• Here is a link to the full explanation: https://rollitup.org/t/welcome-back-did-you-try-turning-it-off-and-on-again.1104810/

Negative Pressure Issue

JeosOne

JeosOne

Member
Yet another question..(sorry guys)

Hello everyone, I am seeking some advice.

I have a 5x5 grow tent I made myself using poly and a wooden frame. It looks fucking beautiful, I'm pretty proud of myself.

Here is the issue. I installed a 6in exhaust fan that spits out roughly 435 cfms. My 6in intake is through my window, with a bug screen over the duct. When I turned the fan on, the tent nearly collapsed itself because of the negative pressure. If I install a carbon filter, will it lessen the pressure? Or will I need another intake? Thanks.

 
S

SticherVA

Member
I like a small amount of negative pressure in my tent, just to make sure I don't get smell leakes into the room. however, I would recomend a speed controller (~$20) for your exhaust fan 1st so you can better balance the airflow, then I would get a 6in booster fan (~$25) for your intake.
 
lakew00d

lakew00d

Well-Known Member
yea like both people said above me,
Passive intake and a fan controller, and a carbon filter will help also,

like ive been told Growing is all an experiment for even experienced growers, just play around with stuff, see what fits your needs.
 
JeosOne

JeosOne

Member
Haha, thanks guys. And yes highflyr, thats how I feel. Like once i think im almost ready to start, NOPE. somethin' else i gotta buy/install/prepare. i dont mind though because its for a good cause :P
 
Tamorin

Tamorin

Active Member
Dude, just a heads up I've been buying Shit for the last year and a half. No matter what it never ends. Good luck though building your own stuff is always cheaper and sometimes more efficent. Their right about another opening plus you'll pull in fresh air which is co2. Keep asking questions. Tamo
 
brokenturtle3102

brokenturtle3102

Well-Known Member
I had the same issue and I taped up my supporting beans with duct tape, helped slightly. I just ended up buying the fan speed controller for 30$ and it was the best 30$ ive spent in my room. One suggestion, keep your eye out on the humidity from outside. If it rains, your room will be very humid and carbon filters DO NOT work efficiently when it's humid. Pay attention.
 
M

madmikeri

Active Member
I would not recommend outside air for a intake. Like broken turtle said you are exposing your room to the outside weather. Not only that if you think about the air you dealing with over the next few months it is very cold and the room is warm this will result in condensation.
Think of your duct as the glass, and the air is the cold beer inside.
I had a setup very similar to yours and needed to change it up to eliminate the rain falling from the duct work.
 
JeosOne

JeosOne

Member
shit, i gotta come up with a better solution then...what would be the wisest way to to get fresh air in there? the air from my room would be pretty stuffy, as i'm in the basement and mold would also possibly be a factor, although i do my best to keep it clean. i've definitely considered the cold air temperature of winter coming through my intake...i don't wanna freeze my medicine out and kill it.. i was curious that if i added another intake from the air in my room, if it would balance out the temperature...because the air in my room is gonna be warm as im exhausting the hot air into it.
 
JeosOne

JeosOne

Member
i suppose i could always shut the window according to the temperature/weather..argh. just when i thought i could get started soon...the doh just cashed my check a couple days ago so im anxiously awaiting the next step..patience....
 
M

madmikeri

Active Member
[FONT=&quot]Here is a common heat transfer formula[/FONT]
[FONT=&quot]cfm = 3.2 x Watts / temp change[/FONT]
[FONT=&quot]now let’s put that to work, [/FONT]
[FONT=&quot]I will assume you are going to put a 1,000w in that tent.[/FONT]
[FONT=&quot]3.2*1000=3200[/FONT]
[FONT=&quot]if you want to keep your temp say +5 degrees above intake air[/FONT]
[FONT=&quot]3200/5=640 CFM [/FONT]

