too much negative pressure?

While setting up my grow (which has been a monumental undertaking) I've continuously told myself to do everything absolutely perfect, might not have a large space, but enough I'm sure I could be pulling out some top shelf bud.
but it's getting so expensive, I was wondering, I have an intake and exhaust fan. I've probably said this in the majority of my posts on riu to the point that most of you probably have my setup memorized, vortex 6 inch s line intake, active air 6 inch outtake. The active air fan is too loud, If I'm using that one I don't want to use another fan. Been thinking of just replacing it with another vortex, but today it occurred to me that a lot of you just use an exhaust fan and bring air in through negative pressure. I know my fans and filter are large for my tent, though. whenever I've plugged it in with just an exhaust on the negative pressure is really intense (no pun intended), so I worry that that much negative pressure might cause it to suck in enough to strain the seams on the zippers and possibly cause light leaks. Also, the light is off center so I wonder if I would risk burning the side of the tent, but I kind of doubt that, since it's a 315 watt cmh and this is a grow tent, probably designed with some heat resistance.

u may be over thinking at this point, get ur grow going then be prepared to deal with smell. have a filter or other means to hide smell. get her going , ull find time to research as ur going

the effect your after is actually passive intake, and you aim for a slight negative pressure, as a general rule aim for 4-6 times the opening of your extractor, so say 6inch fan then try to allow at least 4 times that for passive intake, if your tents is still sucking in to much then more is needed, if you have no tent suction then reduce it, to much neg is bad, i get it so you can just see the tent sucking in , fresh air etc is very important to healthy plants so do take the time to get it right, but as tomascat said dont overthink it,

I know it may not factor too much into your design, but I've been building computers since the 90's and intake/exhaust are HUGE factors in case design, and I imagine that the same principles apply to any closed space with in/out airflow. That being said, my feelings on negative pressure are rather... negative. Positive pressure systems have numerous benefits, but the single most important benefit is dust prevention. Negative pressure is an absolute dust magnet. A positive airflow computer case will stay clean FAR longer than a negative pressure case with the same CFM, and the more negative pressure the more dramatic the rate of dust deposit. I would imagine that in a small grow area like yours, under certain conditions this could rather quickly become an issue, as dust can carry mold spores and such. If you are going to use negative pressure I would highly suggest using an intake filter as well, unless dust is a total non-issue for you (if, say, you hepa filter the air previously to it being drawn into the grow area).

You never mentioned the sized of your tent either.

Maybe get a fan speed controller and slow it down if its too much. I only run my 4" at 50% on my 3x3x6' tent.

You would probably be fine using whichever fan only as an active exhaust, and let the air enter the space passively for the intake.

That's what I do with my tent, one fan is removing air, and the bottom vents are just open for air to move in passively...

You never mentioned the size space you're working with. Is it 4x4 tent? If so I think you'll be perfect with the 315w cmh. Is it a sunsystem lec 315?

Give some more details and point me towards your grow thread if you have one, I want to follow as many cmh grows as I can!

I have two 315w CMH in my 4x8 space no problem. In fact I have my fan dialed down to low and it's still too cool in my tent.

In my experience these lights run pretty cool, so if both fans are just about comparable then go with whichever you want and you'll be fine.

Cool, great info, left the laptop closed all day and didn't get a chance to see it all. it's a 3x3x6 tent, I think I'm going to have to be even more intensely patient on this shit, which really sucks, but this tent is just too small for what I'm trying to do I think. Prolly gonna see about getting a gorilla 4x4, then I gotta get soil, and nutes, and some kind of method of supplementing co2 I'm guessing since I'm probably just gonna do passive intake in the house and exhaust outside. Ace yonder made a great point about dust though. And then I have to estimate the electricity cost of the entire grow and pay it up front.
Jesus Christ I won't have plants in dirt until march at this rate

and yeah, it's a sunsystem lec 315. I think I want some sort of small side light as well at some point

borbor said:

Cool, great info, left the laptop closed all day and didn't get a chance to see it all. it's a 3x3x6 tent, I think I'm going to have to be even more intensely patient on this shit, which really sucks, but this tent is just too small for what I'm trying to do I think. Prolly gonna see about getting a gorilla 4x4, then I gotta get soil, and nutes, and some kind of method of supplementing co2 I'm guessing since I'm probably just gonna do passive intake in the house and exhaust outside. Ace yonder made a great point about dust though. And then I have to estimate the electricity cost of the entire grow and pay it up front.
Jesus Christ I won't have plants in dirt until march at this rate

and yeah, it's a sunsystem lec 315. I think I want some sort of small side light as well at some point

Click to expand...

