Grow Room Ventilation 101-Calculating Fan Requirements for Incoming Air.
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- čas přidán 25. 08. 2024
- We show you how to purify your growroom's incoming air using Ideal Air duct collars and couplers, Phresh intake filters and Hurricane inline extraction fans. The main point we're trying to get across here is the importance of sealing gaps and drawing only filtered air into your indoor garden to protect it from potential bug invasions!
Hey Rhizo Tonic. Thanks for your comment and sorry for any confusion! This video covers just the incoming air for my garden. Please take a look at my subsequent videos for further details on the set-up including the exhaust and ducting. Negative pressure in my garden for sure-I have a 10" Hyper Fan and Phresh carbon filter taking care of the extraction-and an 8" Hyper Fan and intake filter for my inflow. I hope this clears things up. Maybe I need to make a composite video showing the whole set-up? In any case, please consider subbing the channel so you receive updates on my progress! Peace out. E.
That tape is hell foil tape is crazy to remove
I used a plant pot for mine and sealed it with bathroom sealer.
There should be thermal insulation on all non-glass surfaces to ensure heat from the light is exhausted rather than just radiated inside the tent/room.
And don't forget to account for any bends from elbows or turns in the ducting. It looks like you have a few. Those usually drop the cfm flow anywhere from 1-4% depending on many factors, such as how smooth the elbow is and its sweep angle. For anyone looking to get even more serious, or needing info on much bigger systems, I suggest you look into buying the ANSI/ACCA manual D.
Thanks amigo.
Just an observation but the duct couplers are designed to flow like a rain gutter. The input side should be on the inside diameter. Otherwise dust and air will escape between the two sections of the coupling.
1:55 -- Man, dremel makes easier and more accurate hole cuts than a jigsaw. The $10 circle cutter attachment is so worth it. Smoothest, roundest cuts outside of a hole saw bits.
this was very helpful,thank you for the video.
subscribed
0:40 -- It would be smarter to have both, as a hybrid set up with manual dampers (motorized or not) to control which side the air comes through. In the winter/cold season when the outside air (I am giving benefit of doubts by surmising you are smart enough to run flower at night and not during the day when it is potentially 100+ outside) is much lower than the inside/conditioned air, you can save money by turning the grow room AC off. Some places, the dead of winter air can be TOO cold, so having iris dampers on there too would allow fine tune throttling, allowing you to mix the intake with some outside cold air and some inside conditioned air. And of course when the temps/weather call for one or the other damper/intake side to be open or closed, you have the open to do that with either side.
Cool Vid!
great vid man
+Earth.Hawk Thanks EarthHawk!
You do nice, clean work though, and I like a lot of what you did here. Just a few more tips.
- Use the insulated (no lower than r6, and stick to r8 if you can get it) ducting that you can get at the hardware stores. You should especially be using this for exhaust lines because any duct line carrying hot air that is not insulated is just going to be dumping radiant heat right back into the environment you are trying to remove it from.
- I think you could stand to use more light. Throw out those 1000s, because they suck. You can't get close enough to the canopy with them, they get way too hot, and their hot spots are way too intense and damaging. Unless of course you have a massive warehouse and/or are using the double ended bulbs. I think you would have been better served by two dual lamp raptors, or four radiant 8s stocked with red and blue 600 horilux bulbs.
dude you could make a pro look like a noob...this is almost art. duuuuuuuuuude you're awesome
Matt Holy Thanks so much Matt! I'm really glad that you are finding the videos useful. First and foremost I do my best to look out for beginners, as we were all there once. I live and breathe this stuff and always do my best to answer viewers' questions, particularly if they come from a subscriber-so, on that note, thanks again. ;-)
Matt Holy I agree Matt In a way it is Art.
My grow room is being exuasted just under once per minute. I have 2 intake vents with filters on the opposite side of the room. My question is do I need to attach a fan to push air into my grow room or is the negative air pressure from my grow room enough to just pull the air in? Thanks
Aaron .
Could you use an air tight duct collar in plastic?
How do you cool your garden ? Does the ac unit from your house do the job or do you have a second ac for your garden?
Why is room set up with no exhaust n all positive air pressure I'm so confused
What is that white connector you use for securing the ducting to the hurricane fan?
