Another ventilation help thread please

[growsmoke]

Hi icmag this will be my first post here and would like to just gige a quick hello to try and keep this short if possible
I'm having a hell of a time trying to figure out my ventilations system
I understand negative pressure and that in most situation a passive intake is a better choice .. With that being said I don't know if I should plan on using a fan on my intake as well as exhaust .ok so my setup

• 3.5'x2'x5' (LWH) custom wooden box
• Ipower 400 watt dimmable with cooltube
• 4" sunleaves wind tunnel inline fan (exhaust)
• 2 to 3 plants SCROG (if it matters)
• oscilating desk fan just above the canopy
• soil grow

ok so there's my equipment minus the smoke detector and temp humidity gauge stuff like that

everything is setup so I'm not looking for buy a bigger this or different that . That is what I'm working with
now to my question(s)
this will all be in a sliding door closet which is why is only two feet deep the exhaust will go as such
carbon filter--light--1ft ducting---fan--ducting to ceiling
so I will be trying to scrub the air and cool the light on the same exhaust fan
the part in having problems with is my intake
my intake will be a hole in the wall with a foot or two ductwork connecting the hole to the box drawing from the room the closet is in ambient tempts are 73 74
I understand that if I was using a passive intake I would need ether one 8" or two 4" i would have to go with the two 4" so I don't have a huge 8" hole in my bedroom wall
so setup A would go something like this
• 4" hole in wall ---Y connector or splitter of some sort -- 2 4" hole in the box
my concerns with this is there would need to be a bend or two to make this fit
• setup B 4" hole in wall - duct booster fan (100 cfm)-- ductwork -- one 4" hole in the box with a 90"degree or hood on the inside to block light from coming in the box
keep in mind that there will only be maybe 2' at most ductwork between the hole in the wall and the box
my concerns with this is negative pressure

please if you have EXPERIENCE with something similar can you lend me a suggestion on which way to go

thanks in advance

 

[growsmoke]

Edit : The sunleaves wind tunnel is rated at 200 cfm

 

[r2k]

First off, two 4" holes is not the same area as one 8" hole. An 8" hole has 50 square inches of hole, a 4" hole has 12.6 square inches of hole. You need four 4" holes to get the same amount of hole area.

If you are running 400 watts with 73*F air temperature and you want to limit box temp to 80*F, that means you need 180 CFM moving through the box. If you set up the coolTube well with the right diameters and such, you might get that down to 130 to 150 CFM. Pushing that much air through a filter with an inline fan is going to be difficult. I don't know details about your filter and fan operation but inline fans aren't really meant to push against much resistance.

You should try it to see how it all works before you start planting. I think you may find that a smaller bulb will be easier to achieve the temperatures you will need.


-r2k

 

[growsmoke]

First off, two 4" holes is not the same area as one 8" hole. An 8" hole has 50 square inches of hole, a 4" hole has 12.6 square inches of hole. You need four 4" holes to get the same amount of hole area.

If you are running 400 watts with 73*F air temperature and you want to limit box temp to 80*F, that means you need 180 CFM moving through the box. If you set up the coolTube well with the right diameters and such, you might get that down to 130 to 150 CFM. Pushing that much air through a filter with an inline fan is going to be difficult. I don't know details about your filter and fan operation but inline fans aren't really meant to push against much resistance.

You should try it to see how it all works before you start planting. I think you may find that a smaller bulb will be easier to achieve the temperatures you will need.



-r2k

thanks for the reply can you explain how you came to those sq ft numbers and how you came to the 180 cfm forgive me i hate math hah. Besides that I've decided to go with a 6" inline 440cfm i believe and a passive intake but now I'm confused with how large my intake holes should be with a 6" fan? From everything I've seen at least twice the fan size?

 

[Mikell]

Ventilation 101..

