Ventilation Chart and Calculator

[Flo Grow]

Ventilation is a serious concern for us growers.
Lots of ppl have questions about :

Passive intake
Active intake
Active exhaust
Fan size
Passive intake hole size(s)
How much CFM's for a light of a certain size
How close can I get my lights to my plants

And so on.

Hopefully the info below will help clear most of the confusion.

---------------------------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.

 

[pazuzu420]

Very nice post Flo Grow as I was just going to hunt up the formula for some other posts it will be nice to redircet people to this thread for chart. I wish I would of had this about 3 weeks ago before I cut my intakes as I had been asking around for a formula, although my situation is alittle less cut and dry as most would be.

I have two boxes that I want to vent with one filter and fan using passive intakes and ontop of that I need to save money as I"ve spent enough at the moment. I used a 12x24x1 HEPA filter on the main box which is 95ix42 x24 HxWxD. Then when I added the veg cabinet I 'guessed' and covered up alittle more than half the filter with a board that had a 6" 90 degree fitting in it. From there I used 6" insulated duct that connects at the veg box with another 90. The veg box is 48x24x22 with a Eureka HEPA filter that is roughly 5x5x1. The system runs though a 6" supersun master 2 400w light with a 6" vortex rated at 450 CFM plus a 6x20 carbon filter.

It seems to have excellent neg pressure in the main cabinet and currently draws in all round the veg box as it isn't sealed yet. When the flame is put up to the crack you can see it suck it in when the fan is running appox 20%.
Do you think these openings are adequate? I was concidering putting in a duct booster from the veg box to the flower box for when the flower box light is off as the fan doesn't kick on because the box doesn't get warm enough during that time.

Sorry the windiness but people don't often provide enough information when asking questions...

 

[faithfulmastiff]

hey flo, call me stupid, but i didn't get the degree numbers in yellow, only 1 - 10 ?

 

[pazuzu420]

The yellow number represents the number of degrees the room will rise at the given wattage and CFM's. So as an example according to the chart my 400 watt light combined with my 450 CFM fan will increase the ambient temperature 3 degrees or so that is the theory. Wow, with the 4" at 175 it would of risen 8 degrees, quite the difference and quickly explains the temperature differences I'm seeing in a lot of cabinets.

I notice the post just says active exhaust and passive intake but does that mean through an air-cooled hood? If not I could of bought a cheaper hood

 

[greenops]

Flow Grow, I have a 250w HPS and a 4 inch180m3/hr exhaust, which should be about 105 CFM? My cab is at 21 degrees when light is out but then rises to 29 - 31 when its on. So your graph applies to my situation.
I need to lower temps to 26 - 28 degrees. What would you suggest? Bigger exhaust? Or will a cooltube be better to lower temp? THANKS...

 

[infinitescrog]

I'm running a 600w Ultrasun HPS bulb, on a Lumatek electronic dimmable ballast (set to 600w), in a cooltube with an open end (nothing restricting airflow except a mesh screen to stop debris from entering, the screen provides negligible resistance. My grow area is only 2 ft x 5 ft x 5 ft and I have a 550 CFM fan attached to the cooltube. It pulls air from inside the flower room and exhausts it to the outside. My ambient temperature in the room is 68, and even with my fan turned down via speedcontroller I should still only experience 3 degree swing, but temps go up to 82. Anyone have advice? This is even with the door completely open providing almost no resistance to the fan, and I only have 1 90 degree bend in the ducting.

 

[Flo Grow]

Flow Grow, I have a 250w HPS and a 4 inch180m3/hr exhaust, which should be about 105 CFM? My cab is at 21 degrees when light is out but then rises to 29 - 31 when its on. So your graph applies to my situation.
I need to lower temps to 26 - 28 degrees. What would you suggest? Bigger exhaust? Or will a cooltube be better to lower temp? THANKS...

Sorry I haven't dropped in lately.
Your temps are 29 - 31C (84.2 - 87.8F) with lights on, and need to drop to 26 - 28C (78.4 - 82.4F).
Had to convert it so I can relate.
Bottom line, cool tubes suck and are the worst "hoods" to use for growing due to the curve of the tube actually reduces the amount of usable light than say a flat glass lens in a "traditional" air cooled hood.
I started with the exact same set up : Cool tube and 250w HPS - classic newb mistake.
It's now in my garage collecting dust because for pretty much the same price, I upgraded to a Sun System Yield Master II Supreme 6 inch and the yields increased.
The 400w Horti also helped....lol
What are the dimensions of your grow area ?

 

[Flo Grow]

I'm running a 600w Ultrasun HPS bulb, on a Lumatek electronic dimmable ballast (set to 600w), in a cooltube with an open end (nothing restricting airflow except a mesh screen to stop debris from entering, the screen provides negligible resistance. My grow area is only 2 ft x 5 ft x 5 ft and I have a 550 CFM fan attached to the cooltube. It pulls air from inside the flower room and exhausts it to the outside. My ambient temperature in the room is 68, and even with my fan turned down via speedcontroller I should still only experience 3 degree swing, but temps go up to 82. Anyone have advice? This is even with the door completely open providing almost no resistance to the fan, and I only have 1 90 degree bend in the ducting.

