FANS-pushing, pulling, and neg pressure theory...

[tjmccoy]

smurfin'herb,

Originally Posted by tjmccoy
IMO

You'll be better off to have 2 fans pulling than 1 pulling 1 pushing imo.

The intake should be at least twice the size of out-take.



Why are you laughing your ass off???? No I'm not kidding it only makes sense to someone that might be half way mechanically inclined.

If you have 2 fans blowing 100 cfm 1 in and 1 out you'll only be moving 100cfm total air out of the room. If you have 2 fans blowing out you're moving 200 cfm total air out of the room which also means than 200 cfm is moving into the room from the intake provided that the intake is large enough to allow easy flow.

Excuse me for trying to help a fellow out... So you just do what you ever kind of non-sense you can come up with this is the last I will post on this subject.

 

[smurfin'herb]

If i only have 2 fans to work with, then i will have an intake and an outtake so that i can filter my air on both ends! If i put the two fans as the outtakes, the intake would have to be passive, preventing the use of a hepa filter! I see what you are saying kind of, but i like to keep my rooms pest/pathogen free.

 

[hoosierdaddy]

When you put two fans together, exhaust to exhaust, you are creating a pressurized vessel in between the two fans. The pressure being dictated by the total power of the two fans. Add a pipe in between the two, and you are simply adding pressurized area. That area will still be pressurized the same as if there were no pipe, or a 50ft pipe.
The only difference there would be with a pipe and no pipe, would be the time factor it takes to pressurize the area between the two fan blades.
As far as how the system would react to this situation, imagine how a vacuum cleaner acts when you block the intake. For a few miliseconds there is an increase in negative pressure in the chamber, until every bit of the air has been exhausted from the canister, and no more air can enter in (because you are blocking it with your hand).
At this point we achieve a vacuum and the motors increase in noise.
Turning two fans against one another creates just the opposite situation as a vacuum cleaner with a blocked hose.

I can see having a pre-filter to keep out insects. The pathogens I question even a need. But all adding a pre filter material over the intake does is restrict the flow of air in, so to achieve the same CFM you had before the filter, you need to increase the power of the exhaust.

Now, if we connect two 250 cfm fans at the exhaust end-to-end, even though common logic would say that you now have 500 CFM, it doesn't really work that way.
Stacking two 250 CFM fans will not increase the CFM rating at all. It will still flow 250 CFM out of the fans, but with an increased pressure rating.

Now if you add two of the 250 CFM fans side-by-side, then you have increased the air flow and you will truly have 500 CFM of air now moving.
BUT...only if you added enough passive intake to account for the additional fan.
See, with one fan (let's assume 6") you need an additional opening the same size as your original opening.
This all goes back to you can only exit the mass of air that can flow into the box.

Folks, I have spent a whole lot of time researching and putting together a thread concerning each and every one of these issues. I highly suggest we take a good read and try to follow along. I did that thread because of all the mis-information that I was seeing being passed along like so many bad farts. And people were just sniffing them up as if they were valid points. People putting out bad info is something that we really shouldn't have to take. It is hard enough for us to enjoy our sport without having idiots trying to confuse us and making things harder (and more expensive) than they have to be.

Now, if we read through my thread on air flow, and we see that there are some mistakes, or bad info....PLEASE, let's get the information corrected. It just isn't fair to any of us to rely on bad tutorials, or even a bad post by someone who is either simply wrong, but with good intentions...or wrong because they are blowhard assholes that simply want to state things are only they way they see them...because they have a hard time seeing past their own ignorance, mistakes, and shortcomings.

 

[Bozo]

lotta chest pounding on a math problem eh? This is a simple math equation and people are acting like this is a new subject . 600 cfm out 400 cfm in =neg pressure is that really that hard to grasp?
And smurfin if you had been doing this for any time at all youd know there are too many variables and each room is different .I can tell you dove head first into the deep end by your recent posts ,GL with yer grow .I'm sure by this time next month you will be giving people advice like you know what you are talking about ,and past the making a post for every question you ever had about growwing

 

[Albert Hofmann]

This thread is funny...I wonder if anyone has taken into account the load factors on the fans...There is a lot of info being spread around, most of which is incorrect and pointless to the construction of a growroom. This thread is why I stopped helping people on this site...Too much arguing with people that read something and think they are experts. I will go back to my perfectly functioning rooms and leave you all to this debate...I don't like typing anyway.


albert

 

[mexilandrace]

If i only have 2 fans to work with, then i will have an intake and an outtake so that i can filter my air on both ends! If i put the two fans as the outtakes, the intake would have to be passive, preventing the use of a hepa filter! I see what you are saying kind of, but i like to keep my rooms pest/pathogen free.

pests is one thing, but pathogen free?

