# Negative Pressure

• None
O

#### Obtuse

What equation do I need to pick my intake/exhaust fans for my area to be sure I'm creating negative pressure in a cabinet? I've used the search button, but can't find anything that tells me how to figure this out. I see all sorts of different fan combinations for the same sized rooms/ cabs and am getting lost in it all.

#### OctoberDee

You can't really get true negative pressure without a seriously air tight room. You're talking about a vacuum right? If you compare the CFM (cubic feet per minute) of the fans you're using you could easily assume that if the intake fan is 80CFM and the out-take fan is 120CFM then you are effectively moving more air out than in and would then get a strong vacuum created. I don't know if that helps.

#### qupee

You can't really get true negative pressure without a seriously air tight room. You're talking about a vacuum right? If you compare the CFM (cubic feet per minute) of the fans you're using you could easily assume that if the intake fan is 80CFM and the out-take fan is 120CFM then you are effectively moving more air out than in and would then get a strong vacuum created. I don't know if that helps.

Except that resistance in the system is often not even.

That is, your exhaust fan will often be pulling or pushing through a filter, and often will have a longer duct run. That results in a drop in cfm.

Your intake fans will often be less restricted, and loose less cfm.

I doubt there's a general equation, but I would probably consider going more than 50% over. So if intake was 80 cfm, I'd maybe think higher than 120 cfm. I'd have to know just what all my duct runs look like to be more specific - and even then it's a bit of a guess.

#### OctoberDee

Except that resistance in the system is often not even.

That is, your exhaust fan will often be pulling or pushing through a filter, and often will have a longer duct run. That results in a drop in cfm.

Your intake fans will often be less restricted, and loose less cfm.

Right, my example was exclusionary of any real values I was saying that if you are using trying to determine how much air you want in and out you would use a fan with a higher sucking power for out than in because the pressure from within the room will suck the air in if it's forced out at a greater rate creating a vacuum. If you want to include feet of duct and filters of course this would change how much air is being pulled out and you would need a stronger fan for out. The question was not how much cubic feet a fan should be but what equation would be used to determine this. How about taking your room size and figuring how many cubic feet you have and starting there.

G

#### grnmountainboy

i usually go by a 1 to 4 equation. that is for every 100 cfm incoming you have 400 outgoing. this will usually create the negative pressure that you are looking for.

#### BCrocker

20x20 sealed room, 750cfm fan for exhaust. I turn the fan down to about 400cfm and it creates negative pressure always. It is tough to open the door to the room, and when you do you can feel/hear the air being sucked in

T

#### tat2edangel

So as I have read. A fan need to be rated to completely replace the air volume in 5 minutes. Using my own tent set up:

5 x 9 x 6.5 = 292.50 cubic feet

292.50 / 5 min = 58.5 cubic feet per minute

The exhaust fan needs to be rated to at least 58.5 cfpm

So as mentioned any fan rated for and running above that number and all other things being equal will create a NEGATIVE AIR PRESSURE.

Why would i regurgitate all that?

My 292.50 cf are being exhausted not with a 58.5 cfm but with a 600 cfm and there is a VERY NOTICEABLE NEGATIVE AIR PRESSURE FROM THE LOOK OF THESE WALLS!!! There is at least 3" of bowing on all the walls. This fan is a 8" 600 cfm, I have it neck down to a 6" (port size on tent). There is one 3' x 6" screened flap open for intake.

O

#### ODGG

^^^ What tat said...... If your tent bows in, or your doors have a natural suction to them on your cabinet, you have negative pressure. Very easy to achieve, and if you get a speed control for a nice sized fan, you dont even have to run it at full throttle to achieve negative pressure, you can have it nice and quiet.