[FONT=&quot]Now that you know how many CFM you need lets figure out how much "passive intake area you need"[/FONT]
[FONT=&quot]The Home Ventilating Institute recommends one square foot of open air inlet per 300 CFM of ventilation fan capacity.[/FONT]
[FONT=&quot]640/300 that’s 2.13 sq feet or 306.72 sq inches of open space[/FONT]
here is how to calculate the hole sizes:
1. Take the total area in square inches needed, in this case 306.72 square inches, and divide by the number of holes you want.
2. Then divide by Pi (3.14).
3. Take the square root of that value.
4. Then multiply by 2.
The answer is the diameter that each hole would need to be to make up the total area needed for intake.
1 19.76 in hole
2 13.97 in holes
3 11.4 in holes
4 9.88 in

Now if you add a carbon filter you are going to drop your fans CFM by at least 20%, you will also loose CFM over the length of the ductwork. The longer the duct the less CFM you have, you also loose about 15% for every 90’ turn.
If you go passive, you will need to add some large holes that will need to be light blocked. You need to figure out about what CFM you will be running after everything is considered (filter and duct length) then get an intake fan to = that. You want to take out just a little more air then you put in to maintain a slight negative pressure.
 
JeosOne

JeosOne

Member
god damn, you really are mad hahaha. honestly my head is spinning from all this. i appreciate all the help man, im trying to figure it out..
 
brokenturtle3102

brokenturtle3102

Well-Known Member
Jeos, in plain English. If you put a carbon filter in your intake, with your fan that has a controller, you will be great! Adjust the amount of filtered air going in your room depending on humidity and temperature outside and inside. Get a small 130cfh fan for your hood, and put a small cloth at the other end of your light for filter. With this setup, you have intake through a filter from the outside, and winter months will be your saver here. Your extake out of your tent is through a cloth filter, through your light and hood, through the 130cfh fan, and out your room through ducting. This is one way of controlling your room. BUT you now need a second means of odor control.
 
H

HghFlyrJD1

Active Member
madmikeri said:
[font=&quot]here is a common heat transfer formula[/font]
[font=&quot]cfm = 3.2 x watts / temp change[/font]
[font=&quot]now let’s put that to work, [/font]
[font=&quot]i will assume you are going to put a 1,000w in that tent.[/font]
[font=&quot]3.2*1000=3200[/font]
[font=&quot]if you want to keep your temp say +5 degrees above intake air[/font]
[font=&quot]3200/5=640 cfm [/font]

[font=&quot]now that you know how many cfm you need lets figure out how much "passive intake area you need"[/font]
[font=&quot]the home ventilating institute recommends one square foot of open air inlet per 300 cfm of ventilation fan capacity.[/font]
[font=&quot]640/300 that’s 2.13 sq feet or 306.72 sq inches of open space[/font]
here is how to calculate the hole sizes:
1. Take the total area in square inches needed, in this case 306.72 square inches, and divide by the number of holes you want.
2. Then divide by pi (3.14).
3. Take the square root of that value.
4. Then multiply by 2.
The answer is the diameter that each hole would need to be to make up the total area needed for intake.
1 19.76 in hole
2 13.97 in holes
3 11.4 in holes
4 9.88 in

now if you add a carbon filter you are going to drop your fans cfm by at least 20%, you will also loose cfm over the length of the ductwork. The longer the duct the less cfm you have, you also loose about 15% for every 90’ turn.
If you go passive, you will need to add some large holes that will need to be light blocked. You need to figure out about what cfm you will be running after everything is considered (filter and duct length) then get an intake fan to = that. You want to take out just a little more air then you put in to maintain a slight negative pressure.
Click to expand...

real data folks.......
 
JeosOne

JeosOne

Member
Okay, so I've decided to switch the intake and exhaust so I'll be exhausting out of my window instead of taking in freezing cold air. Going to invest in the controller for sure.
 
Tamorin

Tamorin

Active Member
Don't sweat it, it seems complicated now but it is a no brainer. Literally when u get thru your first grow youll know what's up.
 
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