Hey virgin grower here.... please be gentel. Building first grow closet, but I have spent countless hours searching forums for info but I can't find a clear answer to my exhaust . My space is 3 ft by 2 ft by 6 ft tall ( 30 cubic feet) I'm looking to run a 400watt hid (mh for veg, hps for bloom) the light will have an air cooled reflector so I will pull my air through the light to exhaust heat and spent air. I will also have about 20 ft of 6 in flex duct taking the exhaust away. This set up is for medicinal purposes only so 2 or 3 plant max. Not sure if I want to run a carbon scrubber yet as the exhaust being pumped into the soffit straight outside. Math says 60 cfm (double room size)fan is a start but seems very small as I need a negative air pressure to contain the stank! . And 6in inline 400 cfm fan with speed contro ( which will only lower to arou d 200cfm) maybe over kill and way to loud. Also trying not to break the bank. I just want to really think this build through so I can make stupid mistakes later and learn from them lol looking for an idea that will be quiet and let me have control over temp which is hard to figure what this light wI'll produce in this small space Ps.will have passive intake (cold winter and airconditioned house in summer) a small oscilating fan for air movement and plan a coco coir grow medium set up. Any suggestion is helpful.... happy trails
exhaust needs
- Thread starter wme1980
- Start date
nomofatum
Well-Known Member
1. Activated Carbon of some kind is an absolute requirement!
2. Venting should not exceed the volume of your space every 2 minutes.
The 60 CFM fan you think is too small is actually 4x too big.
15 CFM is what you actually need and 15 CFM will do as good a job of containing the stink as 400 CFM.
At 400 CFM the plants would think they are stuck under a hurricane perpetually. You don't want to move the air so fast to create an extreme low pressure, you only need a very mild low pressure in the closet.
2. Venting should not exceed the volume of your space every 2 minutes.
The 60 CFM fan you think is too small is actually 4x too big.
15 CFM is what you actually need and 15 CFM will do as good a job of containing the stink as 400 CFM.
At 400 CFM the plants would think they are stuck under a hurricane perpetually. You don't want to move the air so fast to create an extreme low pressure, you only need a very mild low pressure in the closet.
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I'm planning on running scrubber, into aiaircooled reflector, ducted to fan ou tside grow area, fan to 15 foot flex duct out. So garden exhaust, carbon filter , and light cool. 25 cfm will move through all that? Def sounds like u know your stuff, just wanted to make sure I was painting the right picture.
nomofatum
Well-Known Member
Depends on the fan. A regular fan, no way no how. You are going to need one designed for backpressure and rated to about 15-20 CFM with the back pressure of your system (or able to be turned down to that level.)
http://www.ebay.com/itm/181144406542
That is my guess. The better style doesn't seem to have a small enough unit for your use.
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Smokenpassout
Well-Known Member
I have the same size space. I use a 4" 170 cfm fan and phresh carbon filter. I also use a speed controler to slow it down to maybe 60 percent power, which really quiets it down. I get ample fresh air intake, zero smell outtake.
Fish Weed
Active Member
If you are doing passive intake, make more passive air holes than you think you will need and then tape over some of them to seal them off. If you start to have circulation issues or heat issues it's easier to just cut away the tape over a hole than it is to cut a new hole while your grow it is going. I did this with a cabinet grow and it worked out perfect. It really helps to dial things in after you get everything up and running. I used some of that silver heating duct tape (not regular duct tape).
WeekendSupervisor
Well-Known Member
I'm sure I'm missing something here. The OP says the space is 3x2x6 (36cuft). Air flow is to go from a carbon scrubber, to aircooled hood, out the closet, out the soffit, having passed through up to 20 ft of 6" flex.
How have you come to such a low number cfm?
NanoGadget
Well-Known Member
If you use a can style carbon filter you have to be careful about letting your cfm get too low because if you go too far below what the can is rated for you won't be pushing enough air through it for it to work effectively. This may our may not be the case with a disk style scrubber, as I have no experience with them.