Hi
I accidentally(stupidly) put my hand in the Aluminium outlet of my fan(Prima Klima 2 speed 125) to insert an ONA BLOCK inside but it blocked the airflow for a small time 2-3 seconds max and it made the fan sound louder for that period of time . Now I think that the fan has got Louder while operating . is this the result of a quick outlet blockage or that blockage won't affect the fan function at all ?I would like to know if its only in my head or the fan can get damaged/louder if the outlet is blocked for couple seconds ?
Thank You for considering my question
Is it bad to go with too large of a fan? For example I want to purchase a 4 x 8 x 7 grow tent and an ac infinity cloudline T8. But the fans 740 CFM while the T6 I fear is too underpowered at 351 CFM, what do you think? The fan has adjustable speeds so idk if that helps.
Wish you were still making content.
Taylor Working on it :)
Sooooo I'm in the process of converting from a small grow facility ( two rooms) to a larger grow facility two buildings 2400 sq ft + 8000 sq ft. It is intimidating to bootstrap this with a life worth of savings and design a commercial operation. Selfishly I hope you might do a video around design/scaling...
should the size of the fan blowing air in be bigger than the one pulling it out or vice versa ? I'm growing in a 4' long×2' widw×5' tall.
How cold were you able to get the room with ,the two 1000 and the fans going full blast?
Jeez. It’s going to cost me like 400 dollars (including seeds) to get a grow started. That is nuts.
Times like these, I wish I lived in the country
Once you buy the setup, it's really cheap to maintain. Also, $400 for a setup that will allow you to grow over a pound of weed per cycle will pay for itself several times over after the first grow. Also, when you live out in the country you'll constantly have other people scouting out and stealing any crops they find. You won't harvest unless you plant them in the middle of a cornfield (which makes for shit weed), or deep in a forest.
you didnt think very far there. Once you buy you dont have to buy again and maintaining it is even less. most your cost will be electric, seeds, soil, and nutrients depending how you do that. i spend 3k and got way more back so it ends up funding itself. So either you are not really interested in growing or your are lazy
What about the outtake, especially if you’re using a carbon filter 🤨 how often should you have that on ?
For a space that is W=2ft 6in. , L=4ft 6 in. H=5ft 10in. and only one 2ft x 2ft window with a 1ft x 2ft space to use when window is open for a intake/extraction point. What would be the best set-up for intake and extract in a scenario like this. I'll be using one 400w hps for both veg and flower until I can get a 400w Halide blue spectrum that I will use for veg. What would be a good fan for this setup with the calculation being about 65 cf exactly? I found a 4" inline fan that says it's 65 cfm would this be sufficient enough? also should I only use one fan for the extract and just let the intake air get pulled in from the extractor fan pushing out the extraction hole?
It's a small set up but I'm making use of this space for data reasons so that I can apply the gathered data to large scale projects. And with that being I have to make sure the air circulation is optimal for this size of a room so I can control the humidity and temp to get accurate data.
Hi Kevin. Thanks for your question. It's very interesting and there's lots to consider. First, some numbers. If you're planning to use a carbon filter then you should add 20 to 25% to your MINIMUM CFMs bringing us up to around 85. I will assume that you are running open (non-AC) hoods and you are growing light-loving, flowering plants through to maturity and harvest in the same small area.
Now, given that it is so compact and you will have mature plants in there (which probably prefer a lower RH of 40 - 45% while transpiring heavily!) I would personally spec your fan a little larger just to give yourself some wiggle room. A 6" Hyper Fan moves 315 CFM and you might think that's overkill - but it's not, especially as you can dial it down to 35% sped with the included speed controller and remember you have to reduce that number by at least 20 - 25 % due to pressure drop (filter, ducting, etc). The hyper fan uses even fewer watts when dialed down.
I don't know where you live, but if it gets hot and / or humid you'll be glad you have some "throttle" power left with a 6". A passive intake will be fine - make the hole at least 4" in diameter and cover with a bug mesh.
Good luck and keep in touch. Everest.
+Everest Fernandez (Just4Growers) Ok thanks! and I'm In the United States over in Oregon. getting ready for winter over here! lol. anyways with a 2 x 2 window and 1 x 2 opening in the window to use do you think it would matter if the extraction and the passive intake were to share that space being that close together if only pushing for that small of a space?