1) What do you already have as far as supplies, lighting, etc?

2) what are you trying to accomplish?


Let's try to simplify some things for you...

Sounds to me like you have a oversized fan for your tent if you are drawing more air than the (hopefully open) passive vents can allow in. Do you still have the intake duct attached and allowing intake without the fan? You really should just get the exhaust on a fan controller and lower it down to the lowest setting that maintains your temps on the hottest day. (the lowest speed that works for your worst case scenario) If you are exchanging the air more than once every 2-3 minutes you are overdoing it. 4x4x6 is 96 CF. If you are running a 400 CFM 6" I can see that imploding, you are trying to exchange the air in the tent every 15 seconds.

a senile fungus said:

1) What do you already have as far as supplies, lighting, etc?

2) what are you trying to accomplish?


Let's try to simplify some things for you...

Click to expand...

Cool man, it will totally hijack my own thread, but I kinda woke up in the morning with a headache and thought it'd be best to take all the grow shit down for some reason, now I'm kicking myself a little because I know there's always a solution

this might be more than you wanna read though, but you just asked the right questions

my supplies so far-
sunsystem lec 315 (~25 lbs)
vortex s-line 347 cfm inline fan
hydrofarm active air 400cfm inline fan (I don't like it because it's noisy, might sell it and buy another vortex)
secret jardin dr90 grow tent 3x3x6 (max weight support 65 lbs)
can 66 carbon filter 412 cfm exhaust@0.1sec contact time (31 lbs+flange)
25' of ducting, as well as another 6' section and 3' section
4 1 gallon smart pots, 4 5 gallon smart pots
hygrometer
bug screens for the ducting
3 neoprene lined silencing duct clamps
rockwool to start the seeds in (my favorite method)
kill a watt meter
a 6" clip on fan for circulation
a shitload of ratchet hangers (8 I think)
and at least six S hooks of every size imaginable

What I wanna do:
I live at home, I'm 24 and pay rent and shit, it was just a lot easier than finding another place after my last lease. I'm told I can grow, but there's a lot of impossibly picky demands my mom is making about it, I'm not gonna argue, she's the homeowner and it's cool she might let me do it in the first place, but,
there can't be any smell, there can't even be a chance of it, and she can't hear it (or not loudly) around the house. the house is 1000 sq feet, one level.
The tent can't be in front of the window, it can't take up a large part of the room, it can't be half-in and half-out of the closet. There can't be a shitload of visible ducting going to and from the tent. It's not too bad if there's ducting running under tables and the bed and shit like that, just don't make the room look like there's a robot octopus (roboctopus) in it.
the window is 5 feet wide but it only opens like 10 inches wide on the side right next to the closet, so I really have a difficult time getting the tent close to fresh air.
as stated in my last post, I gotta overestimate the electricity cost of the grow and pay it up front. paying for the highest tier of power usage, and the summertime rates, so it works out to $0.15/kwh which is nuts, I know.


and what I want is to grow 4 plants, under 18/6 for about 6 weeks, then flower them. I have the seeds already, gonna do 707 truthband by emerald triangle from humboldt seeds, starbud from hortilab seeds, midnight kush from g13 labs, and querkle from tga for my first run.

I would like to add another light during flowering, and if nothing else, then two weeks into 12/12 I'll go get a cheap 125 watt cfl reflector at the grow store

I'd like to be able to pull about four ounces on the first run
I wanna be a perfectionist and do a little bit of overkill in the right places, pulling four ounces on the first run would come in second place to pulling three ounces of some 10/10 grade A.
but after doing a couple plant cycles and getting a little better it'd be cool to be able to pull like 8 ounces per run
I know what everyone's gonna say about guessing yield and all that, I know. I'm just talking about having a setup that a noob could conceivably pull a qp with and a decent amateur might pull a half pound with

the plan was to do some combination fim and lst, but I think I might be going too far into detail at about this point.

after I get one or two grows under my belt, I'll probably find a place to live that'll let me grow (that's the hard part, why I'm staying here for now.) but if not, perhaps the idea of grow tents and fans and reflectors won't be so foreign to mom, so either way that's when I'll start thinking about a second tent for a more perpetual type of setup.

nomofatum said:

Sounds to me like you have a oversized fan for your tent if you are drawing more air than the (hopefully open) passive vents can allow in. Do you still have the intake duct attached and allowing intake without the fan? You really should just get the exhaust on a fan controller and lower it down to the lowest setting that maintains your temps on the hottest day. (the lowest speed that works for your worst case scenario) If you are exchanging the air more than once every 2-3 minutes you are overdoing it. 4x4x6 is 96 CF. If you are running a 400 CFM 6" I can see that imploding, you are trying to exchange the air in the tent every 15 seconds.

