passive air intake question
- Thread starter axiom420
- Start date
Green Troll
Active Member
Doesn't need to be that much at all. So long as the intake hole(s) are the same surface area as the exhaust, your fan will suffer no resistance other than your carbon filter. So lets work this out.
4 inch duct surface area is 3.14(2)^2 (pi r squared, and you thought that high school math would never come in useful) so 12.57 square inches.
Now you need either 1 hole the same size, well thats easy, just another 4 inch hole, or even better, a series of holes spread evenly. So lets look at 1" holes. 3.14(0.5)^2 = 0.79 square inches. Hm, kinda small. You would need 16 of those to make it work (12.57/0.79 = 16). Swiss cheese grow box!
Lets look at 2 inch holes. 3.14(1)^2 = 3.14 (if the radius is 1, its the same as pi). So 12.57/3.14 = 4. That's more like it!
One near each corner on the bottom (i am assuming you are taking about a DIY grow box here as tents already have the intake vent cut out to size) Make sure the exhaust is at the top, the intake holes at the bottom (to draw in the coolest air) so it pulls it across your plants. Light proof the holes by putting some 90 degree plastic pipe bends and pointing them down or use some flexy hose and make an S shape with it, then stretch some panty hose over the ends of the pipes and rubber band those in place. This will keep out the bugs and dust. Light proofing is very important. Check the rest of the box by turning on your light inside and closing up the box. Switch the light off in the room and if you can see light coming out the box, light can get in! So seal these gaps up.
Then hey presto, you have a light proof, well ventilated grow box. Happy growing!
axiom420
New Member
great info man!!!!
jondamon
Well-Known Member
http://urbangardenmagazine.com/2010/07/planning-your-grow
Just over halfway down in the link it talks about FRESH AIR and also how to work out passive and powered intakes.
They agree with me 2-3 holes the same size as your fans spigot size.
J
Just over halfway down in the link it talks about FRESH AIR and also how to work out passive and powered intakes.
They agree with me 2-3 holes the same size as your fans spigot size.
J
ASMALLVOICE
Well-Known Member
All the math is correct except the fact that there was not one mention of cfm rating. You can have a high powered vortex fan and you are going to need to figure that into the equation. There you will find that 2-3 even 4 times the amount of intake as exhaust is required. If this is not figured in, it will have negative effects on the space, either too much negative pressure(hard to open the door) or working the fans ass off.
The amount of intake is directly related to the amount of outflow.
Peace and Great Grows
Asmallvoice
The amount of intake is directly related to the amount of outflow.
Peace and Great Grows
Asmallvoice
axiom420
New Member
i was planning on using this.... http://www.ebay.com/itm/4-x-14-Carbon-Air-Filter-Value-Combo-4-Inch-Inline-Fan-Exhaust-F016-/281102466057?pt=LH_DefaultDomain_0&hash=item4173033c09 it says 190cfm
woody333333
Well-Known Member
they agree because youre right
ASMALLVOICE
Well-Known Member
I have 2 4x18 from the same place( gotham) and I have 1 exhausting and 1 scrubbing. You going to need at least 2 4" openings, cause the 190cfm will be knocked down a notch after adding the filter. More like 80-100cfm tops. My room works flawlessly with 1 4" intake and 1 6" intake (both passive and filtered )
On a side note, you can upgrade to the pro model, which is 4x18x 2" thick walls and refillable for just $10 bucks more than that one, with the same fan. I have 1 grow on mine, and I like them a lot.
Hope this helps
Peace and Great Grows
Asmallvoice
contraptionated
New Member
I agree with asmallvoice. One way of comparing the 4x intake/1x exhaust area idea is to just look at an air handling unit for a central air conditioner. The inlet (intake)area (where the air filter is located) is far greater than the duct that the air is pushed through.
There was also a test done with a smoke bomb in a sealed grow room with an exhaust fan . The fastest evacuation of all the smoke was achieved when the intake diameter was 4x the exhaust outlet diameter. When it was increased to 5x there were no appreciable gains in smoke evacuation which indicated that if you go much more than 4x, you could defeat negative pressure.You'll have to use the search engines to find the minutes of that experiment, but you could just take our word for it.
I also know for a fact that an exhaust fan will work much harder (draw more current;higher amps, more electricity,) if the passive intake is not sized in such a way. When I was a noob my 10" can fan would draw 285 watts before I enlarged the intake. It should have been drawing about 250 watts, which is what I reduced it to when I sized the intake correctly.
It is also worth mentioning that you will need to refer to ACCA Manual D to size the duct properly at the outlet of the exhaust fan if you want your exhaust fan to perform up to its max cfm rating. Look up the table with the max cfm capabilities for round flex duct. For example: A 10" canfan (or vortex brand) needs a 14" round flexible duct at the outlet if there are no bends in the exhaust trunk and possibly 16" round flex duct if there is a bend or offset.
But also take into consideration that you will greatly extend (while diminishing heat extraction) the life of your carbon filter if you just keep the duct diameter the same as the exhaust outlet diameter. It's a give and take. You could make up for the lack of heat removal by having multiple exhaust fan/filter combos. Redundancy is a good thing if you ask me.
There was also a test done with a smoke bomb in a sealed grow room with an exhaust fan . The fastest evacuation of all the smoke was achieved when the intake diameter was 4x the exhaust outlet diameter. When it was increased to 5x there were no appreciable gains in smoke evacuation which indicated that if you go much more than 4x, you could defeat negative pressure.You'll have to use the search engines to find the minutes of that experiment, but you could just take our word for it.
I also know for a fact that an exhaust fan will work much harder (draw more current;higher amps, more electricity,) if the passive intake is not sized in such a way. When I was a noob my 10" can fan would draw 285 watts before I enlarged the intake. It should have been drawing about 250 watts, which is what I reduced it to when I sized the intake correctly.
It is also worth mentioning that you will need to refer to ACCA Manual D to size the duct properly at the outlet of the exhaust fan if you want your exhaust fan to perform up to its max cfm rating. Look up the table with the max cfm capabilities for round flex duct. For example: A 10" canfan (or vortex brand) needs a 14" round flexible duct at the outlet if there are no bends in the exhaust trunk and possibly 16" round flex duct if there is a bend or offset.
But also take into consideration that you will greatly extend (while diminishing heat extraction) the life of your carbon filter if you just keep the duct diameter the same as the exhaust outlet diameter. It's a give and take. You could make up for the lack of heat removal by having multiple exhaust fan/filter combos. Redundancy is a good thing if you ask me.
axiom420
New Member
well the plan is im going to use a cool tube and ducting out both sides to cool it with its own 6" 240cmf duct booster so most of my heat should be gone there. then ill use the 4" inline fan filter combo to scrub and exhaust straight out.
Green Troll
Active Member
Well i guess you guys know. All i know is my tent has a 5" exhaust and a 5" intake and it doesn't cave. When i close the vent, it caves like a bitch. Plants are fine, fan is fine, it is 215 cfm...what can i say? The votes have it, go with 3-4x the size of the exhaust.
jondamon
Well-Known Member
This will work but its still putting pressure on your exhaust fan if you're using passive vents.
Its about creating enough surface area so that your exhaust isn't working as hard.
You or I would never be able to tell if your exhaust is working harder or not without having the testing equipment the manufacturers have.
But the information about passive vents is out there to be read.
J
Green Troll
Active Member
Well i shall retract what i have said and stand corrected. And open another vent when i get home 