Originally Posted by Fman

I am not sure if this helps.
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.

Originally Posted by Dradden

What is the CFM on the 6 inch and proposed 4 inch? Air pressure is tricky when you decide to add intake fans. It has to be very close to balanced. For that size area depending on CFM of the 2 six inch fans a 12 inch passive intake should be fine.hey dradden they are 150 cfm rated each and the 4 inch intake is 80 cfm's rated so your saying of i run a passive intake and my two exaust are 320 cfm = 12"total my passive should be almost equal say 10" or so..? also ill be using one strictly for cooling the light and the other for cab exaust..

Edit: FMan do you play L2?...sry but had to ask :P