Here’s how you can calculate the amount of ventilation you need. If for instance you wanted to keep you grow room temperature from getting any more than 5° warmer than the intake air temperature, and you were using 400 watts of power, you’d make the calculation below in blue. The chart is based on the following formula. It is a well-established heat transfer formula.


(3.2 × 400) ÷ 5 = 256






---------------------------Calculating the passive intake.-------------------------

The Home Ventilating Institute recommends one square foot of open air inlet per 300 CFM of ventilation fan capacity.

If you were going to use 256 CFM, you’d want 256/300 square feet of intake area, which is 122.88 square inches.

Here are some options for the intake area for a 256 CFM ventilation fan:

1 hole - 12.5 inches in diameter.
2 holes – 8.84 inches in diameter.
3 holes – 7.22 inches in diameter.
4 holes – 6.25 inches in diameter.
5 holes – 5.59 inches in diameter.
6 holes – 5.11 inches in diameter.


Here is how to calculate the hole sizes:

1. Take the total area in square inches needed, in this case 122.88 square inches, and divide by the number of holes you want.
2. Then divide by Pi (3.14).
3. Take the square root of that value.
4. Then multiply by 2.

The answer is the diameter that each hole would need to be to make up the total area needed for intake.

A large number of small holes will create more backpressure than one large hole of equivalent area. This would be negligible unless you’re using a huge number of holes or you’re using ducting to supply the air to each intake hole. If you’re just cutting them in a wall you should be fine using 8 or less holes without having to take into account the extra backpressure.

wow video, you are posting away today, good stuff!

i know you just explained all that video. but i suck at math and lets say my FOAF had a rubbermaid setup 5 feetaway. Right now theres 2 1.5 inch holes for passive intakes. running 5 "100w" 26w CFL's Temp is at 81 with the window open and its like 70 outside. It has been a temp battle for me with 5 n 6 CFL's in the sockets. running a big exhaust with a rather big hole too.

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i am a pathological liar, and have illusions of grandeur. I post pictures of pot plants i found on the internet and i have never nor will i ever consider the cultivation of marijuana, as it is illegal.

I just double checked the numbers and the formula seems to be off a bit for my setup. I flipped your formula though. I have a 400w HID and a 120cfm bathroom fan for my exhaust with a 4" output and about 15' of ducting on the on it.

So 3.2 x 400w = 1280 / 120cfm = about 10.6666667°F temp change. Correct?

In my room with the light off the temp is about 20-21°c (68-69.8°F), when the light has been on for a while the temps are about 24-25°c (75.2-77°F). I have never seen it change more then 4 or 5°c (9°F). I also have a 'active' air intake rather then a 'passive' air intake, would this be why the formula doesn't work for me?

Thanks.

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No, as far as i see this is about passive intake here and not active - since you then being blowing air forcefully into your grow this has to be as much as you evacuate from the room too, that is all you got to check with your setup then.

as far as i get video this is about how much air do you need to cool a bulb/grow room down - the second about how big your passive intake has to be.

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Green dude, is there a question in there somewhere?

Squirt, most probaly yes. I have been using this formula for a long time and it's always worked for me. The formula does no take into account a active intake, its assuming passive.

nevermind i figured it out ;P

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i am a pathological liar, and have illusions of grandeur. I post pictures of pot plants i found on the internet and i have never nor will i ever consider the cultivation of marijuana, as it is illegal.

okay let me spell it out for you all, if you have a 400 watt light, multiply that times 3.2 which equals 1280 than divide by the cfm of your fan, lets assume a 500 cfm fan for this scenario, so we have 1280 divided by 500 equals 2.56

This means that using a 400 watt light and a 500 cfm fan your temp will rise 2.56 degrees above ambient temperature.
Peace

Instead of making a ton of holes, just buy a stanley blower for like 40-50 bucks or 8 to 40 bucks when on sale. The Stanley blower is ideal for ventilation fans, is very quiet, and moves a good amount of cfm.
the cfms are
1282 on low
1746 on med
2181 on high
There is also a mod you can make to it so you can use it as an inline rather than just a squirrel cage. The cfms are before the mod, so expect it to drop once you add ducting. Overall, the results will shock you for the $50 invested though. Click Here to see the mod and more pics of the fan.

Attached Images

	stanley.jpg (47.5 KB, 15 views)
	blower.jpg (27.3 KB, 14 views)

Dankster,

That thing looks like it could do some serious work....good bang for your buck. thanks for sharing.

Ice

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The smoother the inside of the duct the better as air creates friction and if something sticks in like a bump or screw or whatever it is creates turbulence which creates a resistance to flow. Curves and 90's are what really slow the air down so try and have a straight a run as possible.