 

[Granger2]

Yes. Read the Ventilation 101 sticky. It's pi [3.142], times r [radius] squared. So a 4" diameter hole, the radius is half the diameter [4/2=2], squared [2x2=4], times pi [4 x 3.142=12.6] Now try it for a 6 inch =28sq in. Your intake doesn't need to be round. For a 6" exhaust you need 2x28=56 sq in intake. That can be any shape. It can be a 7.5 x 7.5" square, a 7x8" rectangle, 2-6" round or 1-8" round. Or it could be 4"x 14". Good luck. -granger

 

[Ganoderma]

First off, two 4" holes is not the same area as one 8" hole. An 8" hole has 50 square inches of hole, a 4" hole has 12.6 square inches of hole. You need four 4" holes to get the same amount of hole area.

If you are running 400 watts with 73*F air temperature and you want to limit box temp to 80*F, that means you need 180 CFM moving through the box. If you set up the coolTube well with the right diameters and such, you might get that down to 130 to 150 CFM. Pushing that much air through a filter with an inline fan is going to be difficult. I don't know details about your filter and fan operation but inline fans aren't really meant to push against much resistance.

You should try it to see how it all works before you start planting. I think you may find that a smaller bulb will be easier to achieve the temperatures you will need.


-r2k

A circle times Pi will give the space measurement. 8 inch X 3.1415 will give you how many inches of space the circle is. Half of the radius times Pi will give you the inches of the of the outer diameter of the circle. An 8 inch duct is 25.132 inches of total space with in the circular space. An 4 inch duct is 12.566 inches of space. A 6 inch Duct is 18.849 inches. 2 4-inch ducts= 25.132 inches, 2 6-inch ducts= 37.698 inches, 2 8-inch ducts= 50.264 inches of space

You are going to be creating a negative pressure in your space with your fan. A single hole the same size as your fan/duct work allows only so much air to pass, this creates a resistance to the air flow. To decrease this amount of resistance, if you double the amount of vent holes you have decrease the resistance and allow twice as much air to pass. I have seen a drop in room temps by adding a 2nd vent of the same size with out the need of an other fans.

 

[r2k]

Yes, I agree with what they said regarding area of a circle. If you use a 6 inch diameter fan, you have 28.27 square inches of space for the fan. That means you need an intake area of about 56.5 square inches if using passive intakes. The rule of thumb is to use about twice the fan area for intakes to reduce incoming air resistance. This is what Ganoderma and Granger2 mentioned previously.

The ventilation 101 thread is a good place to start for basic knowledge of what kind of temperature rise to expect with a given fan size and bulb wattage. The basic formula is:

CFM = 3.16 * wattage / tempDifference
tempDifference = (desiredBoxTemp - incomingAirTemp)

In your case, tempDifference = 80 - 73 = 7 degrees

If you increase the fan to 440 CFM, you will see a temperature difference of about 2.9*F, so that's good. Of course, that assumes you can actually move 440 CFM with all the resistances you have. It is wise to reduce the effective fan CFM to about 80% of max to have some margin, so that that means 440 CFM x 80% = 352 CFM. With that fan CFM, you will see a temp difference of 3.6*F. That's still pretty good, in my opinion. It means if you want 80*F in the box, your incoming air temperature could be as high as 76*F and still meet your target.

Of course, all of this involves lots of guessing on my part. Inline fans are known for being bad at moving air when there is resistance like a carbon filter, and that is going to hurt you. The best way to figure out what you can get is to build the system with all pieces and holes and then get your thermometer out and measure some temperatures after everything has been running for a few hours. You don't have to put plants in it, just the bulb, ducting, filter, and holes. Everything but the plants.

-r2k

 

[Granger2]

Ganoderma,
You're on the right track, but some of your math is wrong. Check it over. Good luck. -granger

 

[growsmoke]

Thanks for all thr help guys I've been trying hard to understand thr ventilation 101 the best I can but some of it is definitely too technical for me and way over my head so especially thank you if you posted from it and then explained instead of being a jack ass and saying google or vent 101.. No shit I don't post without researching until I'm ether confused and need a personal explanation or nothing I find is similar enough to my situation but I think I have it figured out as best as I can without it being in action while trying to keep it as simple as possible I upgraded my setup to a 6" hyperfan rated at 315 cfm yes I know I could get a six inch with higher cfm but I also need it to not be bulky and the design and compactness of the hyperfan seem to be highly rated ... I've decided completely against an active intake now .. My concern now is my carbon filter in still not confident 325 cfm rated fan will sustain enough of that cfm rating to cool my box efficiently sp if anyone could recommend alternatives to odor control from personal experience thatd be awesome I know about ozone generators and thr absorbant gels but need to do some research

 

[Mikell]

So I'm a jackass for directing you to a resource that not only answers your questions, but explains the basics of ventilation?

