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

[hoosierdaddy]

Albert, I have constructed a thread concerning these issues. I invite you to read through it and show me where I am screwed up on this.
http://www.icmag.com/ic/showthread.php?t=100823
I will be the first to admit that I was wrong, and I will go out of my way to make sure that we collaborate and provide the PROPER information to our fellow growers.
Let's not qualify our assertions with our personal pedigree's, but rather let the real scientists do the vouching for us. Let's cite our sources so we aren't steering folks in the wrong direction, and not simply spouting off half baked opinion, basted in ego.
Let's depend on the laws of physics, instead of the assumptions of theories, be they yours or mine...

 

[mexilandrace]

lol at how complicated people make things sometimes, you can tell a lot about a person by how much they think or over think things.

I over think just about everything too.

all that matters in all this is to have more air being pulled out of the room than you have being pushed in, put the filter on the "outgoing" air and you are fine.

There is no reason to pre-filter the air going into the room. Maybe if you were trying to create a sterile environment... but why on earth would anyone growing a plant that was born in dirt would try to do that is beyond me.

 

[hoosierdaddy]

No lint in the macros of course. (s)

 

[mexilandrace]

ya I suck at macros anyways so I guess I am just bitter about it...

 

[Whatever]

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

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

I'd generally agree with that first statement but there are a few factors involved that will have an influence on how matched fans will function as intake/exhaust, mostly minor though, such as the use of a carbon filter on the exhaust side and the length of duct runs on the intake and exhaust side and how the run is actually configured like sharp bends like elbows on either end. Another thing would be something like are you pulling air from within a closed house and dumping directly outdoors or vice versa and there is the issue of how 'leaky' your room is but sealing a space properly is soooo easy.

The easiest way with matched fans to create negative pressure is use a speed controller on the intake side and dial down slowly till achieved and you really only need a touch of negative pressure and an excess just means your exhaust fan is being put under greater load than necessary.

As for the second statement...while not an HVAC person I have designed and built quite a few rooms, all open systems, and found intake fans necessary for the most part (but right now just running a 400 watt in a closet with a passive intake and 4" Elicent). This had to do with where the grow room was in the structure, the amount of watts being run and also to help with seasonal adjustments in an enviro with temps of 90+ in the summer and below freezing in the winter. The idea of using an intake fan 2-4 times smaller than the exhaust is bad advice and a total mismatch IMO unless you also used a passive intake and used the much smaller sized intake fan to flow more air to deal with excess heat issues. So a small passive intake allowing constant flow, you have a strong neg pressure in the room and when the thermostat kicks in the fan blows air in to relieve the heat issue. This is just one possible scenario to help with season hot/cold issues. One time I had warm house air as the main intake source and had a cold air intake fan on a thermostat as part of the system and in another had an 8" Elicent intake and 10" Elicent exhaust on a scrubber, so only a 59 cfm difference, in a 1080 sq ft room and still strong neg pressure.

I learned to let the space dictate how best to configure the system and also someones budget. My rooms never had more than a 2f variance lights on with constant air flow winter or summer and used the same equipment year round.

 

[Albert Hofmann]

Albert, I have constructed a thread concerning these issues. I invite you to read through it and show me where I am screwed up on this.
http://www.icmag.com/ic/showthread.php?t=100823
I will be the first to admit that I was wrong, and I will go out of my way to make sure that we collaborate and provide the PROPER information to our fellow growers.
Let's not qualify our assertions with our personal pedigree's, but rather let the real scientists do the vouching for us. Let's cite our sources so we aren't steering folks in the wrong direction, and not simply spouting off half baked opinion, basted in ego.
Let's depend on the laws of physics, instead of the assumptions of theories, be they yours or mine...

JUst from a quick glance at your link I noticed a problem. The link is very informative as to how a vacuum cleaner works and is useful in comparing a completely sealed room to a vacuum. But, again it doesn't take into account that we are talking about an unsealed room that has 100's of points where air leaks in (intake holes). As most growrooms are in makeshift areas and airleaks are abundant, you actually want the intake that you cut in your room to be considerably smaller than your exhaust. My room runs a 12" vortex exhaust with 1 8" passive intake hole and then the cracks.


albert

 

[Whatever]

My room runs a 12" vortex exhaust with 1 8" passive intake hole and then the cracks.

LOL...see that's how different peeps are. For me I seal every room like I was gonna run CO2 so I only have air going in and out of the room where I know it can only move through...the fans. Also just in case CO2 will be run cause have come close a few times and think my next room will be sealed.

