I have the Lasko 12" high velocity floor fan for my fresh air fan, which runs at 480 CFM. How many CFM's would my vent fan need to be for negative pressure? I'd prefer 6" as I have 6" duct already. http://www.lowes.com/lowes/lkn?action=productDetail&productId=153139-39646-4914&lpage=none
its simple math. 3.5 x 6 x 8 is 188' of volume, so to maintain negative pressure anything above 188CFM should be sufficient.
why would you use an intake fan? just use one powerful exhaust fan and have passive intakes to ensure negative pressure.
if its in an environment, then no, having open holes wont effect your odor control, so long as the fan is on. If the fan pulls enough air to create neg pressure, that means air is being pushed out faster than it comes in, so the air coming in is accelerated....due to the lowered air pressure in the box/tent/cabinet. (remember, air diffuses from higher pressures towards lower pressures, to equalize) so, as long as the fan is on, air will ONLY enter the cabinet/tent/box through the passive openings when the fan is turned off, pressure will equalize, and THEN and only then would odor be an issue coming out of your intakes
So unless you have a carbon filter on your intakes or leave your fans on all the time, your gonna have a smell. I have my fans hooked up to the same power strip as my lights, so the timer shuts everything off. I guess I could get another power strip for just the fans and leave them on all the time.
But what if I wanted to keep the fresh air fan, just to increase the amount of freshair into the room, how would I calculate the amount of CFM's Id need? I thought by taking the 440 total CFMs of the unit, dividing it in half cause it has 2 intakes and the other intake is directly from the grow room, and adding it to the minimum CFMs needed for negative pressure in my grow space, so 220 CFM + 206 CFM = 426 CFM. Make sense to anyone? Feedback?
I'm not following your math so much.... Negative pressure is achieved by pulling more air from the room than is able to be taken in. I don't think the size of your room would matter, as long as the exhaust fan is capeable of pulling more air out than your intake fan is able to push air in. I'm no physicist, but anything that moves more than your 480 cfm intake would be sufficient. But remember, a fan's cfm rating isn't the same pulling air as it is pushing air... As your system sits right now, you have 168 cubic feet of grow space, your 480cfm fan is replacing the air inside with fresh air 2.8 times per minute, given no drag on the airflow... Thats brand new air every 21 seconds or so. I don't think you need to add another fan, you're exchanging air faster than your plants can use it. My advice is to move the fan you have to the exhaust side of things and worry no more.
in a basic form, thats correct...~426 but, it depends on a ton of things firstly, you most likely have reduced your inline cfm by a factor other than 1/2. Area of a circle is pi*radius^2, so heres an example to illustrate, itll be easier 8" inline fan, 737cfm. area (planar) of air moved with one iteration could be estimated (its not accurate, but it would be off by the same factor later, so it doesnt matter) at ~50in^2. thats 4" (the radius on an 8" fan) squared, multiplied with pi. a 6" fan would yield an approximate ~30in^2. So, if you have ducted an 8" to two 6", then estimate at half, or slightly stronger. 6" to 4" is worse though ~30 down to ~25 or so, so its under half...you lost some efficiency. see how that works? so, you have to figure how much air your actually moving, then you can compare with your intake. If you want a true negative pressure (I say low pressure, since its lower pressure in the cab than the grow room) then your exhaust needs to be greater than your intake. get your reduction factor (using a method like my example) to approximate it, and see for yourself. I will say this, forcing an intake is going to require more work from your exhaust to achieve low pressure...you could just remove your intake and do it just fine most likely