As for how big you need to determine the cubic ft capacity of your room. Then buy a fan that will change all of the air in that room in 7 minutes. For instance you have a room with 245 cu.ft capacity you want to buy a fan that moves all of that out in 7 minutes, so 245/7= 35 so you would need a 35 cfm fan capacity = L x W x H.
Now for a bong hit. Hope this helps.

How did you get the 122.88 square inches?? Wouldn't it just be 1 sq ft or 12 sq in of intake area?

"""""If you were going to use 256 CFM, you’d want 256/300 square feet of intake area, which is 122.88 square inches."'"""""

Please explain.

BTW

I have a 150 watt hps in a 30 cubic ft grow closet with a 4 inch, 70cfm panaflo computer fan pulling air from the top. My intake is a 2in X 20in long rectangular intake at the opposite end of where my fan is.

My temps get slightly above 80

World Peace,
ILL

most 4 inch inlines draw 170cfpm......and ive never heard of anything smaller other than pc fans. for the seven minute rule that adequatly ventilate most peoples grow space (unless your like me).dont forget that you have to have unrestricted intake as well as exhaust. i use passive venting in my room, at a rate of double my exhaust. theres no reason for this beyond fresh air, and the fact that the excess pressure from having more intake than exhaust helps keep my reflective material from ballooning inward towards my growspace, helping to elimuinate hotspots. hope this helps and not confuses you

I've added to this post to finish the rest of the equation. I'm just trying to explain videoman's formula a little better because a lot of you still don't get it.
Yea no prob, it definitely cranks the air in and out fast. I've invested in four of these bad boys for my rooms' ventilation system. I think these guys could surpass some of those cans fans easily, especially in price. You can't find them at Wal-Mart in this season but you can order them at ace hardware or online.

I want to hopefully simplify things for everyone because everyone seems to be confused with what to do in the heat transfer formula. You can use your computer calculator with it if you'd like. First I want to make this clear.

Length x Width = Area (expressed in Sq. Feet)
Length x Width x Height = Volume (expressed in cubic feet)
CFM= Cubic Feet Per Minute (volume)
Squared= Number multiplied by itself (ex: 12in. squared = 12in x 12in.= 144sq. inches(area)

The first part is fairly simple. I think 3.2 is the radiant heat factor produced by one bulb. If you have more than one bulb I think the formula would probally be:

3.2 x (# of bulbs) x (# of watts per bulb) ÷ (# of maximum degrees above intake temp desired)=to get the CFM needed to maintain your desired temp.

So if your intake air is 75F and you don't want the room to go over 80F thats a 5 degree difference. If you are using 4 bulbs with 400w each bulb the example would be.

ex: (3.2 x 4 bulbs x 400w each bulb)= 5120 ÷ 5F degrees difference maximum above intake temp = 1024 CFM needed.

You can use a stanley blower on the lowest setting for your exhaust fan. Now to calculate the size of the hole for your PASSIVE air intake. We will base it on this statment.So we need to take the 1024CFM needed ÷ 300CFM to get= 3.41 Sq ft. needed for intake opening.

(I am rounding to the nearest hundredths place on these equations.)If you don't have enough space to make a 3.41 Sq foot hole, you can make 3 and 1/2 holes that are 1 sq. ft each. If you still don't have enough space for the holes, you can convert it to square inches and add more smaller holes.

12inches in a foot so 12in. x 12in.(squared)= 144 sq inches.
144 sq inches x 3.41 sq feet = 491.04 sq. inches. total area needed.

You can break it down into smaller circular holes with the rest of the formula using the 491.04 sq in. if you'd like.Lets say you want 7 holes

1. 491.04 sq in./ 7 holes = 70.15 sq in.
2. 70.15sq in. / 3.14pi. = 22.34in.(for circular holes)
3. √22.34= 4.73in.(radius) Use this number if you are using a compass to make the hole outlines. I recommend you do it this way.
4. 4.73in. x 2 = 9.46in.(diameter) mesasure this length both ways(horizantal&vertical) in a cross and draw a circle around that.

Or you can guesstimate.LOL Wow I hope I didn't make things more complicated. Another option is to get an intake fan equivalent to the exhaust. Good Luck.

First off this is a thread over a year old and the person who started the thread has been banned from the site so there is no use asking him any questions

Second I do not know why you make pot growing so difficult. I grow it the simplest way possible and would pit my smoke against any other. The 7 minute change of the air is an industrial calculation, me being a retired maintenance guy I have had to cool some pretty big rooms. In one pressroom for example I had to install 66,000 cfm of fan to follow the 7 minute rule. That was 6ea 11,000 cfm fans.
The room intake needs to only be as big as the fan the same with the exhaust.