+Kevin Farmer Your intake and extraction points should ideally be at diametrically opposite points in your grow room. Extraction at the top, intake at the bottom, on opposing walls-that way your plants stand the most chance of benefiting from the fresh air rather than creating a sort of 'slip stream' that bypasses your plants in the case of intake and extraction points being located too close to one another.
No need for 2 extraction fans. You can suck the air through the reflectors on it's way out. I know it will work well because I have done it before.
Hello there amigo. It's typically recommended to BLOW air through air-cooled reflectors in order to create a positive pressure inside of the attendant ventilating ducting system. This means that any small leaks in your air-cooled light ventilation system will need up blowing air out back into your room, whereas if you suck, you risk sucking unfiltered air into your ventilation system which could disperse terpenes into areas where they are not expected or welcome.
When are you coming back we miss you💚👊
I released a video just this week.
How did you arrive at the 1 air change/minute rate?
Hi John, thanks for your question-it's a really good one. Firstly, to answer you directly: twenty years ago, if you were to attend an "indoor gardening" trade show (as they were marketed back then) you would hear fan manufacturers and professional growers recommend one air exchange every two to three minutes. Then, as lighting became more powerful, it gradually came down to one exchange per minute-and, due to the simplicity, this recommendation kind of stuck. Of course, it was born out in the real world too. Now, THIS VIDEO is eight years old and, during that time, we've seen LEDs take the market by storm. Now the air exchange recommendations are even higher. TWO exchanges per minute and more! Of course, these are massive generalisations that don't take into account the ambient environment / climate. But it's ALWAYS a good idea to over-spec your fan (if you're trying to decide between a 6" and an 8" EC fan for example, always go for the 8" - not because I'm taking a commission on all brands' fan sales - but because you can run it slowly, without additional whirring noise, and you have the extra bandwidth for moving a lot of air if you need to, on hot days / nights, etc, and it will always run more quietly than a smaller fan moving the same amount of air through narrower ducting. Seeing how long this reply is already getting, I think I need to completely revise and update my grow room ventilation series for LEDs - as they have completely changed the goal posts. Before, the main bogeyman was HEAT and that was the main reason we vented-now, it's more about humidity and CO2 replenishment - as LED rooms like to be run a little warmer to make up for the lack of IR in the LED grow light spectrum. So much to discuss! You can always reach out to me personally at just4growers@gmail.com or we can continue the discussion right here if you prefer. Interested to hear your thoughts on this.
Which one ventilation take a air in and exit.
You should make a foliar and how it helps video if that makes sence
Mike P Absolutely! It’s been on my list for as long as I’ve been making videos! I’ll get it done this year :)
Not sure if you still check comments but I posted this way of doing things on forums and the people blasted me saying “how are you going to control RH? How are you going to control temperature? How are you going to control CO2?” No exhaust. No clean air. Period.
U rock dude
andrea a Thanks for the smile Andrea-have a great weekend. :)
hey Everest. do you recommend a smaller intake than exhaust size and what ratio should i have them running comparatively? thanks
+Ruben Chavez yes, input should shift around 70% of your output / extraction
Damn! those things look like an F15 turbins
Jammin Dave Lol! It's a cheeky set-up, for sure Dave, and I hope that I'm fully future-proofed for planned expansion and / or hot summer temperatures outdoors! Thanks for sharing my videos and I hope you subscribe to the channel for regular updates on my F15s :)
ja ja ja very funny. Good luck with your growing plans...later!
Can I use a bigger intake or ventilation fan than my grow tent needs? I'm gana start my first grow tent
yes with a fan speed controller, bigger fans are quieter too on lower speeds.
Will you grow tomatoes?