Ok. If you actually did read it, definitely give it another browse. Duct sizing would be something to pay particular attention to. If it's not covered there, it is very researchable.

 

[r2k]

If you really want to know what to expect, you need to find some hard core design data on the fan and the carbon filter. Every fan has a set of curves that show the relationship between static pressure and amount of CFM you will get out of the fan. The filter will also have a curve that shows static pressure vs CFM you can get through the fan. You need to find both of these curves to really know how the two will react when used together. The manufacturer will need to provide these. I tried to google it, but I can't find this data for the hyperfan you describe. Once we know this data, we can accurately predict how many CFM you can expect with the fan and filter combination. Once you know CFM and bulb wattage, we can predict the temperature rise you will find when you build the system. Until we have curves for the fan and the filter, we are only guessing at what you can expect with these components.


-r2k

 

[Mikell]

http://www.canfilters.com/max_fan_6.html

Should be a rough equivalent. Somewhat skeptical a cheaper fan of the same size and wattage is going to draw more air, though it might draw less under load.

 

[Ganoderma]

Ganoderma,
You're on the right track, but some of your math is wrong. Check it over. Good luck. -granger

Its been a while since I've had to pull the PI out of the closet and use it.

 

[Oti$]

I'm going to make it easy for you. You want to exchange the while rooms volume of air 2/min. Use passive intake to make things easy. You want your intake holes twice the size of your exhaust hole to maintain negative pressure. The area of a circle is pi x radius squared

 

[r2k]

... You want to exchange the while rooms volume of air 2/min.

Let's look at that approach. From what growsmoke said above:

3.5'x2'x5' (LWH) custom wooden box
Ipower 400 watt dimmable with cooltube


Following the “2 air exchanges per minute” theory, the room volume is:

Volume = 3.5 x 2 x 5 = 35 cubic feet
2 x Volume = 35 x 2 = 70 cubic feet per minute

Using the ventilation 101 formula, that means:


TempDiff = 3.16 x wattage / CFM = 3.16 x 400 watts / 70 CFM = 18*F temperature rise

If you want to maintain 81*F as the upper limit, this formula means you would need incoming air temperature to be (81 - 18) = 63*F.

If you have incoming air at 63*F or lower, then that old 2xVolume formula will work. I suspect you don’t have incoming air at this low temperature (I think it is an indoor grow) and so you can’t achieve a reasonable plant temperature with a 400 watt bulb. Maybe 150 watt bulb could be used, but 400 watts is too much using that formula (unless you have incoming air at 63*F)

I never understood how rules like "2xVolume" could ever work. It all changes based on bulb size and incoming air temperature. Why would it matter what the volume of your box is? If I had the same bulb and grow setup in a barn, do I need 80,000 CFM because the barn is 40,000 cubic feet? That makes no sense. For the same bulb and same incoming air source, you should need the same ventilation to achieve desired results no matter if the box is 35 cubic feet or 90 cubic feet (or whatever).

-r2k

 

[Oti$]

You are right, I don't grow in boxes, I do rooms and 2 air exchange keeps me in 5-8° of ambient temps in the houses. I'll defer to the cabinet guys. I should have read the specs before opening my mouth. I was trying to make it simple bc the op sounded like he needed it simplified. 2x formula had always worked to keep my temps in line and the air in my rooms nice and fresh, exchanging the air twice a minute makes sure there is fresh co2 available to the plants as well as keeping temps more or less in line with ambient.I usually run multiple lights in 10x10 rooms and it's always been more than adequate. Good luck with your grow!