DDub
Well-Known Member
I have noticed many people on this site who have questions about Ventilation. Specifically how many CFM's are required to Exhaust their grow area. This is a very important thing to take into consideration when planning your grow room.
Many people don't know that there is a proven heat transfer formula to figure out your required CFM's.
**This Formula DOES NOT APPLY TO CFL LIGHTING**
First you must know 3 thing's before using this Formula:
1. We will call this (W) The amount of Light you will be using in Watts.
2. We will call this (V) The amount of Variance you would maximum like to have between your Normal Room Temperature and Growing Area again in Fahrenheit. (So if your Normal Room Temp. or Intake Temp. is 68 degrees Fahrenheit and you would like to have a maximum of 75 degrees Fahrenheit, you would use 7)
3. Of course to know #2 or (V) you must know the Temperature in Fahrenheit of your Normal Room Temperature or Intake Temperature.
The Formula is as follows:
3.2 x (W)
------------- = CFM
(V)
So if you have 1200 Watts of light, an Intake Temp. of 65 and would like to keep the Grow Area at or below 75. (W) is 1200 and (V) is 10.
3.2 x (1200) = 3840
3840/10 = 384 CFM
So 384 CFM's would be required to Exhaust this grow area and keep it at 75 degrees Fahrenheit max.
So now you have your Exhaust set up. What about the Intake. If you are going to use a passive intake there is another Formula to figure out the size of Intake you will need.
For every 300 CFM of Ventilation strength you will need One Square Foot of Open Air Inlet for a Passive Intake.
Here is the Formula, you must know (A) and (H)
A/H/Pi=Z Sq Root of Z x 2 = The Diameter each hole would need to be for Intake Area Needed.
(A) will be your Area in Square INCHES, this is how you calculate your sq inches needed from CFM used. You need to know the Sq Inches for the Formula. We will use the number used above of 384 CFM.
So 384/300 = 1.28 this is the sq ft needed
Take 1.28 and Multiply by 144.
1.28 x 144 = 184.32 square inches needed
(H) will be the number of Equal Circle Holes wanted
(Pi) is 3.14
A/H/Pi=Z Sq Root of Z x 2 = The Diameter each hole would need to be for Intake Area Needed.
This one is complicated so we will do a few.
So like above say we have 384 CFM of Exhaust. And we only want one hole.
First we must calculate the sq inches of ventilation required. I did this already for 384 cfm which is 184.32 sq inches needed. Now we divide by the number of holes wanted and we only want one, so the number stays the same. We take 184.32 and Divide by Pi or 3.14.
So 184.32/3.14 = 58.70
Now we take the Sq Root of 58.70 which is 7.66
Now we multiply 7.66 x 2 = 15.32
So we would need One 15.32 inch hole to act as a Passive Intake for our 384 cfm Exhaust Fan.
Now we'll do another say we have an Exhaust of 747 CFM. And would like 4 equal size Holes.
747/300 = 2.49
2.49 X 144 = 358.56 This is the amount of Sq Inches
Now 358.56/4 (Since we want 4 Holes) = 89.64
89.64/ Pi (3.14) = 28.54
The Square Root of 28.54 is 5.34
5.34 x 2 = 10.68
So we would need 4 holes of 10.68 inches.
Many people don't know that there is a proven heat transfer formula to figure out your required CFM's.
**This Formula DOES NOT APPLY TO CFL LIGHTING**
First you must know 3 thing's before using this Formula:
1. We will call this (W) The amount of Light you will be using in Watts.
2. We will call this (V) The amount of Variance you would maximum like to have between your Normal Room Temperature and Growing Area again in Fahrenheit. (So if your Normal Room Temp. or Intake Temp. is 68 degrees Fahrenheit and you would like to have a maximum of 75 degrees Fahrenheit, you would use 7)
3. Of course to know #2 or (V) you must know the Temperature in Fahrenheit of your Normal Room Temperature or Intake Temperature.
The Formula is as follows:
3.2 x (W)
------------- = CFM
(V)
So if you have 1200 Watts of light, an Intake Temp. of 65 and would like to keep the Grow Area at or below 75. (W) is 1200 and (V) is 10.
3.2 x (1200) = 3840
3840/10 = 384 CFM
So 384 CFM's would be required to Exhaust this grow area and keep it at 75 degrees Fahrenheit max.
So now you have your Exhaust set up. What about the Intake. If you are going to use a passive intake there is another Formula to figure out the size of Intake you will need.
For every 300 CFM of Ventilation strength you will need One Square Foot of Open Air Inlet for a Passive Intake.
Here is the Formula, you must know (A) and (H)
A/H/Pi=Z Sq Root of Z x 2 = The Diameter each hole would need to be for Intake Area Needed.
(A) will be your Area in Square INCHES, this is how you calculate your sq inches needed from CFM used. You need to know the Sq Inches for the Formula. We will use the number used above of 384 CFM.
So 384/300 = 1.28 this is the sq ft needed
Take 1.28 and Multiply by 144.
1.28 x 144 = 184.32 square inches needed
(H) will be the number of Equal Circle Holes wanted
(Pi) is 3.14
A/H/Pi=Z Sq Root of Z x 2 = The Diameter each hole would need to be for Intake Area Needed.
This one is complicated so we will do a few.
So like above say we have 384 CFM of Exhaust. And we only want one hole.
First we must calculate the sq inches of ventilation required. I did this already for 384 cfm which is 184.32 sq inches needed. Now we divide by the number of holes wanted and we only want one, so the number stays the same. We take 184.32 and Divide by Pi or 3.14.
So 184.32/3.14 = 58.70
Now we take the Sq Root of 58.70 which is 7.66
Now we multiply 7.66 x 2 = 15.32
So we would need One 15.32 inch hole to act as a Passive Intake for our 384 cfm Exhaust Fan.
Now we'll do another say we have an Exhaust of 747 CFM. And would like 4 equal size Holes.
747/300 = 2.49
2.49 X 144 = 358.56 This is the amount of Sq Inches
Now 358.56/4 (Since we want 4 Holes) = 89.64
89.64/ Pi (3.14) = 28.54
The Square Root of 28.54 is 5.34
5.34 x 2 = 10.68
So we would need 4 holes of 10.68 inches.
wowwee???
