VENTILATION, is more CFM really necessary? Take a look

Alright, hypothetically speaking, a tent that measures 3'x3'x6 1/2'
If I had a 6" 250 CFM inline duct fan, and 6" x 20" carbon scrubber to exhaust my room. Set up like this:
Scrubber--->light--->fan>out. I will be using passive intake. There are three vents on the tent that measure 17.5" x 5".
The light is a 600 watt HPS/MH cool-tube with a reflector. It has its own fan and wattage controller on a digital ballast.

The ambient temperature around the actual tent, will be 70-72 even in the middle of summer. It's a room in the garage that receives air conditioning and heating.


So, is it really necessary that I get a higher rated CFM fan? Many people told me I would need a much higher one.
I calculated like this: (excuse any math mistakes, math is my worst subject; but I get by).
-3x3x7 = 63.
-63 x 5 (times air needs to be circulated) = 315
-315/60 (minutes) = 5.25 CFM needed???
Extra calculations:
-5.25 + 63 (20% of 315 for the scrubber resistance) = 68.25
-68.25 + 15.75 (16) (5% for the 600W light) = 84

So would I only need a total of 84 CFM to exhaust my room and keep temperatures at a good range?
I'm not really sure if this is right, but I found a formula online and I THINK I followed it correctly. I don't remember the link of it though.

Umm no... you dont divide by 60. 3x3x7=63 Cu f. You dont need to go 5 times a minute. 2-3 times a minute is sufficient. 63 cu ft 2-3 times a minute = 126-189cfm which is why you keep getting told you need way more (than the 84)

Account for static losses in ductwork too. Bigger fans run on a variac can run quieter at reduced speed and offer tuning ability to avoid outrunning your carbon filter.

LordRalh3 said:

Umm no... you dont divide by 60. 3x3x7=63 Cu f. You dont need to go 5 times a minute. 2-3 times a minute is sufficient. 63 cu ft 2-3 times a minute = 126-189cfm which is why you keep getting told you need way more (than the 84)

Click to expand...

thank you. ;D But if it's 189 cfm needed then, and you factor in 25% of that for the filter and light, wouldn't it still only need 236.25 CFM?
189 CFM
(20% for filter, 5% for light) 25% of 189 = 47.25
47.25 + 189 = 236.25

EDIT:
Okay, so factoring in another 5% for ducting,
It would be 245.7 CFM.
Should I factor anything else?

My fan is rated for 250 CFM

I use 4" on 5x5. Outside air comes in blows through dust shroom. And exhaust is phresh 4" vortex fan thus cooling and adding fresh air to adjacent tent

For ventilation.and ventilation only (which is nothing more than fresh air intake) you multiply your widthxlengthxheight to get total cubic feet....

3x3x6.5 = 58.5cfm

You want an air exchange 2x per minute to control humidity so...

58.5x2 = 117cfm (after all pressure losses from connecting ducting,and carbin filter and any bends in the ducting).

Now if using one fan to both cool the light and ventilate the grow area it is different and you'll need to get a more powerful fan.

In that area with a 600w light you'll want a 6" 440cfm inline centrifugal (not a duct booster fan from HD or Lowes,duct boosters don't hsndle pressure losses at all).

Read ventilation 101 in it's entirety,the link.is in my sig below.

Ou8aCracker2 said:

For ventilation.and ventilation only (which is nothing more than fresh air intake) you multiply your widthxlengthxheight to get total cubic feet....

3x3x6.5 = 58.5cfm

You want an air exchange 2x per minute to control humidity so...

58.5x2 = 117cfm (after all pressure losses from connecting ducting,and carbin filter and any bends in the ducting).

Now if using one fan to both cool the light and ventilate the grow area it is different and you'll need to get a more powerful fan.

In that area with a 600w light you'll want a 6" 440cfm inline centrifugal (not a duct booster fan from HD or Lowes,duct boosters don't hsndle pressure losses at all).

Read ventilation 101 in it's entirety,the link.is in my sig below.

Click to expand...

If I get this fan
http://www.amazon.com/VenTech-IF6B-Inline-Floor-Mount/dp/B005KMPN0K/ref=sr_1_1?ie=UTF8&qid=1395540730&sr=8-1&keywords=6"+centrifugal+fan
ad set it up with the scrubber on the floor--->cooltube light---holeintent--fan>out... Would that work? I plan on setting the fan on a shelf. I wouldn't know how to make it fit in the exhaust hole the tent provides. Im too scared it would fall or something.
If I buy this fan, are you sure it would work, and not get too cold or anything? I don't want a controller if I don't need it. I've spent way too much already.

nevermind. I just bought the tjerlund m-6 fan with speed controller

Someone got excited. lol