Still searching for the right variety to grow indoors
80% or more of the final product is down to your rooms environment air movement and humidity and temperatures and so on some people just don’t realise they all think it’s down to a massive lights and pumping them full off plant food and That just is not the case at all
Hi Everest, at last an expert in this stuff on CZcams. :)---
My specs are --- 2' x 4' x 5.8' Gorilla shorty grow tent - Advanced LED Diamond Series DS XML 350 light.---I was considering getting the Gale Force 6" Inline Fan, 412 CFM - with using a variac transformer I already have, as a speed controller.......and using an iPower GLFILT6M Air Carbon Filter (6 inch x 16 size) to filter and exchange the air.---The tent has a built-in micro-mesh filter on the bottom middle back of the tent, for air intake.Will this Gale Force 6" fan be over-kill, or will it give me 'wiggle-room' for hot humid days?Will the variac transformer control the Gail Force fan speed?---Do the carbon filters work better with the air flowing slowly thru them, not putting carbon dust into the air?Or does that not happen if the fan is full-blast?---Also, if the built-in micro-mesh filter is on the back bottom middle area, where should the filter be hung, and which tent duct hole should the air be sent out thru? (Upper left, right, or back-left side of the tent)
+John Smith Hi John, thanks for your question. So your tent is just under 48 cubic feet. Your fan is rated at 412 CFMs. Attach a carbon filter and that will reduce output by 20%, so now we're at 330 CFMs, but ducting arrangements could bring this down further to around 275 CFMs. We're still way over your required minimum of 48 cubic feet per minute but if you are living in an area of high humidity then it might be a good idea to have the extra power-especially if you already have a fan speed speed controller. I'm not familiar with the Gail Force brand and its compatibility so I'd have to get back to you on that one. The slower air moves across the carbon bed inside a carbon filter, the more contact time the air has with the activated carbon, and the better it will work. Your chosen carbon filter (iPower GLFILT6M Air Carbon Filter (6 inch x 16 size)) is rated to filter air moving at 400 CFMs or less so you should be okay. The filter should be positioned as high up in the tent as possible, ideally vented out through the roof port. If you have mesh inputs at either side of the tent then the middle is the optimum position. You could use some clip-on wall oscillating fans to help circulate the air but be sure to choose a premium brand that's rated for constant use as the cheap ones tend to burn out and can present a fire risk. I hope I answered all or most of your questions-let me know if you have any more. Peace out, Everest.
+Everest Fernandez (Just4Growers) yes, you've answered, with great info too. --
The slower air speed thru the carbon info, and its placement, is very helpful :) -- will the slower air speed thru the carbon extend the carbons life, or use it up faster?
...! ... :-o ... :) ...
I'm glad I'll have so much 'wiggle-room' with the CFMs because I recently found out the built-in micro-mesh filter on the tent will leak light, so I'll need to make an upside-down 'U-bend' with ducting, thru one of the tents lower ports as an air input, to block any light leak...I hope...will that work?. -- are there any micro-mesh 6 inch duct intake filters for sale to put on the bottom of that U-bend?
+John Smith Hi John. The slower airspeed will not extend the carbon's life-that's determined largely by the amount of surface area on the activated carbon's micropores. For your 6-inch duct intake check out something like Black Ops' bug screens ( goo.gl/3WlOSa ) or a Phresh Intake filter ( goo.gl/MZgYCd ) - the latter wouldn't require a U-bend.
Which is better a Carbon Filter or HEPA filter for intake? Do they do the same job? Is one better than the other?
Hi Phillip. Thanks for your question. For intake, use a specialized "intake filter" - e.g. Phresh or Hepa - they just remove particulates. Carbon filters do this, but also remove odors, and are generally used as air scrubbers or as part of an extraction system.
+Everest Fernandez (Just4Growers) okay I thought Presh was a type of carbon filter for intake. That's what I was confused about. Thanks for the help. Your videos are the best.
+Phillip Tussey Any time! Phresh make both intake and extraction filters.
how calculte the air exhust suppose room size is
45 feet long x 30 feet wide x 20 feet height what size of fan or how mans are to be used. ,nice video
SUKHDEOSINGH RAJPUROHIT Hey Sukhdeosingh. You ask a very interesting question, because your ceiling height is so unusually high. Higher ceilings give you a larger "buffer" of air and do not necessitate larger fans as some formulas might suggest.
If your room is full of plants (that is, your active growing area is 45 x 30 ft) then I would suggest using fans that move 12,000 - 16,000 CFMs in total and attaching them to speed controllers. Move towards the upper limit if you are using carbon filters which lower the CFMs by around 20%. I believe it is preferable to over-spec, use speed controllers and be ready for spikes in ambient temperature than having your fans on maximum all the time and not being prepared for external environmental extremes.