In fact most of the time the ventilation gets cut off lights off for better humidity control mainly and the way I've configured things no odor leaks.

Just the way I do things.

 

[hoosierdaddy]

This may be hard to grasp, but it doesn't matter how many cracks and openings there are in the room, as long as there is at least enough intake hole, crack, etc...to provide enough air volume to satisfy the CFM that you are trying to obtain.

Too little opening, and you have a system that is not performing efficiently. (many have a very inefficient system, but it's what they have learned to live with)

Too much opening? There is no such thing...including the cracks.
And back to what I said before, as long as you do not push in a greater volume of air than the exhaust fan and set-up can handle, then negative pressure WILL exist in the box or room.

A perfect example of upsetting the balance and pushing in more air, would be the situation that whatever described of placing a scrubber at the end of the run. That would make the exhaust see a restriction that the intake fan would not see, until after the fact of the exhaust anyway. That situation would indeed create a positive pressure in the box, and the extra air that can't get out the exhaust fast enough, will seek the cracks and crevices for an escape.
To the point of the OP, it is not a very good idea to have a fan on the intake, that way you will ALWAYS avoid positive pressure in the box.

 

[Whatever]

I don't know what's hard to grasp cause that's just simple air flow dynamics.

as long as there is at least enough intake hole, crack, etc...to provide enough air volume to satisfy the CFM that you are trying to obtain

Or as in Albert Hoffmans case a big honkin exhaust fan strong enough to pull more air (higher velocity) through any available openings. I would not do it the way he did but does not make me right and him wrong...and also don't know all the factors he's dealing with. I had issues with trying to run a passive intake and 1 12" exhaust with 4800 watts in an 8 x 18 x 6' room. I was limited by a 12" exhaust hole and only way I overcame it was with active ventilation...and a few other tricks to allow for seasonal adjustments.

To the point of the OP, it is not a very good idea to have a fan on the intake, that way you will ALWAYS avoid positive pressure in the box.

Going back to the original question I don't believe that putting matched intake/exhaust fans in a box will create negative pressure. It doesn't frikkin matter if you have an intake fan or not regarding negative/positive pressure just that the system be dialed in properly.

 

[Albert Hofmann]

This may be hard to grasp, but it doesn't matter how many cracks and openings there are in the room, as long as there is at least enough intake hole, crack, etc...to provide enough air volume to satisfy the CFM that you are trying to obtain.

Too little opening, and you have a system that is not performing efficiently. (many have a very inefficient system, but it's what they have learned to live with)

Too much opening? There is no such thing...including the cracks.
And back to what I said before, as long as you do not push in a greater volume of air than the exhaust fan and set-up can handle, then negative pressure WILL exist in the box or room.

A perfect example of upsetting the balance and pushing in more air, would be the situation that whatever described of placing a scrubber at the end of the run. That would make the exhaust see a restriction that the intake fan would not see, until after the fact of the exhaust anyway. That situation would indeed create a positive pressure in the box, and the extra air that can't get out the exhaust fast enough, will seek the cracks and crevices for an escape.
To the point of the OP, it is not a very good idea to have a fan on the intake, that way you will ALWAYS avoid positive pressure in the box.

This back and forth with you is kinda weird...by the simple fact that you go between proving my point and then disproving your own point tells me that you are not fully understanding the subject. Reread your posts and you will see what I was saying.
I continue to stand by my original post...


albert

 

[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.

The only time I would use a fan to push through the intake is if I cant get the intake to a large enough hole and create to much strain on the out take fan.

Dont forget about the importance of sucking out high in the room as possible I usually build a simple box around the out-take so it skims from the highest point possible this really does make a huge difference.

A separate closed system running Cool tubes are also effective but I dont use them as I dont run co2 and I dont like the extra glass filtering out uv.

 

[hoosierdaddy]

Look, Albert. I know this subject as well as any that we are concerned with.
Until you can show something more, you are simply wrong.
Actually, I don't care what you show, you are giving out bad info.

Now, if there is something that I specifically said or did to contradict myself, please point it out...don't be a squid and tell me to read back and I will magically catch what you are referring to. See...you need to stop pointing fingers until you actually have a clue, years of hanging duct or not. Try designing some duct then get back to me.

 

[hoosierdaddy]

Going back to the original question I don't believe that putting matched intake/exhaust fans in a box will create negative pressure. It doesn't frikkin matter if you have an intake fan or not regarding negative/positive pressure just that the system be dialed in properly.

The same forces that cause a fan to work best at the exit are at work when you have two identical fans, one at entrance and one at exit. There are very slight resistances seen by the air going through the entrance fan, than seen by the exit fan. And it is these slight variances that allow the exit fan to perform slightly better than the intake fan. This causes a negative pressure.

