Well I am in the process of building my veg cab, and I am a bit confused regarding the PI size. If I have a 4 inch 160 cfm CPU fan as an exhaust, then theroretically, my PI should be 8 inches? That just doesn't seem correct. Should I just have 2 - 4 inch PI's? Shit, maybe I even have it backwards, but I thought the PI should be twice the size of my exhaust.... Thanks in advance!

An 8" circle is not double a 4" circle, it's four times bigger because the area is calculated using the square of the radius. So if you double the diameter you also double the radius which translates to four times the area. The area of a circle is pi x r-squared. So a 4" circle has about 12.56 square inches of area. For a circle with twice that area, run the formula in reverse, which is to say first multiply it by 2 (because you want it twice as big), divide by pi and then take the sqare root. That gives you a radius of 2.8. So...a circle with a diameter of 5.66" is twice the area of a circle with a 4" diameter. That 8" circle has an area of 50.24 square inches, exactly four times the 12.56.

hey where did you get a 160 cfm cpu fan?? cuz im interested in using cpu fans as exhaust and intake fans also

right Toasty. Because air is cubic, one 8" is better than two 4". But, I don't think he needs an 8". Two 4" holes allow you to get airflow on both sides of his grow and balance out the intake a bit more. He's only pulling 164CFM of exhaust. I would get a bigger fan.

Totally agree. He was wondering about needing an 8" intake, and I got so wrapped in explaining the math that I probably wasn't clear in saying that 8" is more than the size he needs to match his exhaust. The easiest way to have double the intake of a 4" exhaust hole is to have two 4" intake holes and forget all the slide-rule stuff.

What does PI stand for? Searching for it brings up a ton on the mathmatical constant pi, which isn't helpful I'm guessing Passive Intake but not too sure...

well if a Active intake is a AI then I guess a PI is a Passive intake.. so then your assumption would be correct joint...

Yes, sevenofspades and Up4 meant passive intake. Later I used pi in the math talk literally meaning the infinite number that is the constant ratio of the circumference of a circle to the diameter =3.14159...

Thanks for all the help and math that I thought I would never see nor hear about again! I will just use 2 3" PI's. Here is another issue that I have come across. If you need to keep the seedlings as close as possible to the cfl's, I would have to install a shelf. Simple enough. Now what do I with my PI's? The shelf would be prohibiting them to function properly if they were placed in the standard bottom corners. Should I put a hole in the shelf? Or just make 1 PI up top, and one below? Any suggestions would be a major help. Thanks again!

Either: Leave air space around the shelf so it still draws intake from the bottom. Obviously the shelf has to be either the full width or the full depth of the box, just don't make it both. Or... Instead of raising your plants why not lower the lights?

I wouldn't mind lowering the lights, but I am running CFL's in my veg box, and I have no clue how to go about erecting something that would be somewhat difficult for me. I guess I will just have to use a shelf out of peg board. That would work I believe? There should be enough little holes to have the box stay at the proper temps. Thanks again guys! also, if anyone has a link to actual box materials and building instructions, that would be great! I am lacking heavily in the craftsman area, so any help would be spectacular! lata

So if I had a 6" exhaust with 400 cfm sucking air out how big of passive intake would I need? Would 2 6" holes work or can I use 4 3" holes and where would be the best place for them. My cab is raised 2 inches off the floor could I just cut the holes in the bottom or would the 2 inch air space in the bottom not draw in enough fresh cool air?