Doesn't need to be that much at all. So long as the intake hole(s) are the same surface area as the exhaust, your fan will suffer no resistance other than your carbon filter. So lets work this out.2x-3x the surface area of your exhaust.
2-3 holes that are 4" each.
J
great info man!!!!Doesn't need to be that much at all. So long as the intake hole(s) are the same surface area as the exhaust, your fan will suffer no resistance other than your carbon filter. So lets work this out.
4 inch duct surface area is 3.14(2)^2 (pi r squared, and you thought that high school math would never come in useful) so 12.57 square inches.
Now you need either 1 hole the same size, well thats easy, just another 4 inch hole, or even better, a series of holes spread evenly. So lets look at 1" holes. 3.14(0.5)^2 = 0.79 square inches. Hm, kinda small. You would need 16 of those to make it work (12.57/0.79 = 16). Swiss cheese grow box!
Lets look at 2 inch holes. 3.14(1)^2 = 3.14 (if the radius is 1, its the same as pi). So 12.57/3.14 = 4. That's more like it!
One near each corner on the bottom (i am assuming you are taking about a DIY grow box here as tents already have the intake vent cut out to size) Make sure the exhaust is at the top, the intake holes at the bottom (to draw in the coolest air) so it pulls it across your plants. Light proof the holes by putting some 90 degree plastic pipe bends and pointing them down or use some flexy hose and make an S shape with it, then stretch some panty hose over the ends of the pipes and rubber band those in place. This will keep out the bugs and dust. Light proofing is very important. Check the rest of the box by turning on your light inside and closing up the box. Switch the light off in the room and if you can see light coming out the box, light can get in! So seal these gaps up.
Then hey presto, you have a light proof, well ventilated grow box. Happy growing!
i was planning on using this.... http://www.ebay.com/itm/4-x-14-Carbon-Air-Filter-Value-Combo-4-Inch-Inline-Fan-Exhaust-F016-/281102466057?pt=LH_DefaultDomain_0&hash=item4173033c09 it says 190cfmAll the math is correct except the fact that there was not one mention of cfm rating. You can have a high powered vortex fan and you are going to need to figure that into the equation. There you will find that 2-3 even 4 times the amount of intake as exhaust is required. If this is not figured in, it will have negative effects on the space, either too much negative pressure(hard to open the door) or working the fans ass off.
The amount of intake is directly related to the amount of outflow.
Peace and Great Grows
Asmallvoice
http://urbangardenmagazine.com/2010/07/planning-your-grow
Just over halfway down in the link it talks about FRESH AIR and also how to work out passive and powered intakes.
They agree with me 2-3 holes the same size as your fans spigot size.
J
I have 2 4x18 from the same place( gotham) and I have 1 exhausting and 1 scrubbing. You going to need at least 2 4" openings, cause the 190cfm will be knocked down a notch after adding the filter. More like 80-100cfm tops. My room works flawlessly with 1 4" intake and 1 6" intake (both passive and filtered )i was planning on using this.... http://www.ebay.com/itm/4-x-14-Carbon-Air-Filter-Value-Combo-4-Inch-Inline-Fan-Exhaust-F016-/281102466057?pt=LH_DefaultDomain_0&hash=item4173033c09 it says 190cfm
well the plan is im going to use a cool tube and ducting out both sides to cool it with its own 6" 240cmf duct booster so most of my heat should be gone there. then ill use the 4" inline fan filter combo to scrub and exhaust straight out.I have 2 4x18 from the same place( gotham) and I have 1 exhausting and 1 scrubbing. You going to need at least 2 4" openings, cause the 190cfm will be knocked down a notch after adding the filter. More like 80-100cfm tops. My room works flawlessly with 1 4" intake and 1 6" intake (both passive and filtered )
On a side note, you can upgrade to the pro model, which is 4x18x 2" thick walls and refillable for just $10 bucks more than that one, with the same fan. I have 1 grow on mine, and I like them a lot.
Hope this helps
Peace and Great Grows
Asmallvoice
This will work but its still putting pressure on your exhaust fan if you're using passive vents.Well i guess you guys know. All i know is my tent has a 5" exhaust and a 5" intake and it doesn't cave. When i close the vent, it caves like a bitch. Plants are fine, fan is fine, it is 215 cfm...what can i say? The votes have it, go with 3-4x the size of the exhaust.