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