# calculate passive intakes



## old timer 4311 (Sep 10, 2012)

Hey guys,

Planning next grow box and was wondering if there was a formula out there to calculate the area of passive intakes. 

Box will be approx 63 cubic feet. (3x3x7). Exhaust fan will be 6" 400 cfm hydrofarm. 

Thinking there must be some math formula to calculate the most efficient area of passive intakes needed.


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## Old_SSSC_Guy (Sep 10, 2012)

Its figured from the volume of air you move out of the unit.  According to my vent chart a flexible 6" duct can move only (designed for) 75 cfm, and a 6" round metal duct can move 85 cfm.  A 400 cfm is really big for 63 cf.  You could do the cabinet with a small bathroom fan, or run the 400 fan at maybe 25% with a fan dimmer switch.  Makes it a lot quieter too.

Give yourself a 6" round intake hole and you ought to be fine.  If you run the fan high speed though, you'll need a lot more and it will get noisy.


View attachment FIELDDUCTSIZINGCHART.pdf


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## sMACkaddict (Sep 10, 2012)

I can't make heads or tails outta that chart...


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## Old_SSSC_Guy (Sep 10, 2012)

The top charts show the inches diameter in the left column, and the the right column is the number of CFM (cubic feet per minute) which can move through that size.  The upper left chart shows the sizes for flexible stuff (like dryer hoses) and the right corner chart shows the ratings for solid round metal pipe.  Metal pipe permits more air flow than flexible pipe.  The bottom charts show that ratings for square duct work, like what's usually used for AC/Heat ducts.

A 6" flexi hose duct is rated to move 75 CFM, a 12" solid duct is rated to move 525 CFM.  

Most folks don't know that it takes more duct size than you first think.  Usually a LOT more.  That never stopped me though... heh... I will run a 6" fan through a 6" duct.  No way its moving only 75 CFM, but it sure makes a lot of noise when you crank the fan up.

What's not to understand?  What am I missing?


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## sMACkaddict (Sep 10, 2012)

thats what I am saying... those ratings seems a little arbitrary... Volume is cubic, so it needs more than one dimension to be calculated, and all they are basing it on is the width of the duct, not the length of the run.  Plus it matters the speed at which the air is moving which is set by the fan...
I know I saw a good calculator for this kinda thing, just can't find it anywhere..

sMACk


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## sMACkaddict (Sep 10, 2012)

I dont know what the calculations are based on, but this seems interesting at least:

hMPp://megaswf.com/serve/71961/


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## sMACkaddict (Sep 10, 2012)

found the calculations, taken from rollitup.org user gotdank,
"
LxWxH of room= CuFt. Divided by X minutes for air exchange(5 or lower suggested) = Unloaded CFM required.(we will call this uCFM)

uCFM + (Duct Length in feet x 10%) = Duct Loaded CFM(dLCFM) (example 12'*10%) = 120% , uCFM + 120% = dLCFM)

x=# of 90deg bends

(dLCFM x (1.5^x))= Bend Loaded CFM(bLCFM)

if its filtered then 

bLCFMx1.3 = Filter Loaded CFM

then to add for heat from the bulb

For a 250HPS- multiply by .75
For a 400- multiply by 1
For a 600, multiply by 1.3
For a K, multiply by 1.6.
"


sMACk


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## Old_SSSC_Guy (Sep 10, 2012)

Ok Smack, but I am math challenged (smile).  Why not take oldtimer's figures and put them through your formula for him?  

Maybe our brains are reversed.  The chart made perfect sense to me but I can't make heads or tails from your formula.   :batman:


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## old timer 4311 (Sep 10, 2012)

Interesting. 

Makes sense in theory but when i run my 400 cfm at about 50% power and 100% power it seems like a larger volume of air travels through the box. 

Ill do a test tonight and see if my temps fall based on 50% vs 100%.

Wouldnt the max cfm also be a product of velocity? So my fan spooled up to full power move air at a higher velocity? 

I have a small box i can test it on. Have 4 1" intakes with 6" exhaust so should plateau with temps running at fairly low speed on the fan.


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## old timer 4311 (Sep 10, 2012)

So what does that formula calculate?

Came up with 28.7

6' duct run
2 90* elbows
Filtered and 600 hps


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## sMACkaddict (Sep 10, 2012)

i think its easier to use the link I gave, it is a flash program where you just pop in the numbers.

