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Extraction ideas for small, stealth grow

Hi all! 


As it stands I'm planning on building a small grow box (approx 35cm x 35cm x 100cm) to grow a couple of plants in.


For air flow I had been working on the idea of using a passive intake near the top which would effectively be an opening leading to a light baffle which leads to the LED heatsink I will be using. Unconventionally, I then wish to draw the warm air (warm from the LED heatsink) DOWNWARDS and extract at the bottom of my box where I would have another light baffle and a PC extraction fan. I was considering the NF-A4x20 FLX (40mm PC fan, 9.4 m^3/h, 14.9dB/A, 2.26 mmH2O) as it seemed to fit the bill. The fan is quiet and ideally would exchange all the air in the box 1.3 times per minute, sounds pretty good?


Pressure loss:

1) The two light baffles on either end will cause pressure loss. 

2) Pulling warm air down and extracting at the bottom will add further pressure loss


Can this little PC fan cope?


Just to explain a couple of points in a bit more detail. I'm planning on putting this grow box outside in a shed where the lights will run at night (warmer months only May - Sept). The box will draw ambient air from outside which will heat up from the light source and then this waste heat energy will be drawn down through the box in an effort to make the grow box a bit warmer. Ideally utilizing the waste heat to warm the grow box!


Whatever extraction choice you suggest please keep in mind I am looking for:

1) Stealth: quiet and hidden. Though I will be extracting straight outside the shed, I shouldn't need to filter the air for 1-2 small plants?

2) Low power consumption 


I will await your ideas!


Cheers all.

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Hey there. Good luck with your grow. I use a small space too. Computer fans are often quite loud when on full pelt. I use a 5 inch inline fan as extraction, which is way, way more than I need but it runs very lower power so I can't hear it. Maybe with comp fans it will keep temps correct but the noise will still be there which effects your stealth. Also how will you filter the smell of these couple of plants as comp fans struggle to pull air through CF?

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I've grown in small spaces and had to be inventive. One of the best fans for them are good bathroom extraction fans which are easy to mount, have specified flow ratings, can have very low db ratings and work brilliantly well in small spaces.


They take up a far smaller space than many small budget in lines and have been developed for decades to work in damp enviroments. They can be speed controlled by a basic plug in controller if needed. 


I even ran one on a 1.2m tent for a year and it was absolutely fine. 


Just make sure you buy one that can be directly controlled as well as on timer. 



Edited by 2Fat2EatThat

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29 minutes ago, Stoned_physicist said:

I shouldnt need to filter the air for 1-2 small plants?


I missed this bit. Yes you will need one. I grew two plants in Australia without a CF, which was stupid, but you could smell em when you got out of your car on the street. Mate seriously think about this before starting out. Two decent plants will stink the joint up in my opinion. Wait and see if anyone else agrees.

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I agree with @ROCKSTEADY6 You will definitely need a carbon filter as even one plant will stink out the area, it's a very distinctive smell. In my opinion a dedicated Ruck fan or similar is needed aswell to pull the air through the carbon filter as bathroom or certainly pc fans do not have the power, never heard of anyone using these fans for extraction through a cf.

A 4" fan would do but you'd need a controller to get the speed down, thats another minefield though. The best controller's i've used and still do is the 5 speed stepping voltage controller or a proper Variac, both are completely silent.

All the best.

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A few ideas for you- 


PC fans of the same size and t even amperage can vary greatly in output. I've bought at least 4 cheap ones which were totally useless.


A buck controller from China is a very cheap way to control the speed of a PC fan.


You'll need to control the odour, but this doesn't mean a 4 inch fan and filter. You could use an ozone generator plumbed  into your exhaust, or I've seen folk making DIY filters with activated charcoal and biscuit tins. Or cooker hood filters. 


Dunno how effective these DIY filters are but worth researching. Ozone will definitely work, I used to use it when I was under the influence of Ed Rosenthal's Marijuana Growers Handbook.



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Agree defo need a filter whether its 1 plant or 20.. A 4 inch twin speed fan and filter would help, i wouldn't even connect it to the light just extract from the top and have the filter inside the grow area with the fan outside it, 4" fans are quiet its the whooshing air outside that is noticeable, put a 5" reducer on the outtake fan and connect it to 5" ducting to lessen the whooooosh..Also consider doing autos during the colder months and all through the cycle have them on 24 hours, this way you don't need to worry about heating during the colder months because there's no lights off period.

