# How much CFM Ventilation required?



## accid (May 28, 2009)

Good Morning,

My box is 48 cubic feet (3x2.5x6) (LxWxH), I am curious how much air flow would be proper for a box this size. I have looked into 6 inch inline fans and high powered pc fans. What is the MINIMUM cfm i should use?

Thanks Gentlemen.


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## NYC_Diesel x Jack_Herer (May 28, 2009)

you want to exchange your air at least once every 5 minutes, more is better.  Any more than once a minute is overkill IMO.  At only 48 cubic feet pretty much any fan is going to do the job.  6" would more than likely be overkill, but the lighting you use and the amount of heat it creates is also going to have an impact on how fast you need to vent your grow space.
Just my $.02

What lighting you using?

PS-Welcome to the forum


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## accid (May 28, 2009)

I am using 400 Watt MH/HPS digital ballast bulbs. They do emit some eat. I have no hooked them up yet so I am unsure as to the amount of heat it will generate. Those 6 inch inline fans put out 200-250 CFM... thats basically 5 times my box Cubic ft. Would you suggest going with it anyways? I mean how much heat does a 400w HPS emit? 

Also what do you recommend for removing scent. It is not my primary concern right now, just curious. My concern is the fans, reason being is that I am going to buy them tomorrow.

Let me know


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## SPEARCHUCKER (May 28, 2009)

Since there are these questions going around. 

Where is the best place to look for ventilation fans?

I got a couple Honeywells today. Air purifier for outside the room, (hollowed out filter, put in a carbon middle.) and got a little fan for inside.
Whats the best place to get them for intake and outake. Something that will adapt to 4 inch piping. Cause I really didnt see anything at Lowes, less I was in the wrong sections.

Accid. Check out carbon filters for scent.


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## NYC_Diesel x Jack_Herer (May 28, 2009)

sure it will do the job.  I use two 600 watt lights in a closet 4 times the size of your box, and I vent the whole thing with one 4" fan rated at 170cfm.  For smell removla there are many products, but the best is a carbon scrubber.  It will attach on the same line as your exhaust fan and must have a cfm rating that matches the exhaust fan.  If you search for exhaust fan carbon filter combo, you can see some that are sold together.  The one I use for my closet was about $65 plus shipping, they last 12-18 months before you have to replace the carbon in them.


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## The Hemp Goddess (May 28, 2009)

Actually, you want to be able to exchange the air in your space 3-4 times a minute.  This ensures a good supply of CO2 for your girls.


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## DirtySouth (May 28, 2009)

You guys exchange air alot,y?I have a larger tent (9'L x 4'W x 7'H) I exchange air every 15 minuites with much succes.Of course I have 1 isolating and 2 "air movers" pushing the air all over the place.
 It just seems to me that its overkill.I'm not knockin it,just wondering why your workin the fan so much?


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## BOYSENBERRY (May 28, 2009)

I would never skimp on the intake and exhaust. I have seen lots of people do this and they have mold problems and have to use sulfur burners yuk. I will always use over kill when it comes to the fans I purchase and lights too the more the better. And if you put a carbon filter in the mix you will lower the cfm of the fan. I will post a link for some fans very cheap and you wont find anything better these are the best inlines out there you can take them apart to clean them and you wont have to dissconect your ducting. One of the key factors in growing is air movment this will make happy strong plants. And you will want to buy a dehumidifier for your grow you can get a small one for your grow and the water you capture collect it and use to feed your plants. This is a general rule that I go by 500 cubic foot grow would have atleast 1500cfm for intake and a LIL more for exhaust if you run a carbon filter but I run two exhaust one for the lights and one mounted high in the room to suck out that hot air. And four 18'' oscilating fans.:holysheep:
http://www.hvacquick.com/spfnc onfig.php?fm=td


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## BOYSENBERRY (May 28, 2009)

I was reading your signature and you say I have never and will never be involved in the distribution of controlled substances I LMAO it's not meth man it's *medical marijuana controlled substance makes it sound like it's bad or illegal well I live in Cali don't know where you live but if you have a script and go by the county laws your all good. Just my thought we need to have people not look at it like a controlled substance. *


			
				NYC_Diesel x Jack_Herer said:
			
		

> sure it will do the job. I use two 600 watt lights in a closet 4 times the size of your box, and I vent the whole thing with one 4" fan rated at 170cfm. For smell removla there are many products, but the best is a carbon scrubber. It will attach on the same line as your exhaust fan and must have a cfm rating that matches the exhaust fan. If you search for exhaust fan carbon filter combo, you can see some that are sold together. The one I use for my closet was about $65 plus shipping, they last 12-18 months before you have to replace the carbon in them.


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## NYC_Diesel x Jack_Herer (May 28, 2009)

I changed my signature because I had a couple people emailing me asking to buy MJ.  I do not live in a state that has medical MJ laws, and distribution or cultivation on any level is illegal, hence the comments   I will change it back in a few days when they are leaving me alone.  But where I live, MJ is definitely a controlled substance.  I don't even want their message in my mailbox.  I ignore them but I still don't want some agency going through the email and seeing the messages.  I'm not saying they could, but better safe than sorry.  Fortunately I am moving this year so I will not have to worry as much


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## BOYSENBERRY (May 28, 2009)

I cant beleive people emailing you for MJ WoW I hear you 100% move to del norte cali 99 plant limit LOL sorry to mess up this post
hXXp://www.pcfrm.org/how_to_grow_pg2.php


			
				NYC_Diesel x Jack_Herer said:
			
		

> I changed my signature because I had a couple people emailing me asking to buy MJ. I do not live in a state that has medical MJ laws, and distribution or cultivation on any level is illegal, hence the comments  I will change it back in a few days when they are leaving me alone. But where I live, MJ is definitely a controlled substance. I don't even want their message in my mailbox. I ignore them but I still don't want some agency going through the email and seeing the messages. I'm not saying they could, but better safe than sorry. Fortunately I am moving this year so I will not have to worry as much


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## BOYSENBERRY (May 28, 2009)

I dont know if you saw the link I posted for inline fans here you go the best out there hope this helps 
hXXp://www.hvacquick.com/spfnconfig.php?fm=td


			
				SPEARCHUCKER said:
			
		

> Since there are these questions going around.
> 
> Where is the best place to look for ventilation fans?
> 
> ...