WeekendSupervisor
Well-Known Member
As your garden is smaller than mine, but similar in that it is aircooled, and carbon scrubbed, I would suggest going with a 6" Hyperfan. At 100% its 315cfm, but it comes with a built in speed controller (no hum at lowest setting)that you can dial down to 35%, rheostat type dial for control. It is tiny, inline, and well suited to the size space you are using. I assume you are going to get a can type carbon scrubber, which I've heard reduced your efficiency by 25%. Every bend in your flex line is another reduction of 15-20%. You can see why you'd want to have greater cfm than the measurement of the space, when you take into account where the air is going, and that you're expecting it to cool a light and create a slight negative pressure.
I went with the Vortex S-600 but I had to purchase an autotransformer to dial it down, since the microcontroller style speed controller makes the fan hum loud. Go Hyperfan imo.
I went with the Vortex S-600 but I had to purchase an autotransformer to dial it down, since the microcontroller style speed controller makes the fan hum loud. Go Hyperfan imo.
nomofatum
Well-Known Member
OP edited original post extensively.
Though he does still actually only want 15-20 actual CFM, a fan will need a much higher MAX rating to push through all that stuff.
WeekendSupervisor
Well-Known Member
I understand better now. Learning for everyone 
nomofatum
Well-Known Member
Do you think he could actually get a 315CFM fan to run as low as 15 CFM actual? I expect not. Results won't be as nice if he is clearing the air more than every other minute. The system needs to have a minimum speed of less than 18 actual CFM to be sized correctly. Otherwise you are stuck with a low pressure hurricane and plants won't do as well.I understand better now. Learning for everyone
I think he needs to use a 4" fan otherwise he will pay more for worse results.
Thanx for all the knowledge. I am currently looking at a 4 in inline rated at 190 cfm with speed control, and my duck work should be relitivaly straight btw. Is this something I should send to my garden spot or ship to some friend that could be a blabbermouth. Use ing cash prepaid card and an eBay store
I think you're ahead of the curve by being new and being concerned about sufficient ventilation, it's often overlooked because it's not sexy like lights and nutes. What I've read in books says you want a minimum capacity to cycle your air through the space ever 5 minutes. That seems way too low for me. I set up my rooms with the ability to cycle the air through 2 times a minute, in your case that would be 60 cfm. So a 400 cfm fan with a speed controller turned way down seems about right to me. Plus you have extra power if you get a heat wave or something.
nomofatum
Well-Known Member
2 times a minute is way to fast. You want 1 time every 2 minutes. The 4" 200 cfm fan will need a static pressure (resistance) of 1.5 inches to reduce the CFM to under 25. Check if you can find the static pressure (inches w.c.) rating of your filter. If straight your duct won't add much pressure, a 4" duct adds .65" for a 100ft run, so .13" or so for you.
Here is where I grabbed the fan CFM to static pressure curve, it's actually from the 4" fan I have.
http://www.tjernlund.com/Tjernlund_M-Series_Specifications_8506014.pdf
http://www.ebay.com/itm/181144406542
Check your carbon filter specs, if it's less than .5" of static pressure this fan will work, if it's >.5" you will need the 4" 190-200cfm pressure fan + speed controller and to set it very low.
With the 4" 200cfm at full speed you would need 1.6-1.8" of static pressure to reduce flow enough.
Here is where I grabbed the fan CFM to static pressure curve, it's actually from the 4" fan I have.
http://www.tjernlund.com/Tjernlund_M-Series_Specifications_8506014.pdf
http://www.ebay.com/itm/181144406542
Check your carbon filter specs, if it's less than .5" of static pressure this fan will work, if it's >.5" you will need the 4" 190-200cfm pressure fan + speed controller and to set it very low.
With the 4" 200cfm at full speed you would need 1.6-1.8" of static pressure to reduce flow enough.
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If you say so... but it's been working pretty well for me.