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tbh I would not grow without using a dedicated carbon filter & inline fan, dont get a bathroom fan as theyre crap for our needs, plus they lack the pulling power to pull smelly air through a filter properly,

theyre also noisy & aren't designed to run 24/7 unlike an inline fan is, for example, I've been using a low power 5" systemair fan more or less continuously for over 14 years & have only changed the bearings

once in 2011/2012, theres a thread in the diy section showing you how to change your bearings, also, dont get those cheap £10 fan controllers as theyre also crap, they weren't designed for our needs

& are actually meant to be used as lamp dimmers, theres also a few members on the site who have had close calls when using them & the controllers burnt out, I would recommend a basic controller called

the xpelair wac1 which costs about £30 delivered & its totally safe when use with 4" or 5" fans. as for using a carbon filter, my advice is to buy the best you can afford ie Carboair or Mountainaire, not only are

they tried & tested, they also do their job for between 2 to 3 years of continuous use. you mentioned pc fans, if you do use these, only use them as active air intakes.


seeing as this will be your first attempt at growing, do not underestimate how stinky plants are when growing compared to the smell when smoking it, they smell 100 x times more smelly so please get a filter.


another thing that you'll struggle is growing plants in a small space, sure some folk get away with successful small grows but to make it worth while & for taking the risk, I'd increase your width & depth at the very least.

also, your better putting your extraction at the top of your grow area, dont forget heat rises. you also dont want to be blowing extracted heat air out into your shed as the air will be stale & be full of condensation which

isnt good inside a shed. the best thing you can do is do more reading in the grow room section & in the grow diaries, these will give you a good idea of whats possible & whats a waste of time, energy & the cost.

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Hi again everyone! Thanks for all of your input.

I have put a lot of thought into what you have all said and I understand now that I need a fan that can overcome a certain amount of pressure (carbon filter) whilst still delivering the required # of changes of air within the grow space per minute.


Assuming I want to be somewhere between a full air change of my grow space every 2-5 minutes I have calculated that I need an approximate air flow of 0.87 - 2.16 (cubic ft/min). I have no idea what the total static pressure of my grow area will be (I haven't built it yet...) but I have been browsing some centrifugal 'blower' type fans. I'm interested in some fans that Sunon produce, particularly the GB1206PTV2-AY. According to the datasheet this fan can deliver 7.3 CFM under no pressure and has a max static pressure of 0.58 inH2O, this sounds much much more powerful than I'm going to need and I am hoping that I can actually run this fan on a lower voltage to find a sweetspot for my growbox.


I haven't bought this fan yet as I would like to hear your opinions. 

I've had a look at some DIY carbon filters but I'm in a position to purchase one if anyone has any particular brands they highly recommend. If these filters have available data on pressure requirements of fans that would be even better as it is very unclear to me what level of pressure drop to expect from a carbon filter.


Sorry if this is getting a bit unorthodox for the most of you. I am very limited in space, power but not in creativity! :bong:

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Hi Stoned P. Hope you are not working on a Saturday like me. Your proposed box size is not too dissimilar to mine in dimensions - yours is taller though. I think you might be overthinking your extraction needs, the £12.50 fan you are suggesting will still be too small. To put things into perspective I use a 5 inch inline fan that apparently moves 310 m3 per hr. Once you put a carbon filter on the end my fan is more like 220 m3 per hour. Then you have a couple of bends and it is 175m3 per hour. Then you want to use a fan controller and run it at about 30 - 40 % if possible so its actually 52 - 70 m3 per hour. 


I presume I am not alone as I have bought loads of grow related crap (under powered or just generally useless) that now resides in the loft not being used. Do not be like me. Buy better quality from the start. 

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the best advice I can give about a diy carbon filter is DONT do it, your seriously risking your liberty all for the sake of saving some money, stop being a tight arse !

as its already been posted but its worth saying again, buy the best filter you can afford & as for that make of fan "Sunon" never heard of it but your better getting

an inline fan like a Ruck, RVK or a Systemaire & not a blower type as they're generally more noisy than an inline fan.


its also been pointed out about getting a fan controller, not only do they allow you to slow the fan down for stealth reasons, they also allow you to control your growing area's

environment, having a good environment is just as important as using the right light & feeding the plants the right food, if its summer time, you can turn the fan down slightly

& come winter time when its colder, you can turn the fan down more, this'll help retain some heat/humidity. 


forgot to add, just in case you dont know, when your sucking in or blowing out through a carbon filter, you can lose up to about 15%-20% of the fans air movement rate,

this is caused by dragging the air through the carbon & it creates a drag effect, thats why its important to get the right fan from the off instead of having too go & buy another

fan when you finally realize that you fucked up by trying to save money, I've seen at least a 1000 other growers on this site who have made the same mistake :ouch:


the general motto is - buy something cheap & you end up buying twice  :doh:

Edited by Joint hogger
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