# Someone please settle the debate on the correct size intake and exhaust



## Rickety Rocket (Jun 5, 2012)

I have been through so many posts in several different forums and its back and forth with the correct calculations for the exhaust, filter, and intake. can somebody please give me the correct info. someone posted the ventilation info below and from everything else i've read its incorrect and the exhaust should move x2 the cfm as the intake. the only thing i think i'm sure of at this point is the size of the exhaust fan should match the size of the filter and the cfm of the filter should either match or be higher than the exhaust. is that correct? also is it better to have an inline fan or should i do like the article below says and just poke a hole in the wall? somebody with experience please help.Detailed vent addition
*How do I add ventilation to my grow area details?*
*Should I place the fan in the exhaust, intake or both?* 
The fan should be placed in the exhaust, and the intake should be a simple hole (or light trap, if light getting out is a concern). 
This type of system is known as an Active Exhaust, Passive Intake System. 
Mounting the fan in the exhaust, sucking air out of the room accomplishes a couple of things... 

· Since the exhaust is at the top of the area, the fan will suck the hottest air out of the area first. 
· The fan is actually lowering the air pressure inside the area. Any incidental pinholes or leaky seams will simply draw air in. If the fan were blowing IN, those pinholes and leaks would allow potentially smelly air OUT. 


*How big should the passive intake be?* 
It should be slightly larger than the exhaust. Remember, the volume of air being blown out, will be replaced through the intake. Using a bigger intake hole allows the incoming air to be at a lower velocity (speed), which minimizes mixing up of the air in the area. It will also allow the fan to operate more efficiently. 


*How big should the fan be?* 
Fans are rated in either cubic feet (CFM) or cubic meters per minute in North America. In Eurpose, metric fans are rated in m3/hr - cubic metres per hour (m3/hr). 

That means a 70CFM fan will move 70 cubic feet of air in one minute. 
Your fan should be big enough to move the volume of your area 2 to 3 times every minute. A 70 CFM fan would be adequate for a 35 cubic foot area, and would be optimal for roughly a 23 cubic foot area. 

· To figure your areas cubic volume, multiply (in feet) the length by the width by the height. 


*What if I have more than one fan? Should I use one to blow air in and one to suck air out?* 
Not if the object is to provide as much ventilation and cooling as possible. 

· If you have two 3-inch diameter fans, and you mount one in the intake, and one in the exhaust, you have a total intake area of one 3-inch hole and a total exhaust area of one 3-inch hole. 
· If you use both fans as exhausts, you have TWO 3-inch exhausts and two 3-inch intakes (actually, two 3.3 inch intakes. They should be bigger than the exhausts, remember?). 
· Twice as many holes, twice as much airflow.​


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## ImAgIaNtInDaGrOwWoRlD (Jun 5, 2012)

Just get a can-fan combo. The size is up to you and the size of the garden. I use a can filter 66 with 6" ducting for a 12 x 12 room. The a/c is your intake for the summer and I use a can-fan 6" high output with a backdraft damper for the intake in winter


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## Green Chi (Jun 5, 2012)

Heya RR,

I'm a newbie grower, but I am well versed in HVAC design which includes air movement.

This is what I have for my areas...
Veg - 4" duct attached to the light casing (or just wide open at a high point in your area if you don't have a venting light)
Duct travels outside area to a blower, then a short piece of duct to direct the air where I want. I don't use a filter because I don't seem to have odor issues from this area.

Flower - Charcoal cylinder filter hung high in area, 4" duct travels outside area to blower, and a short piece of duct to direct air.
(If you have a vented light, you could do... filter-duct-light-duct-blower)

Both areas allow incoming air in very low near floor.

As you already mentioned pulling the area into a slight negative pressure keeps odors inside, I don't completely close my areas but have no odor when blowers are running.
I have both setups on thermostatic control ($20 Home depot plug-in t-stat) to come on when inside temp reaches about 85f, they typically shut off about an 1/2 hour after lights out. The blowers I have are about 100 cfm's for a 4x4x6 area with a 600 watt MH light, and they work well.
I would not increase the cfm to area ratio even if it were given to me for free, there is no need and charcoal filters will be used up faster.

CFM's is one of three factors in air movement, static pressure and velocity are the others. If you increase cfms and don't compensate duct size and filter area your pressure and velocity get higher than they should be. Which will waste watts and filter life, as well as increase noise.

The three things the ventilation accomplishes are
- Ventilating heat (which is likely the factor that will dictate your minimum cfm's)
- Bringing in CO2 (My plants don't get any new air overnight and they are very healthy)
- Controlling odor

Sorry if I've said anything obvious, that didn't need to be said, just trying to help.


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## Rickety Rocket (Jun 5, 2012)

So I have a pretty big grow space and i purchased a 10" can fan max which is a little over 1000 cfm. I'm going to purchase a 10" phresh filter which has a slightly higher cfm than the exhaust fan. So here are a few questions I have: Is the ducting necessary or can i just mount the exhaust fan to the window attached to the filter? I've heard the less duct the better. . . Also should I get an inline fan to prolong the life of the exhaust fan by moving in more air or just have a hole with some duct coming from my window? And if I get the inline fan I heard it should it be 1/3 the size or cfm as the exhaust - is that correct? Is it worth purchasing a muffler or silencer for the filter? Also is one better than the other? Thanks for all the help guys. 
I'm starting my seedlings tonight. Oh btw this isn't really related but if i'm growing in hydro should i let the seeds soak overnight first or just pop them into some rapid rooter plugs? I'll check back in an hour or 2. thanks


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## zer0ed (Jun 5, 2012)

ive always heard that you want the room to be vented once every 5 minutes.
so it would be the size of your grow area (which you didn't mention?)
Width x Length x High = Cubic feet / divided by 5

the cfm's of your intake should always be less then your exhaust.