In practical terms, you would need one 10" Hyper Fan (for example) for each of those 1,000 CFMs you want to move, so you're looking at 12 - 16 fans. Alternatively, you could go for larger 16" fans that shift more than two and a half times the CFMs of the 10" fans. Larger fans are available but it might be beneficial to extract from multiple points with such a large growing area.
cfm x 60 divided by volume of room - for cannabis you need about 15 ach
great video man, I have one question
my grow tent is 80x80x180 CENTIMETERS
what fan & filter should i pick for this size grow tent
Rafael Rassi Hi Rafael. Thanks for your question. Your grow tent is 1.152 cubic meters. (.8 * .8 * 1.8). You need an extraction fan capable of exchanging the air in your grow tent at least once per minute. Fans are rated in cubic feet per minute (CFM) or cubic meters per hour (CMH), depending on where you live in the world. As you specific your tent dimensions in metric, I'll go with CMH. 1.152 * 60 = 69.12 CMH. However, if you are using a carbon filter then I recommending adding 20% to this figure. 69.12 * 1.2 = 82.94 CMH. (48.82 CFM) A 100mm or 4" diameter extraction fan connected to a fan speed controller (e.g. Zephyr 1 by Titan Controls) would be a good choice for this size grow tent.
I know that was a lot of numbers! Let me know if you have any questions. Always glad to help. Peace and happy growing, Everest.
Thanks a MIL man, i was almost buying this one
www.airfan.com.br/loja/index.php?route=product/product&path=51&product_id=205
because i thought it would do the job... THANKS again
Rafael Rassi My pleasure! That fan you link to is 6 inches (150mm) but the CMHs are low! A 6" Hyperfan with its built-in speed controller would give you more than enough extraction power, but it depends what's available to you locally.
I ended up buying www.growplant.com.br/exaustor-axial-in-line.html
Will it work fine on my tent? With the carbon filter?
Rafael Rassi The specs look fine to me Rafael. Next step is to purchase a 4" carbon filter. For that sized grow tent I would just rely on passive air intake (no need for a fan to blow air into the tent.)
My grow space is 2,304 cubic feet. If the highest CFM most fans go up to is 750 do I need to have more than one fan for my incoming air?
Nice space Greggory! You can get more powerful fans than 750 CFMs but it's no a bad idea to spread your air exchange requirements over multiple fans for a space like that.
Everest Fernandez Thanks for the reply!
With multiple fans for proper air exchange would I need more exhaust?
+Greggory Brandt Normally indoor growers spec their exhaust fans slightly higher than their inputs (be sure to allow for any pressure drop due to carbon filters on your extraction side too, usually around 20 - 25%). By extracting more air than you input you create a 'negative pressure' inside your grow room which makes it easier to control your grow room's environment and any garden odors as air is being pulled into your garden through any leaks, rather than being pushed out.
All that said, I know of a grower in Holland who swears by positive pressure-he tells me that it increases the amount of effective CO2 in his garden. I guess 20% more pressure = 20% more CO2, and-as it's such a crucial input and often the bottleneck of indoor gardens reaching maturity-he may have an interesting point! I'll be looking into this next year.
The only good access to fresh air is from the house vent.its located on the side of the house what are you thing bringing in the air to the grow room?or the other option is from the outside but would be more hot.sorry I'm from other country and English isn't my language.
+Robs Hi Robs-thanks for your question. I enjoy speaking with fellow growers from all over the world. (What is your first language?) Your best option is to bring air in from another room in your house as the temperature and humidity of this air will be more stable than the outside air.
Ok that's what I'm going to do. I was trying not to cut a hole on my house but rather be safe than sorry.my first language is Spanish ,and I have one more question I just bought a filter last night I accidentally let it outside It got wet it rain all night D
Do you think still good?
+Robs Ola Robs.Estará bien. Sólo tiene que dejar que se seque. (It will be fine, you just need to let it dry out.)
Que Bueno.me salve de comprar otro
you really dont wanna use the flex ducting in more then 4 foot sections- it flows terribly- straight solid for the win- up your cfms by a ton
stevecameron85 That’s great advice Steve, thank you.
yeah those ribs inside make for a ton of turbulence. I use the flex ducting only when solid is not an option. also stay away from 90 degree bends- they kill flow too!
dont use your fans to push air. get them to pull. think about a string... is it easier to push a string through a hole or pull it?
What to do when can only take in air from outside. Cold and humid air.
No chance of taking air from an adjacent room? You could consider a sealed environment.