I think it is a huge mistake to try to give plants a sterile environment.

Now, I am a closet breeder, so maybe that is where I am coming from, but I want to know which plants are weak and which are strong. I want the plants that can't hold up to normal stresses to show it so I can pull them out of the gene pool.

If you breed a plant in a "perfect" environment then those seeds aren't going to behave predictably in the less than "perfect" environment 9 out of 10 people will provide.

It's like how people are obsessed with germs and scrub everything down with anti-bacterial this and that, and then the first time they are around someone with a cold they catch it.

 

[smurfin'herb]

lotta chest pounding on a math problem eh? This is a simple math equation and people are acting like this is a new subject . 600 cfm out 400 cfm in =neg pressure is that really that hard to grasp?
And smurfin if you had been doing this for any time at all youd know there are too many variables and each room is different .I can tell you dove head first into the deep end by your recent posts ,GL with yer grow .I'm sure by this time next month you will be giving people advice like you know what you are talking about ,and past the making a post for every question you ever had about growwing

I do have lots exp. with smaller grows. And now ( thanks to all the questions ive asked) i have pretty good exp with larger ones. If you would like to play who's got the better grow room, then be my guest. I would love to compare pics, seeing that the only one i found that you have is a blow up bozo doll. But save it for another time and not here. Im not a know it all (as u can tell by the amt. questions i have) but my pics will speak for themselves (once i get the new ones up, i been having trouble uploading).

 

[Bozo]

These are all posted by Alberthoffman, followed by my responses (in parentheses)....

"If you have an intake and exhaust fan that are the same size, you will not achieve good negative pressure."


("GOOD" neg pressure? you only need a lil bit of neg pressure, so if its already good (meaning you are achieving neg. pressure) then making it better/stronger, really doesnt matter if you think about why you are trying for neg. pressure in the first place, and what you are already achieveing with "GOOD" neg pressure. Its not like its gonna make it even more smell proof or anything by increasing the neg. pressure.)

"Neg pressure is when your exhaust fan draws so much air out of a room that it pulls air in through every crack in the room...unless you plan on putting mini aircleaners at every crack in the floor, walls and trim your "theory" will most definately NOT pan out."

(If im so worried about spores and pathogens and bugs as to use a hepa filter, do u really think i have little cracks and light leaks ect... in my room? nope sorry.)

"If you must have an intake fan (complete waste of money) then size it 2 - 4 times smaller than the exhaust."

(You sir, are an idiot!
Cmon man, you know better than that. (I hope....))

"You are worrying about the wrong things (pathogens, spores, etc), once you have done this for awhile you will understand."

(I HAVE done this for a while, and i have learned to have a hepa filter on my intake, especially when pulling air form outside. You must like to deal with molds,mildews, and pests for some reason?)

I dont know what to say bro, please research and then think before you post.

wow i recon you told me, all these years and I have kept my grows to myself and I should post pics to prove something to you? If it helps you feel better(and I know it will) you win post up all the pics you want hell open up yer front door and show the whole world for all I care
And it is a simple math problem you just feel the need to talk about it
I dont have problems with dumb questions just cocky assholes who ask questions then school people on the subject in same thread .Kinda ironic no?


I didnt come here to troll you but the above post kinda chapped my ass I wont bother to respond to anymore of your questions now that I see how you react, name calling and such .I didnt see anyone flaming you about a 1+1=2 and thats exactly what it is you just wanted to argue about something .The sky is blue........

 

[smurfin'herb]

Well maybe someone in my same situation, will come across this in the near future, and it will help them, and save them the trouble form looking like the idiot who asked the question. Have hell open "MY" front door? First off I do not grow at the place i live, i have learned not to eat where u shit. So there will be no unleashing of hell at my door! Also, im medical bro. Feds can come in on me ya, but they dont want to waste time with medium sized ops (20kw and under) like mine. Especially when it is legal under state law. So dude, go back to your lil closet grow with your cfl's, and quit flaming this up!