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## Green Chi (Jun 5, 2012)

Rickety Rocket said:


> So I have a pretty big grow space and i purchased a 10" can fan max which is a little over 1000 cfm. I'm going to purchase a 10" phresh filter which has a slightly higher cfm than the exhaust fan. So here are a few questions I have: Is the ducting necessary or can i just mount the exhaust fan to the window attached to the filter? I've heard the less duct the better
> 
> Only problem might be noise, the duct tends to prevent the blower noise from bouncing into the area, but the filter might do the same job. Your fan and filter are a good match.
> I've done Air conditioning in whole houses with 1000 cfms, I think you are oversized at 1000 cfm.
> ...


I've been doing the 24 hour soak (in the dark) with mine and have had very good results.


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## Rickety Rocket (Jun 5, 2012)

thanks. thats what i figured. does the intake have to be 1/3rd the cfm of the exhaust? can anybody answer any of the other questions i've asked???


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## Green Chi (Jun 6, 2012)

I've never done a system with a powered intake fan in conjunction with an exhaust fan. Normally (in HVAC) the exhaust fan draws the area into negative pressure and the make-up air is comes in through openings (grills).


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## Rickety Rocket (Jun 6, 2012)

ok so i decided to do an active exhaust with passive intake for now. . . With a 10" exhaust what size should the intake hole be? Also how do I block the light from getting in at the intake hole. Do I just use ducting or make some kind of a box?


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## NetGuruINC (Jun 6, 2012)

Listen, go to ebay and buy a fan + filter combo. And you can also buy the ducting to fit your exhaust seperately on ebay. The one I have which is in the picture below is a 4" Inline Fan + Filter Combo. I also purchased the 4" ducting to fit on ebay as well for $12 bucks.




This fan is inside of a 4 x 4 tent, and its MORE THAN ENOUGH venting, you know youve got good ventilation when you zip up your tent and the exaust fan is causing your tent to suck inward like a vacum. CLEAN AIR BABY!

This is the EXACT exhaust + filter combo I purchased:
http://www.ebay.com/itm/4-Inline-Fan-Carbon-Filter-4-x-12-Combo-scrubber-exhaust-blower-odor-/200715318736?pt=LH_DefaultDomain_0&hash=item2ebb90b5d0


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## BlueBalls (Jun 6, 2012)

Rickety Rocket said:


> With a 10" exhaust what size should the intake hole be?


You'll want the intake area to at least be equal and up to double the area of the exhaust.

For example.

Area of a circle = PI * radius[SUP]2[/SUP]&#8203;

10 inch diameter circle has a 5 inch radius.
PI = 3.14
3.14 * 5[SUP]2 [/SUP]= 78.5 square inches
78.5 * 2 = 157 square inches
radius = square_root (157/3.14) = 7.1 inches

A 14" diameter intake will be a little less than double the exhaust area.


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## Rickety Rocket (Jun 6, 2012)

thanks, but that link is dead. I already purchased a 10" can fan and 10" phresh filter. What i need to know is how big the intake hole should be and how to block the light.


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## Rickety Rocket (Jun 6, 2012)

thanks blueballs. do you know how i can block the light at the intake?


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## BlueBalls (Jun 6, 2012)

There are a ton of ways you can make a _light trap_.
Making an S out of ducting works well.

I use a 90º rigid elbow painted flat black with a bug shield.
It works well in the dark corner I have my tent.


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## Green Chi (Jun 25, 2012)

Correcting bad information I left in message 3.

It is not OK to leave your ventilation system off overnight in your grow area, moisture/humidity builds and can cause problems.
I just found this out first hand, luckily leaving the blower on constant reversed the issue in 4-5 days.
(though I'll still let the T-stat operate the blower for the veg cycle, never again for bloom)


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## Curbcalamitous (Nov 17, 2020)

Rickety Rocket said:


> thanks. thats what i figured. does the intake have to be 1/3rd the cfm of the exhaust? can anybody answer any of the other questions i've asked???


1/4-1/2 of the exhaust as intake when using active intake. as a rule of thumb.
another uel of thumb is dont use active intake. theres a few exceptions. if you don't have mnore than one source of fresh air for example. but generally allow physics to do the work for you. its free.


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## Curbcalamitous (Nov 17, 2020)

BlueBalls said:


> You'll want the intake area to at least be equal and up to double the area of the exhaust.
> 
> For example.
> 
> ...








(or pie times the radius squared - BEDMAS brackets exponents division multiplication addition) your way makes it way more complicated, plus its less useful information, he's clearly asking about an active intake and wants to know what % his intake fan should be of his exhaust fan.... also I'm not sure why you're squaring your area of the circle, it's already in inches squared. 78.5"^2 since you square the radius first before multiplying by pi.... so its 
pi*R^2
(7/22)*(5*5) (Well i dont know a shorter way to write out pi correctly, 
3.14*25
=78.5"^2
if you took this numer and multyiplied it in inches by your ducting you would have the total volume of your ducting and will just need to convert it to cubic feet.

also radius^2 = Area/Pi
so you get
R^2=78.5/3.14
R^2 = 25
R = root 25
R = 5"


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## Samwell Seed Well (Nov 17, 2020)

6.4 cubed


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## pulpoinspace (Nov 17, 2020)

8 and half year old thread, but good info.


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