# how often to exhaust?



## danfinance2008 (Mar 24, 2009)

i looked and could not find a definat answer

how often do you think i should bring fresh air into the grow box?
i am going to focus on autoflower plants, 1-2 plants at a time with a 250 hps light (if that matters, i will also make a cooltube for it ) my grow box is going to be 2.5 x 2.5 x 5 foot high.

also i will run a filter for the smell, how will i know what size fan to buy?
does anyone knoe of a adjustable speed fan?


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## Shockeclipse (Mar 25, 2009)

danfinance2008 said:
			
		

> i looked and could not find a definat answer
> 
> how often do you think i should bring fresh air into the grow box?
> i am going to focus on autoflower plants, 1-2 plants at a time with a 250 hps light (if that matters, i will also make a cooltube for it ) my grow box is going to be 2.5 x 2.5 x 5 foot high.
> ...


Take your 2.5x2.5x5=31.25 square feet your fan should be big enough to replace the air every 5 or 10 mins, not sure the exact number but you wont need a very big one.  You can also hook your fan up to a reostat (I thi k its called that) and that way have it run whenyour temps get hot.  Although I would just get a small fan and run it all the time i think.


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## blancolighter (Mar 25, 2009)

If you're gonna run a filter for the smell, you'll need a good fan to pull the air through, like a centrifugal or axial fan. I've got a grow cabinet about the same size as yours, and I have a carbon filter with a 250 cfm can fan in there. The carbon filter takes the cfm down a bit, but I find that in a small space like that, it's good to have a high air exchange rate as heat loves to build up quickly in that small space and you just gotta keep pumpin it out. I have my fan hooked up to my timer so its only on 12 hours a day (doesn't need to run when lights are off, plants are sleeping and don't need the fresh co2). Is there a specific reason you want your fan to be adjustable? I really see no need as you can't OD your plants on fresh air...


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## The Hemp Goddess (Mar 25, 2009)

I would recommend at least 2 fans--one to cool your light and the other to exhaust your space.   You may be able to use a passive intake.  You want to replace the air in your space 3-4 times a minute to provide enough CO2 to your plants.  So, you want a 125 CFM fan just for air for your girls.  If you have a carbon filter, you are going to have to beef up the fan.  There are too many variables to tell you exactly what you need.  My winter ventilation system and my summer ventilation system are entirely different, due to extreme heat in the winter and potential sub zxero weather in the winter.


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## WeedHopper (Mar 25, 2009)

My exhuast runs 24/7. Keeps fresh air on my girls and I have no smell,, due to negative airflow.


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## danfinance2008 (Mar 25, 2009)

The Hemp Goddess said:
			
		

> I would recommend at least 2 fans--one to cool your light and the other to exhaust your space. You may be able to use a passive intake. You want to replace the air in your space 3-4 times a minute to provide enough CO2 to your plants. So, you want a 125 CFM fan just for air for your girls. If you have a carbon filter, you are going to have to beef up the fan. There are too many variables to tell you exactly what you need. My winter ventilation system and my summer ventilation system are entirely different, due to extreme heat in the winter and potential sub zxero weather in the winter.


 
i agree on two fans, i liked your diy on the cooltube and will follow that. 
you say i want to replace air 3-4 times a minute to provide co2, ect. is this constant? like 24/7

sorry i am a nexbie  

also the cool tube for the lights wont have a carbon filter. will is smell if i seal it ver well? 

and a can fan for the carbon filter

also this is done in the basement and will have kinda steady outside temp from basement going into rook
thanks


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## danfinance2008 (Mar 25, 2009)

CowboyBudsky said:
			
		

> My exhuast runs 24/7. Keeps fresh air on my girls and I have no smell,, due to negative airflow.


 
what do you mean by negative airflow?

thanks


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## danfinance2008 (Mar 25, 2009)

blancolighter said:
			
		

> If you're gonna run a filter for the smell, you'll need a good fan to pull the air through, like a centrifugal or axial fan. I've got a grow cabinet about the same size as yours, and I have a carbon filter with a 250 cfm can fan in there. The carbon filter takes the cfm down a bit, but I find that in a small space like that, it's good to have a high air exchange rate as heat loves to build up quickly in that small space and you just gotta keep pumpin it out. I have my fan hooked up to my timer so its only on 12 hours a day (doesn't need to run when lights are off, plants are sleeping and don't need the fresh co2). Is there a specific reason you want your fan to be adjustable? I really see no need as you can't OD your plants on fresh air...


 
just to be able to control heat, humidity and other issues. that way if i i can guess on the size fan u need ( a 250 sounds right ) and adjust it from there. i dunno maybe i am putting to much into it:confused2:


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## WeedHopper (Mar 25, 2009)

Negative airflow means I have more Fan then I need. I have 100cf of grow space. with a 170 CFM exhaust fan.


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## danfinance2008 (Mar 25, 2009)

THIS I FOUND AT ANOTHER FORUM

a 4 " fan has a area of about 12 square inches

a 2" hole has a area of about 3 squar inches

if you want 1.5 as much, you will need 6 2" holes

my first cabinet had a 4" exhaust and 4 x 2" pipes for intake. i had serious heat issues until i did the math. i aimed for twice as much intake as exhaust, so i would have needed eight 2 " holes. i switched to 4 x 3 pipes (~7 sr in area, or 28 sq inches of passive intake.) the cab temps droped dramatically

i found it on another forum, i dont take credit


if my spelling is of, i have adhd, and just the way it is srry


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