#### OctoberDee

5 x 9 x 6.5 = 292.50 cubic feet

292.50 / 5 min = 58.5 cubic feet per minute

The exhaust fan needs to be rated to at least 58.5 cfpm

....and there it is

#### qupee

So as I have read. A fan need to be rated to completely replace the air volume in 5 minutes. Using my own tent set up:

5 x 9 x 6.5 = 292.50 cubic feet

292.50 / 5 min = 58.5 cubic feet per minute

The exhaust fan needs to be rated to at least 58.5 cfpm

So as mentioned any fan rated for and running above that number and all other things being equal will create a NEGATIVE AIR PRESSURE.

Why would i regurgitate all that?

My 292.50 cf are being exhausted not with a 58.5 cfm but with a 600 cfm and there is a VERY NOTICEABLE NEGATIVE AIR PRESSURE FROM THE LOOK OF THESE WALLS!!! There is at least 3" of bowing on all the walls. This fan is a 8" 600 cfm, I have it neck down to a 6" (port size on tent). There is one 3' x 6" screened flap open for intake.

....and there it is

Lol, or not. There seems to be some significant confusion in that post.

1. An air exchange every 5 minutes is a fairly random suggestion that can be found on the Internetz. You can also find the suggestion of 5 air exchanges per minute - 25 times more. Neither includes much if any justification when you see it recommended.

2. one exchange every 5 minutes is WAY TOO LOW. I would think anyone who's actually put together and ran a grow and thought about these numbers would immediately know that this is a long ways from enough air exchange, even with separately ventilated light hoods. Experience rules here.

3. An air exchange every 5 minutes has nothing to do with negative pressure. I could setup a tent to turn the air over once per 5 minutes, but if I try to force a bunch of air in the intake, it will have positive pressure. I could hook a 1000 cfm exhaust fan to a 1'x1' tent, but if I have a 2000 cfm fan blowing in an intake, I'll have positive pressure.

Thinking a 60 cfm fan would suffice for the OP is a bad joke.

T

#### tat2edangel

Wow, I stand corrected. I was totally wrong.

But WAIT........

Didn't I read in Marijuana Botany by Robert Connell Clark (bought mine the year the Broncos won there first Super Bowl, not on the internet) that marijuana has a set or at least measurable "Rate of Diffusion". Marijuana IN FULL BLOOM is able to Diffuse the air through its stoma with in its own foot print in AS LITTLE AS 15 MINUTES. But Marijuana is able to diffuse the air with in its own micro climate with in AS LITTLE AS 5 MINUTES! but this is a standard that is used as a STARTING POINT that can then be adjusted form there and as needs be.

Since the way I worked out my theory of equation as the OP ASKED.

"What equation do I need to pick my intake/exhaust fans for my area to be sure I'm creating negative pressure in a cabinet?" didn't work.

Can you please educate me on the correct "equation" in this very specific application? I am always open to good conversations and constructive critisizum . I would love to debate Theory of Accurate Air Movement Over Time and Space. I would be willing to use Stephen Hawkins Construction of a a Theory as a basis for the debate and would be willing to allow the Farming community call the dabate.

Oh Yea.......

If you subtract 1 and you add 2 yes Johnny you get a positive.
But we were looking for a "NEGATIVE PRESSURE"
Thank you for raising your hand.

Or not, I could be wrong. It has happened before and will again. Ill just shut up and smoke a bowl.

Todays recipe:

"PICKLE" bowl and a beautiful "Blue Berry Hash" cap.

#### OctoberDee

Sorry, but it would be super funny if this is about a space that's like 4 cubic feet.
:D

T

#### qupee

Nah man, I just re-read that and I was a dick. My bad.

Good brownies. :passingjoint:

M

#### mal

Supporter
Well Handled

Very nice of you, take care

mal

T

#### tat2edangel

WELL, THIS BUDS FOR YOU!

Todays recipe:
HP13 BOWL (this is just the most amazing pineapple/tangerine smell. BRAVO !)

TOPPED WITH A LITTLE SHIVA SKUNK HASH

OH BOY HOLD ON

T

#### tat2edangel

Just so you know qupee you posted a video about ducting in my "new to me tent" post and it is the exact example I have used. So thank you. PEACE