 

[overgrowseeds]

MORE AIR MEANS MORE CO2 MEANS MORE BUDS

 

[smurfin'herb]

Also, i wanted to see what people think bout this..... How are you supposed to maintain neg pressure if your intake/outtake fans arent running 24/7? The general reccomendation is like " on for 5min every 15-30 min" ive read. So when the fans are off, air can be pushed out by oscillating fans in the room and will will create positive pressure. This can also be a prob when you have heat/cool vents (that aret covered) in the room. Anyone else notice this...?

 

[hoosierdaddy]

Fans within the room will not create pressure, only turbulence within the room. Albeit if the fan is close to a intake/exhaust, the pressure is slightly out of balance, but normally this would be very negligible.

What you want to do is replenish the air supply, or turn over the air, so many times in an hour. If there is no AC to cool the room, then we may need to replenish the air more often than what it would take for proper air turn over.
This provides the atmosphere that the plants need.

The only time the air flow should be off, is if you are providing additional Co2.
Now, you can save lots of energy if you thermostat control the fans, and only have them come on when the temps reach a certain level. But this needs to be in conjunction with an AC unit that keeps the temps down.

High-tech set ups can monitor all of these conditions and turn on and off what is needed, when it's needed.

 

[smurfin'herb]

Ok heres example... I have a room with two doorways with the doors removed and replaced with poly doors with zippers. When lights are off, the osc. fans are on, and create pos. pressure. I can tell this, because the poly doors are pushing out. So the pressure on the door is actually turbulence? and your saying no air is escaping when this happens?

 

[hoosierdaddy]

When the flow of air is directed at the poly, you will see what you described. When the fan oscillates to another direction, the poly will no longer flutter from air flow, as the turbulence are directed at another direction.
If the poly does continue to flutter, it is because the turbulence is bouncing off of other surfaces and blowing at the poly, but with less force than before.

And yes, you can in essence create pressure differentials with just a fan in the box, depending on how you have it situated. Like I mentioned before.
But try to picture the same fan smack in the middle between the two openings, and facing the wall at 90deg from the openings. Considering there is nothing else in the perfectly configured box, both openings poly should flutter with equal force...from the turbulence of the fan bouncing off the walls and back to the poly.

Always remember...if any air escapes your room, the exact same amount of air had to first get into the room.

 

[smurfin'herb]

Ok heres example... I have a room with two doorways with the doors removed and replaced with poly doors with zippers. When lights are off, the osc. fans are on, and create pos. pressure. I can tell this, because the poly doors are pushing out. So the pressure on the door is actually turbulence? and your saying no air is escaping when this happens?

 

[hoosierdaddy]

Ok...
Do you have air that can enter the room with ducting, etc?
If so, then when you have your air system off, and nothing but the fan inside running, then it is quite possible that the house air system is pulling from your intake. Or from your exhaust for that matter. Any duct to atmosphere will allow this.

If both polys are bulged out, and air escaping, I can guarantee you that there is air entering to the room from somewhere else.

 

[smurfin'herb]

I have spring loaded dampers on the intake and outtake. so when they shut off, no air goes in or out. The house heat/cool vents are closed in that room. Theres no air escaping because i have the room sealed air tight! But there is pressure being put on the doors (via the fans) when the lights are off. If the room wasnt sealed good, i almost know for sure air would be pushed and pulled through the cracks somehow..

Sorry bout the double post, dont know what happened...

 

[hoosierdaddy]

Then for sure, the only pressure being created is on both sides of the fan. The pressure inside your room will be no different with the fan or without. You are only seeing the turbulence blow around what is already in there.
I really fail to see your point of the whole thing though....??

If you take the time to read through all of my Understanding Air Flow thread, it may bring to light the physics that I am trying to describe. There is no gray area with any of this, it is all text book physics and HVAC.

 

[swampdank]

passive intake will work in larger grows.

passive intake will work in larger grows.

just not too big. I use two 1k lamps and run a 8 inch blower to "suck" the hot, stale air out of the room(and off of the lamps).

Temps outside the room are key. My grows have always been below grade so the temps are pretty low to begin with, even in july.

I woould suggest trying it both ways. You might be able to get away with passive intake. And save a little dough.