Click to expand...

Yeah, at the beginning I may have put my overkill idea in the wrong place and thought it'd be a good idea to exchange air that fast as long as I had an intake fan, but the noise, and all that shit...

Counting your harvest before you ever plant the seeds is a surefire disappointment. Gardening should be a relaxing zen like exercise. You know what's better than the perfect grow that takes years of planning? Growing for years, learning from your experience, and mistakes, and planning better each time. Finished is better than perfect. You sound like you know what you're doing, just do it. Good luck and happy new year!

yeah, I know it's stupid to say "yeah I'm 1 day into veg how many pounds am I gonna yield?" I was just kind of ballparking what I would like to be able to do with my setup. I know there's a lot of unknowns that come into play there, Pretty much not trying to make a setup where it's pretty easy to be able to expect that on the regular.
I mean, a qp would be like 0.35 gpw, not trying to say that's what I'm gonna get, just that if I spend $1500+ on a one-tent setup, I'm gonna be pissed off if I pull 0.25 gpw.

borbor said:

yeah, I know it's stupid to say "yeah I'm 1 day into veg how many pounds am I gonna yield?" I was just kind of ballparking what I would like to be able to do with my setup. I know there's a lot of unknowns that come into play there, Pretty much not trying to make a setup where it's pretty easy to be able to expect that on the regular.
I mean, a qp would be like 0.35 gpw, not trying to say that's what I'm gonna get, just that if I spend $1500+ on a one-tent setup, I'm gonna be pissed off if I pull 0.25 gpw.

Click to expand...

There is absolutely nothing wrong with setting a goal. Just don't depend on your goal or be overly disappointed if you don't make it.

OK a 3x3 space will fit your 4 plants, but, IMO, you will have only a couple weeks to veg them, not 6 weeks. Maybe veg for two weeks then flip, or you could leave the tent on 12/12 and put a seedling in every 2.5 weeks or so... The 12/12 thread has some nice looking plants!

You won't need a 2nd light in there with the sunsystems 315w lec , also don't get that light too close , @GroErr has the same light going. You should look up his thread called GroErr Grows...

Your mom is cool for letting you grow but electricity up front? Wtf? You should just calculate your monthly operating cost and pay that each month when the bill comes in, then split the difference between you and her.

You need to figure out how to either move air in or out that window without it being obvious from outside. In doing so you will probably put in curtains, so theoretically the tent COULD be in front of the window thus eliminating some spider webby ductwork. BONUS: if the tent is in the room and not the closet its easier to access and could be bigger! Like a foot or two or three bigger.

If you're looking to buy gorilla tent I've got a pretty sweet hookup, pm for details. I think he ships free to the attached 48.

Personally I think you could go with a can 33. I have a can50 for my 4x8 tent... My plants aren't super stinky yet but I expect the filter will do fine.

Also, vertical growing is something you may be interested in. I went vertical and I feel it's a good use of light and space.



Ya I know it derails a thread but you may as well consolidate all your convo's and thoughts in one thread rather than have them spread out. I saw you posted another similar thread and that thread has pics that this one doesn't...

put the vortex on exhaust and create a light proof passive intake 3x the area of the exhaust hole size to alleviate the neg. pressure.

I recommend an air intake filter, rewashable

So besides stress on the tent, what are the specific problems caused by excessive negative pressure? If your RH and temps are correct what harm does it cause?

NanoGadget said:

So besides stress on the tent, what are the specific problems caused by excessive negative pressure? If your RH and temps are correct what harm does it cause?

Click to expand...

Unless you get to the point of causing wind burn, none. Shorter filter/fan life is the only one I can come up with.

NanoGadget said:

So besides stress on the tent, what are the specific problems caused by excessive negative pressure? If your RH and temps are correct what harm does it cause?

Click to expand...

Stunting of plants is the big one LOL studies done by NASA for growing plants in space, why my garden is wide open with no intake/exhaust fans or filters. Folks really should try to understand the relationship plants have with natural barometric pressure

RM3 said:

Stunting of plants is the big one LOL studies done by NASA for growing plants in space, why my garden is wide open with no intake/exhaust fans or filters. Folks really should try to understand the relationship plants have with natural barometric pressure

Click to expand...

Good to know! Thanks. I never thought of the possibility that consistently low barometric pressure would impact growth. Glad somebody out there was thinking about this Shit. Lol