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Heat issues: Exhaust CFM vs intake area

S

skorch

Member
So Im looking for some help with a small heat issue.
Currently my setup gets to about 95 degrees when the house is about 65 and I would like to correct this issue before my grow gets any bigger. One thing I should note is that I dont have a oscillating fan yet and dont know how this will effect my temp, if at all. After doing some research Im thinking that maybe my weak point is how big my passive intake is because as far as what I was reading 160cfm fan should be more then enough to vent my box. When doing my research all I saw was that your passive intake should be equal to twice as big as your exhaust. Maybe I'm over think this but shouldn't the cfms be the determining fact not the area of the exhaust?

My current set up
36sqft box
exhaust 6in 160cfm fan with carbon filter
area of exhaust 28.3sqIn
area of intake 28.7sqIn

Im hoping someone can give me some more info on this and tell me what my best option is. Thanks for the help!!
 
Doer

Doer

Well-Known Member
No, that's right, you need a big, preferably filtered intake. What you are not taking into account is the CFM of that fan is multiplying the effective size of the exhaust. So for passive to passive convection exhaust we still want a bigger intake. The velocity change as the air accelerates out thru the passive exhaust is enough to lower the pressure and demand a larger intake.

So, imagine that in your situation. With a 160 cfm, you are significantly lowering the pressure in your box, way more than even a passive convection exhaust will.

The beauty of it is, of course, is you can easily split up the passive side intake, to come from several convenient openings. If you can get some cool air from under the house, that is the best.

Hope this helps. :)
 
S

skorch

Member
Thanks for the quick response. So how much bigger should I go with the intake and still keep my negative pressure?
 
Doer

Doer

Well-Known Member
It is a simple matter of opening it until there is just a good flow. Like my carbon filter, for example.

I can feel it drawing with the back of my hand, but I just want a slight suction and have adjusted it for that. We want long transit time over the carbon.

Adjustment! That is the word I'm looking for. But, what you really want to know is how many time can you change the room volume completely per hr. But, I am also balancing a pair of 600 cfm exhaust fans.

So, if you can pull in 160 cfm via an adjustable intake experiment, how many cubic feet is the room? So, in that case, you are looking for 4-5 x room volume intake per hour minimum, is how you really need to look at this.

You need fresh CO2 from outside air, mainly, and you need fans to circulate the air very well, so there are no stagnate spots, no matter how big the canopy growth.
 
S

skorch

Member
Well i think that will be my experiment this weekend. Cut some holes and until i get max air flow then cover a few up to adjust the negative pressure.
 
S

skorch

Member
So I did a little experimenting with my system. I did find that my issue is that my carbon filter is restricting my flow.
That leads me to my next question where can i get a more free flowing filter.
 
Nizza

Nizza

Well-Known Member
rule of thumb is a passive intake or exhaust should be 3 times the area of the intake or exhaust the fan is on.
also its good to have 5' of run on the exhaust side of the fan before making any turns, it makes less back pressure on your fan.

you can make your own carbon filter with active carbon and wire mesh, just remember that each carbon filter has its own CFM rating and your fan should meet it
 
Doer

Doer

Well-Known Member
And I have to admit I have never seen a 160 cfm carbon filter. Maybe you need to invest in big suction.

I have a tiny tent, for example, only 32 x 32 x 60 inches, much smaller than yours and I use a 400 cfm exhaust, and 2 circulation fans. And that is raw pull, no filters, straight out exhaust. It's inside a filtered room.

I need major air exchange in the tent for heat. It has one suction from the AC and one from an outside fogger tub.
 
Doer

Doer

Well-Known Member
skorch said:
This is what I have. http://www.homedepot.com/p/Suncourt-Inductor-6-in-In-Line-Duct-Fan-DB206/100067594#.UqYjlcRDuSo
When adding the filter it cuts my flow in half.

What fan do you have doer?
Click to expand...
Those suck. :) woops a pun. The fan needs to be much larger than the intake. It is pressuring, so needs more blade width.

I think those are a joke and a complete waste of money....It is not really possible to accelerate much air like that. I have wasted so much, I really feel for you. But, it has to be a horticulture hobby, and we have to find out what will work for us, outside of all these commercial lies. Hobbies cost.

A "real" inline fan looks like this. And as I mention I think they begin at 400 cfm and this one is only $80.
http://www.ebay.com/itm/like/390582965771?lpid=82
 
Doer

Doer

Well-Known Member
BTW, I have 3 exhaust fans, 400 and 2 x 600 cfm. One for the carbon filter and on for the AC hot box collector exhaust and the other for the tent.
 
Ilovebush

Ilovebush

Well-Known Member
That is the best fan choice hands down...you need power to move the air volume required. It will make more noise then in line/booster fans as you are currently using but you have options to reduce this. One option is a fan speed controller http://www.ebay.ca/itm/ACTIVE-AIR-DUCT-FAN-SPEED-ADJUSTER-/251278702186?pt=LH_DefaultDomain_0&hash=item3a8160b66a . You can also box the fan using mdf wood and sound proofing and finally isolate the fan vibration by mounting it using bungee cords or rubber bushings.
 
Doer

Doer

Well-Known Member
Yeah I find they can cool from the inside so I loosely insulate and isolate for sound and to reduce the heat shedding of the outer case.
 
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