active or passive intake?

[headshake]

what's good everyone? i am currently build a new "frank" as my wife likes to call it. anyone who follows my grows knows what's up!

anyhow, i have 4 computer fans that move something like 75 cfm or so.

my question is should i use all four for exhaust and passive intakes, or use 2 and 2 (intake/exhaust)?

thanks in advance for the input/feedback!

good growing!


-shake

[syde00]

IMHO:

use only exhaust fans at the top of the cab where the air is hottest with passive intake hole(s) near the bottom (best to use several small intake holes spread out evenly along the bottom)

heres the first and most obvious reason as to why not to split 2/2

If you are pulling air out the top with 2 fans at 150 CFM, that air *has* to come from somewhere, it will pull it through your passive intakes.

What would adding 150 cfm of in-take air actually do? if you are pushing air in at the same rate you are pulling it out anyways, you haven;t actually gained anything. the exhaust fans might not have to work fractionally as hard, but not a noticable amount. Think about it like this... if you set the cruise control in your car at 10 mph, and then somebody starts pushing your car from behind at 10mph, your still going to be only going 10mph, you might just burn a little less gas in the engine because some of the work is now being done by something else.

The other reason I can think of is that, in general, having a slightly negative air pressure inside the box (you will have negative pressure because the fans will be constantly pulling air through the intakes trying to get them to keep up with the rate its pulling it out, it won;t be able to do this perfectly, which will create slightly negative pressure). THis is a good thing because negative air pressure = less air = less heat capacity which means the hot air will rise to the top quicker and ultimately get exhausted quicker.

the other issue with using combo intake/outtake is that you can create sort of channels of air flow that go directly between the intake fan straight to the outtake. This happens because the air coming in is being pushed into a certain direction at relatively high velocity. ultimately this can create spaces in the box that doesn;t get its air exchanged frequently and becomes "stale" so-to-speak... sorry i know this particular point probably makes absolutely no sense... i can completely visualize it in my head, but i just can;t seem to put it into words lol


the best thing IMHO, put all your exhaust fans spaced out evenly across the ceiling of the box if possible. then drill many many very small holes (in comparison to the out-take holes) all the way around the perimeter of of the box near the floor of it, this should help keep air being exchanged throughout the entire box.

If you want to get like super awesome... you could build like a raised floor inside the box and have all the in-take holes in the raised floor, that would keep fresh air coming directly up through the floor, past the plants, up to the lights and eventually out the exhaust. I actually really wanted to do that when i built my box, but lazyness got the best of me!

[headshake]

sounds good syde! thanks for the info. and the part you are having a hard time putting into words makes perfect sense. it creates a jetstream so to speak. so just the air in said stream is exhausted and the rest remains stale. or something like that, maybe you aren't the only one that can't put it into words!


-shake

[headshake]

this leads me to another question. i have read that your passive intake should be anywhere from slightly smaller to 3X the size of the fans.

what is everyones take on this?

i don't have an extra 12VDC adapter to test this for myself at the moment.


-shake

[syde00]

sounds good syde! thanks for the info. and the part you are having a hard time putting into words makes perfect sense. it creates a jetstream so to speak. so just the air in said stream is exhausted and the rest remains stale. or something like that, maybe you aren't the only one that can't put it into words!


-shake


yes, exactly! i;m glad that made some sense lol

as for the intake/outtake ratio... yeah i;m not too sure on that one. I've heard some claim that intake should be 1/2 the size of the exhaust, and other say your intake should be 2x the size of the exhaust... so tough call.

Obviously the larger the intake, the less work the exhaust fan has to do, but it will also start taking away from the negative pressure situation.

my box ATM, my intake is probably about 1/2 the size of the exhaust... but this wasn;t by design, it was mostly a matter of convienence. my exhaust had to be somewhat large to accomodate the fan i was using, and the intake I just used like a door knob hole drill bit. I've been meaning to make it a little larger, but i haven;t been having any big temp problems yet... but I know with the summer months coming, i;m going to at least need to double my exhaust and intake.

[headshake]

yeah, i would assume that a slightly smaller intake would create the desired negative pressure. but once agian, who knows!

i'm gonna have to break down and do some science experiments one of these days.


-shake