No, 12 sq. in. is NOT equal to 1sq.foot.
1sq. ft.= 12in x 12in. = 144 sq in.
But, you can use 1sq ft. for the hole, it would just be a little extra space considering he only has a 256 CFM draw. This is how he got it.
256 CFM ÷ 300 CFM = .853333333 (85%)
So he only need 85% of the 1 sq ft. per 300 CFM recommended for his hole.
.853333333(which is 85%) x 12in x 12in (which is 1 sq ft.)= 122.88 sq.

If your intake temp is 75F and you didn't want the cab to get over 80F. Use the 5F degree as your difference.
3.2 x 150w = 480 ÷ 5F = 96 CFM needed to maintain between 75F to 80F
The institute recommends 1sq ft. per 300 CFM and you only have 96CFM.
96 ÷ 300 = .32(AKA 32%)
So you only need 32% of that recommendation.
.32 x 12in. x 12in.(AKA 1sq ft.)= 46.08 sq in. hole size for your passive intake. Use video's formula to get the circular hole sizes or get the square root of 46.08 sq. ft. to get your length and width for a square hole.
√46.08 = 6.79in width and 6.79in. height
Step up you exhaust CFM to 96.

Good Luck. Peace

Thanks Dankz.

What if I add another fan of the same CFM? Would that help? Thanks.

ILL

I know all the calculations work and it has a basis in science. I get that. What I have learned is measure the size of the exhaust fan and double the passive intake in to the room. It works however you figure it. SO my 6 inch vortex exhaust fan needs 12 inches passive cold air coming in to equalize the pressure. 2 X 6 = 12. I have 3 straight 4 inch passive cold air vent pipes coming into the grow, low near the floor which pushes the hot air UP. 3 X 4 = 12. Air will not allow a vacuum. If the pressure is not even you will get that sucking affect that pulls plastic IN. So as fast as it is pushing the hot air out, it is pulling cold air in.

Easy peasy in my mind. What I see Iliketolearn, is that you have one small 2 in. hole for the cooler air to come in. Even with such a small fan, 4 inches, you still need 8 inches of passive cool air intake low in the grow. This will by sheer force cool your space better than you have it now. You can complicate it all you want, but it is really very simple. And also, having a fan in the cool air intake can SLOW the air exchange. Air will come in faster with a passive vent, just with the fan. I would also think you might need a bigger exhaust fan by the end of all this. A little pc fan is not extemely powerful but will do at first.

Peace

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My Little Grow LST'ing w/Hardware

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All these formulas get my head spinning faster than my fan!!!

Here's my set up, closet room size 8 ft tall by 27" wide by 51", or about 80 cubic ft of space. I've taken a through wall 4 inch length of dryer duct, stuck it through the ceiling, and ran 4 " dryer duct to a 60 CFM squirrel cage fan mounted directly to the ceiling smack in the middle of the room directly above a 430 watt HPS light. Then in the corner of the ceiling,put another 4 inch dryer duct through the ceiling and dropped 4 inch dryer duct the floor for a passive return. My idea is by mounting the fan above the light, the HPS will have a steady airflow from the bottom up, and the cool air will be sucked through the passive return. I may duct it directly to the light reflector later, since it's setup for that. Since the room is only about 80 square feet, and sealed pretty tight, I expect a fairly rapid air replacement, but this has got me thinking I may want to add another passive in take possibly one in the ceiling without ducting it towards the floor. Being it's the dead of winter I'm a little concerned about sucking cold air around the root system.

I'm new at this, but all this info and ideas are sure helpful. Does this sound like a reasonable setup to you guys? It was pretty easy, just went to Lowes dryer section and you can find all kinds of ready made fittings. The through wall stuff is very easy, about a 2ft 4" tube with plastic 4 hole square wall mounting, just cut a hole in the sheetrock, slide it through to the attic space, which has a ridge vent the length of the house, voila, you're done. Stealthy, and easy!!!

Oh, in the interest of full disclosure,I haven't started to grow here yet!!! Trying to get all my "ducts" in a row before I begin, which I'm planning to sprout next week! Any suggestions greatly appreciated.

I have found at this time of year if you don't want to freeze your plants and are in a cold unheated zone as my grow is, it is not nesacary to have full cool air return. It will get cold in there. IF you are having a problem with the heat then yes, add another vent the same size to your closet. As I said before, 2X the size of your exhaust is what you need in passive air coming in. SO you have a 4 inch exhaust to balance that you need 8 inches passive cool air in. At this time of year I do not have my exhaust running full tilt so I have one blocked off with just a plastic container.


Peace

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Life,
j-angel

aka GP purrie kittie attack cat!


My Little Grow LST'ing w/Hardware

My Outdoor Odyssey 2008 BONSAI MUMS

My Little Grow Indoor To Outdoor 2009