@@Just4Growers hi. No. I isolated the tent but its in a place with no room isolation. And cant take air from inside. I tried to open the top intake passiv. And there is isolation infront of the holes. It seems to do good. Only a few degrees up and down and the same with humidity. But dont know how is gonna react when its colder. Any info about sealed environment. Not many talking about it on the net.
@@user-jm5hg3qi4f Check out monster gardens. (youtube) lots of info on sealed rooms. Hope it helps you out. Good luck.
I am also wondering why you are not just doing passive intake. I have never seen a real need for intake fans on these size rooms, and in my own experience, an 8 or 10 inch vortex or fantech (especially fantech) can pull more than enough cfm to provide passive intake. I have to wonder if you had issues with the negative pressure of the room when you pump intake in with a larger fan that you do exhaust. I assume you know that providing more CFM on the intake (pushing) than CFM on the exahust (pulling) will not result in negative pressure but in fact positive pressure. If you run positive pressure, you better have one bad ass recirc/scrubber carbon filter set up, because a bunch of smelly air is certainly going to make its way out of the room before getting a chance to pass through the exhaust filter and have adequate soak/contact time.
I like to draw my input air through an intake filter to keep things super clean and sterile in my grow room. An active intake fan works well in this situation.
@@Just4Growers For sure. filtering would be a good reason to not do passive. But so many of these are going into little closet set ups, and most people don't bother much with filtration on those.
Although, if you have adequate stand alone filtration, such as the EN 1822 classified IQair health unit, then intake filtration doesn't have to be as stringent. If you have something as high quality as EN 1822 HEPA, then all you really need on the intake is a light MERV for bigger pests and low pressure drop.
Dead link in the description.
Can you link me to your exhaust video? Can't seem to find it.
+x2eXu50x Hey x2-thanks for your question. It's funny you should ask at this present time because I am in the process of creating some follow-up videos on this very topic, so please stay tuned (or rather, subscribe!) so you will be informed as soon as I upload them. Thanks! :)
Everest Fernandez Great will do! Thanks.
What site do you like to buy hydroponic equipment at?
I'm actually good friends with the owner of my local hydro store so I go visit him pretty often!
Do you run the fan will the lights are off?
Yes - 24/7
@@Just4Growers Thanks
@@Just4Growers Just the light turn on and off, in a cycle?
Hector Luis Colon Rosado yes - keep the air moving and exchanging 24/7 and run your lights according to your plants’ preferred photoperiod.
Everest Fernandez thanks your videos are awesome
i gonna get grow tent 1,00x1,00x1,80cm homuch i need?
Hi Damien, in metric? You'd use CMH or cubic meters per hour. 180 CMH would be your minimum or 65 CFM. I'd recommend going bigger and using a fan speed controller. Always good to have some spare horsepower should you need it in the warmer months.
yes metric, Tnx
I’ll show you a duct coupler
Can someone answer me this . Is it better to have 2 exhaust fans or 1 with the equivalent cfm3? I’m considering buying either 2 at 1400cfm each or 1 at 2800cfm , I wanted just the one but realised it would be much heavier to install but then I started wondering if I had 2 they Would possibly fight against each other?
What size fan should I use for an 8x8x7 grow tent?
I'm assuming an extraction fan works differently than the hyper fan?
Replace the entire air volume every minute? Why??
If you have a room full of healthy transpiring plants they will quickly fill the room with humidity and deplete carbon dioxide levels during the day cycle. However, since this video was made, LED grow lights have become very popular and they change everything! Look out for an updated video soon...
@@Just4Growers 7-8 times an hour is plenty. I've never had problems at that frequency. The plants' respirations is simply not that fast to replace the air every minute. That's way too much and is wasteful as one would need much larger appliances to do that.
how to make a easy ventilation system recycling?
I wish u provided phone support lol. I'm just starting out
andrea a I am more than happy to provide you with any help you may need Andrea. It's my pleasure to do so!
That looks like a French house dude?
+Hannible100 Yes, it's French - over 300 years old. Quirky to say the least.
Pretty dusty in there bro.
instead of foil tape just use gap filler or gorilla glue
Ha i was your thousand like .You should like send me an eight or something. Lol
No one likes a mooch. And he doesn't grow cannabis kid.
amateur sucks air from one box into another box suffocating the plants
+wacatz1 to suffocate plants you would need to deprive them of carbon dioxide and / or oxygen. Exchanging your room's air with fresh air is a cheap and effective way of avoiding this.