You could hang the pissiest ass fan on the planet at the exhaust, and speed control it down to about 1 RPM, and it will create negative pressure in the box. PROVIDING it has some air to work with.

You can only move a volume of air out of a box that is equal to the volume of air entering the box. You can never get more out, and when you get less, even the smallest amount less, there is negative pressure in the box.

 

[Whatever]

I continue to stand by my original post...

OK...

If you must have an intake fan (complete waste of money) then size it 2 - 4 times smaller than the exhaust. You are worrying about the wrong things (pathogens, spores, etc), once you have done this for awhile you will understand.

Sizing an intake fan 2-4 times smaller than the exhaust fan when going with active ventilation is some of the worst advice I've seen on the forums in quite awhile and hope people don't pay too much attention to it. In most cases people grow in rooms or areas that do not leak like an outhouse for starters. To say that intake fans are a waste of money is a joke. Every situation demands a different solution...there are no absolutes.

As for the pathogen/spore thing I agree but have personally seen numerous instances where pests, mostly thrips but yes also mites, from the best I could figure were introduced through unfiltered air. Maybe you live in an urban jungle but my friends and I live in the forest and people would be AMAZED at the amount and variety of bugs caught in filters this way. This POSSIBLE pest issue is easy to control by filtering using socks made from material identical to what's used on bubble bags but down under 70 microns.

Honestly I could care less about microscopic stuff like PM cause that's mainly environmental control stuff and genetics play a role also.

 

[Whatever]

You could hang the pissiest ass fan on the planet at the exhaust, and speed control it down to about 1 RPM, and it will create negative pressure in the box. PROVIDING it has some air to work with.

You can only move a volume of air out of a box that is equal to the volume of air entering the box. You can never get more out, and when you get less, even the smallest amount less, there is negative pressure in the box.

Basically no shit Sherlock.

As for the fan stuff answer me this...if you take an Elicent 6" 309 cfm fan and run open, not attaching any ducting like right out of the box, in a perfectly sealed 4 x 4 x 8' room it is exhausting 309 cfm...right? So how many cfm are entering the fan on the intake...309 cfm right? Is there any negative or positive pressure in this situation and if so where is it? There's a force pushing out the exhaust but there's an equal force balancing this on the intake side.

Take 2 of these fans, and for arguments sake say they were 'balanced and blueprinted' so their flow was absolutely identical, and hook them together exhaust to intake and seal the connecting point between the fans with something like aluminum tape...will this setup still move 309 cfm? Is there any negative or positive pressure in this situation?

What about if we added a 6" long piece of solid, smooth bore tube connecting the fans? What happens here?

What about if we used a 1 x 1 x 1' box as the intermediary instead of the tube? One thing I can see is turbulence might start to play a role but just guessing here.

 

[MostHigh]

The debate in this thread needs to stop confusing negative pressure with mass air exchange.

 

[smurfin'herb]

Originally Posted by Albert Hofmann
If you have an intake and exhaust fan that are the same size, you will not achieve good negative pressure.

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

Whatsoevers reply to that post...,

"I'd generally agree with that first statement but there are a few factors involved that will have an influence on how matched fans will function as intake/exhaust, mostly minor though, such as the use of a carbon filter on the exhaust side and the length of duct runs on the intake and exhaust side and how the run is actually configured like sharp bends like elbows on either end. Another thing would be something like are you pulling air from within a closed house and dumping directly outdoors or vice versa and there is the issue of how 'leaky' your room is but sealing a space properly is soooo easy"

my reply,

I agree also, with some of these statements such as carbon filters and ducting runs.
I know from exp, that if you have and intake and outtake the same size, you can just put a reducer, or a 90 degree bend in ducting on the intake, if you feel you are not getting as much neg. pressure as you would like. This will make the neg, pressure more noticeable. But why would you want a lot of neg. pressure anyways when using an intake fan, your probly just putting more strain on it by doing this>

 

[smurfin'herb]

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.

 

[smurfin'herb]

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.

LMAO ur kidding right?

 

[smurfin'herb]

Posted by Whatsoever...
"Take 2 of these fans, and for arguments sake say they were 'balanced and blueprinted' so their flow was absolutely identical, and hook them together exhaust to intake and seal the connecting point between the fans with something like aluminum tape...will this setup still move 309 cfm? Is there any negative or positive pressure in this situation?"

I have idea as to how to answer this, but i think hoosierdaddy has a better one, so i will wait for him to chime in.