Just like GreenOps, you need to ditch that cool tube, especially running the most efficient HPS a grower can which is a 600w HPS.
That 90 degree bend is taking up to 30% OFF (165cfm) your fans rated CFM, or more depending on what kind of ducting you're using.
So now your down to 385 CFM's, not including any other lesser degree bends you might have AND whether you're using flexible ducting (the worst), semi-rigid (BIG improvement over flexible) or rigid ducting (the most efficient), to vent 50 cubic feet.
The fact your max is 82F is NOT a bad thing and is still very good temps to have.
My ambient air temps range from 78F - 82F year round and I pull good dry yields every grow.
But you still need to get a better reflector.
I use a tent that's 31.5" x 31.5" x 63" for about 32 cubic feet with a 400w HPS, and a 279CFM 6inch CanFan that sits OUTSIDE but on top of my tent.
I don't air cool, use my glass lens or have ANY ducting inside my tent because my fan vents STRAIGHT up and into the attic via 3ft of Semi-Rigid ducting.
Then I had to wrap that 3ft of ducting with insulated pipe wrap because the radiant heat coming through the ducting was 92F !!
That is heat that I'm SUPPOSED to be exhausting, permeating my ducting and coming right back into my grow room.
After the pipe wrap my radiant heat dropped to 78F.
I used a digital infrared thermometer for those temp measurements.
Get a better reflector and better ducting, if you're using flexible ducting, with little to no bends and I promise your temps will drop some more.

 

[infinitescrog]

So You're saying I should go with an open reflector? I was wondering if I could get away with an Adjust-a-Wing running sideways with a super spreader, and then just have my inline running exhausting the entire flower chamber.

I am using semi-rigid ducting, and I have insulated flex ducting I could strip the insulation off and throw it on the semi-rigid. I will switch to a different reflector in a few weeks after the next harvest.

 

[Flo Grow]

An open reflector may work for you.
Many ppl have good ambients with the Adjust-a-Wing.
You area is roughly the size of mine, so if you do the same, you should be fine.

Quick pick of my ducting venting straight up.

View attachment 1542570


Look in the top right corner, notice the cheap blue A.C. filter (can cut to size) covering the open to the tent.
RIGHT above it is where my CanFan enters the tent.
Then it's the previous pic where it goes into the attic.
My passive intake is an open floor flap on the back wall and to the left.
The flap right below and to the right is closed.
That way the cool air is pulled across my tent and plants.

View attachment 1542571

 

[azman]

I'm running a 600w Ultrasun HPS bulb, on a Lumatek electronic dimmable ballast (set to 600w), in a cooltube with an open end (nothing restricting airflow except a mesh screen to stop debris from entering, the screen provides negligible resistance. My grow area is only 2 ft x 5 ft x 5 ft and I have a 550 CFM fan attached to the cooltube. It pulls air from inside the flower room and exhausts it to the outside. My ambient temperature in the room is 68, and even with my fan turned down via speedcontroller I should still only experience 3 degree swing, but temps go up to 82. Anyone have advice? This is even with the door completely open providing almost no resistance to the fan, and I only have 1 90 degree bend in the ducting.

i had similar issues as this, it turned out that pushing air across the tubes was the way to go and dropped my temps from 84 to 73 exhausting the air out of the room. lol took me 3 days to finally get it right tho. i went cf,fan,cooltube,cooltube, outside.

 

[oOBe RyeOo]

Ventilation is a serious concern for us growers.
Lots of ppl have questions about :

Passive intake
Active intake
Active exhaust
Fan size
Passive intake hole size(s)
How much CFM's for a light of a certain size
How close can I get my lights to my plants

And so on.

Hopefully the info below will help clear most of the confusion.

---------------------------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.

This is confusing. How did you get 122in. squared for the 256cfm? if you do 256/300=.85333333

 

[MidnightJoker]

Hey FG,
After reading your chart I now see why you recommended going with a 6" carbon filter/fan rather than the 4". There is a huge difference in temperature there for me. Thanks again for the help.

 

[Flo Grow]

This is confusing. How did you get 122in. squared for the 256cfm? if you do 256/300=.85333333

I didn't make the chart, just found it a few years ago, but it appears they skipped a step :

256 / 300 = 0.8533333333333333 sq feet
Now convert to sq inches
0.8533333333333333 x 144 (sq inches in 1 sq foot) = 122.88
Notice the HVI recommends 1 sq ft of open air inlet per 300 CFM, so the 256cfm example is divided by the recommended 300 cfm's, then convert that to sq inches by multiplying by 144.
Then continue using the Pi (3.14), and so on.