I just did your numbers, old timer, assuming you want to exchange your air 1 time/minute, I got:

you need a 383.3 CFM fan and 191.6 sqin of passive intake

apparently the 90degree bends are the real killer.  If you play with the program and change the amount of bends you can really see the difference in the numbers.
These are also not gonna be exact numbers, so I think the fan you have now will be fine, even better if you can get rid of one of those 90 degree bends.

ooo maybe i can embed the program.. im gonna try **EDIT: Not Possible...**

sMACk


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## Old_SSSC_Guy (Sep 10, 2012)

Hey sMack - what duct size are those numbers for?  Thought the rule of thumb was CF/5 minutes?


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## sMACkaddict (Sep 10, 2012)

Old_SSSC_Guy said:
			
		

> Hey sMack - what duct size are those numbers for?  Thought the rule of thumb was CF/5 minutes?



I don't know, he doesn't mention it in the original thread on rollitup.org.  However, he does give a link to the original article with the formulas and stuff I think.  The site seems to be down, which I bet is related to GoDaddy being down(saw that on gizmodo.com).

hMPp://www.gardenscure.com/420/construction/128153-noobs-guide-first-grow-room-set.html#post1262629

About the rule of thumb for air exchange in a grow room, I can't get a standard number.  I've seen anywhere from 3 times/minute upto 1 time every 5mins.  I live in a hot climate and have decided to go for more frequent exchanges.

sMACk


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## sMACkaddict (Sep 10, 2012)

that might be an indication that the ratings in that chart are actually arbitrary... what do you think?

sMACk


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## sMACkaddict (Sep 10, 2012)

this looks pretty helpful as well, i think I've seen this chart on the site, but it was either explained poorly or not at all...

hMPps://www.greenpassion.org/index.php?/topic/3401-what-size-vent-fan-do-i-need-and-intake-area-size/

sMACk


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## The Hemp Goddess (Sep 10, 2012)

I am quite sure that a 6" duct with a 400cfm fan will move far more than 75 cfm--most whole house heating ducts and return air are 6" and that is not even close to being enough air to heat or cool a house.  I will check for positive with aqn HVAC guy I know.  However, regardless, I think you are making this harder than it needs to be.  I simply made the same square inches in intake that I have in sq inches in exhaust.  This creates negative pressure in my space.  I have 2 4" intakes and 1 6" exhaust--approx the same amount of intake as exhaust and it works great.


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## sMACkaddict (Sep 10, 2012)

we are just discussing some ventilation theory... I believe that equal size intake and exhaust works, but I wanna know why and under what conditions, thats all... math is fun!

sMACk


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## ShOrTbUs (Sep 11, 2012)

my older brother is a mechanical engineer, and currently working as a project manager for turner construction. i'll get some answers. he deals with commercial HVAC alot


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## old timer 4311 (Sep 11, 2012)

I would be interested to see what people with hvac backgrounds have to say. The area the calculator spits out is pretty large. Considering the area of a 6" duct is about 28", getting up to over 100 sq in would be difficult. 

I was planning on usong 90 pvc elbows but using 4" pvc i would need almost 10 intakes. And 4" pvc elbows arent cheap!  

Pretty interesting though. 

Thanks for all the input!


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## old timer 4311 (Sep 11, 2012)

Also, did the experiment with my fan at full power vs 1/2 and the temp fell about 4 degrees on full power. Leads me to believe that the 6" ducting can handle more than 75cfm. However its a very short run (maybe 2.5 feet). Im sure it would loose efficiency over a longer run.


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## sMACkaddict (Sep 11, 2012)

ok, well thats good and reinforces what we all thought/knew...


sMACk


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## sMACkaddict (Sep 11, 2012)

old timer 4311 said:
			
		

> I would be interested to see what people with hvac backgrounds have to say. The area the calculator spits out is pretty large. Considering the area of a 6" duct is about 28", getting up to over 100 sq in would be difficult.
> 
> I was planning on usong 90 pvc elbows but using 4" pvc i would need almost 10 intakes. And 4" pvc elbows arent cheap!
> 
> ...



the surface area for the opening of a 6in duct is like 113 sqin, so that is almost dead on track with what Hemp Goddess was suggesting.

sMACk


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## old timer 4311 (Sep 11, 2012)

Isnt area = pie*r squared?

So r=3 area = 28"


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## sMACkaddict (Sep 11, 2012)

ha... that is correct, my bad, forgot that 6" isnt the radius! whoops!

ok so where does that leave us...

sMACk


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## tastyness (Sep 11, 2012)

Oh all you math geeks are having a blast I see. 
Resume calculating.


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## dman1234 (Sep 11, 2012)

I would cut a hole the size i think is right and then just enlarge it if needed during a test run i would do with no plants. jmo


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