms).
For internal circulation oscillating fans are In my opinion the most efficient devices for circulating air in a room. The gentle back and forth sway of the fan is very beneficial for the developing plants. These fans tend to keep anaerobic moulds down by constantly freshening any potentially stagnant air. Home improvement centres carry a large array of various types and styles of air-moving fans, there are wall-mounted styles available and most are relatively inexpensive.
NOTE: To convert CFM to CMH use the folowing formula CFM X 0.03 X 60 = CMH
To help maintain a lower room temperature follow this link detailing how to construct a DIY- Air-cooled Reflector

Great expert info thanks every1 very helpful+reps .

i run a 400hps, with a 6in cool tube on 140cfm outtake fan. i have a carbon filter attached to the end of the cool tube, and the tent is 45 cubic feet.
i also have a 80 cfm cumputer fan between the carbon fitler and tube, right behind a 180 degree bend in 6inch ducting. I have 2 small 30 CFM computer fans as intake.
my temps never go above 80 and usually stay at 75. it may just be how well i set up my tent, but it works amazing. rule of thumbs for the plants sake, and not temps, is one complete exchange every 5 mins.

i hear u on that, so can i have a carbon filter with only one inline fan?

yup, just make sure the carbon filter is not being pulled too fast.

so with only 1 inline, how do i set it up, as an intake with the filter behind it, or an exhaust fan with the filter inside the opposite empty duct? And how do i make sure it isnt being pulled 2 fast?