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## DirtySouth (May 28, 2009)

Big $,go to home depot.(inline fan)@60. tops,they worg GREAT.


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## BOYSENBERRY (May 28, 2009)

post a link i wanna see what your talking about





			
				DirtySouth said:
			
		

> Big $,go to home depot.(inline fan)@60. tops,they worg GREAT.


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## NYC_Diesel x Jack_Herer (May 30, 2009)

I guess the more I think about it the more I think that going by a cfm rating is smart since that is really the only way fans are measured, but the other consideration in the equation is the amount of plant matter per cubic feet of grow space.  If I am growing 4 plants in the Astrodome, well then I would never need to vent that space because the cubic feet to plant matter ratio is so small that the plants would never even make a dent in the CO2 levels of the Astrodome.  
  Scale it down and you can see what I mean.  If your closet is 4x4x6 and you have 16 two-foot tall plants in it, then it seems like your closet is full of plants....but if your closet is 60 feet tall, then the plant to air ratio is very small, and it will take longer for the plants to deplete the CO2 in the air and turning over the air in the room as fast is not as necessary since your grow space is 3.75 times more cubic feet with the high ceiling.  Even though you still have the same number of plants with the same CO2 needs, you are not going to need to exchange the air in your room as fast in order to meet those needs.    

I guess the best thing to do would be to start figuring out what cfm fan you need based on.... the cubic feet of plant matter as a proportion to the cubic feet of your entire grow space.....but I shudder to think at the math.  It is way too late and I am way too high. (The ...'s are there because I had to read that 5 times just to make sure it makes sense, I am in no state of mind to do the math    )

If you think about a small stealth grow box like a cabinet that gets literally FILLED with plant matter, then surely it would need to be vented more often than a larger closet with more empty space and a smaller plant density.  The minimum of once every 5 minutes that I gave you above is what is recommended by the manufacturer of my fan, Vortex.  

PS-It is not often that I disagree with THG, but I do think more than once per minute is overkill.  However this is just my experience.  Perhaps the reason for this is that I have been using grow spaces with lower plant to empty space ratios and therefore do not need to turn over the air as fast.  For example perhaps THG finds that her grow area is 60% plant matter when in full flower and needs to vent a TON to get enough fresh air, and perhaps me being a less experienced grower or having higher ceilings with a lot of empty vertical space is running at 35%, and would get the exact same results with different air movement requirements. 

....my brain hurts, need to hit the :bong: and call it a night.


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## DirtySouth (May 30, 2009)

BOYSENBERRY said:
			
		

> post a link i wanna see what your talking about


 
Here it is hxxp://www.homedepot.com/webapp/wcs/stores/servlet/Search?keyword=inline%2Bfans&langId=-1&storeId=10051&catalogId=10053

simply turn the xs' into ts'


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## LEFTHAND (Jun 8, 2009)

DirtySouth said:
			
		

> Here it is hxxp://www.homedepot.com/webapp/wcs/stores/servlet/Search?keyword=inline%2Bfans&langId=-1&storeId=10051&catalogId=10053
> 
> simply turn the xs' into ts'


 

Why would you tell some one to spend money on crap. IMO dont buy these they dont work... waste of money. ebay has em cheap. hydro stores have em to. 

thoses fans are more or less made for pushin air .


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## alshuray1 (Jun 8, 2009)

IMO 
his set up could be cooled with either an air cooled hood reflector or a cool tube 
with a squirrel cage fan sucking and venting out of the room (pref outside )
a few cpu fans circulating air in the box a few blowing out the box (venting out of the room) 
a few low passive air holes 
and tada 
max 87 degrees 
with c02 your good 
even with out it u should be ok
if you add somekind of intake sucking air from outside and bringing it into box 
you could drop another 10 degrees


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## GeezerBudd (Jun 8, 2009)

The formula I used was LxWx(Hx1.2)=minimum CFM needed to change air in less than a minute.
I came up with around 150 on mine and went with two 75cfm computer fans.
One in-One out.
But that was when I was using spirals.
Now that I have a 150HPS in it is getting a little hot-87F with lights on so I may be getting something bigger.
I use a 25 cfm fan for a circulator also.
I got the comp fans at surplus center.
Hope this helps.
Gb


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## NYC_Diesel x Jack_Herer (Jun 8, 2009)

GeezerBudd said:
			
		

> The formula I used was LxWx(Hx1.2)=minimum CFM needed to change air in less than a minute.
> I came up with around 150 on mine and went with two 75cfm computer fans.
> One in-One out.
> But that was when I was using spirals.
> ...


 
Hey geezerbudd, just use two passive air intake holes (or one large one), and use both of your cpu fans blowing out.  This should roughly double your air flow.


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## GeezerBudd (Jun 8, 2009)

I don't know, NYC.
I'm not sure if I want another hole up top-maybe just a bigger one-I will think about that-Thanks!


Gb


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## Shockeclipse (Jun 10, 2009)

I hav a 145 cubic foot grow room, but I pulled air through a scrubber and through my 1k watt HPS so I went with a 178cfm and a passive intake.  It created a kind of vacuum so no smell ever gets out but I am movin a lot of air around in there.


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