Does the shape of a passive intake in a grow box matter?

T

TerraceFarmer

Member
I'm currently building a stealth (hopefully) grow box from a discarded cabinet I found at a junk pile. After modding, the volume should be just under 4 cubic feet. Tiny, I know, but I plan on doing a ton of LST for ScrOG. I bought an 80 mm. ultra quiet computer case fan that moves air at a rate of ~20 cfm (1100 rpm, less than 10 db!--allegedly), which I thought would be suitable for several changeovers a minute, even with light traps w/ air filters and a carbon filter (before fan, space permitting) at flowering time.

My question has to do with the number, size, and shape of the passive intake vent(s). I'm concerned both about the stress on my fan and the noise level. I've found conflicting rules on the size of an intake relative to the exhaust fan size--1.5x, 2-3x, etc. Which area multiplier would be easiest on my fan while still allowing for the negative pressure required by passive intake?

Secondly, if I have a round exhaust hole, should the intake hole(s) be round as well to keep noise down? I was thinking of just cutting a couple of square holes along the bottom edge in the floor of my box, which is elevated above the floor about 1". In order to limit noise, does each hole have to be larger in area than the exhaust hole, or is it the combined area of the intake holes that is important?

Apologies for the number of questions contained in this post. I'm new at this, and I really appreciate the advice of wise, practiced growers.
 
ASMALLVOICE

ASMALLVOICE

Well-Known Member
Hiya M8,
Your intake needs to be 2-3 times the exhaust size, this can happen in many ways, 1 hole to multiple holes. If you go with pc fans, you should have no problems with the noise. Start with 1, check flow and noise and then add another. should you make 1 too many, easier to plug than 1 large hole, or trying to shrink it down to what you need. Remember, intake low, exhaust high.

Peace and Good Grows

Asmallvoice
 
M

Michelangelo00

Active Member

  • 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.

    Source:https://www.rollitup.org/grow-room-design-setup/419563-ventilation-chart-calculator.html    ​


    ​

 
B

berkman858

Well-Known Member
You could just do an active intake and get another fan. I have matched intake and exhaust fans. This way your intake and exhaust holes are the same size and your air flow just about doubles.
 
M

Michelangelo00

Active Member
The problem with matching exhaust and intake is positive pressure. Pulling through a hose, filter, or lights decreases exhaust. If intake > exhaust, the filter can't do it's job because air is pushed out through cracks. Smells escaping is bad for business.
 
T

TerraceFarmer

Member
In a box this size (~4 cubic feet), will I need an oscillating fan inside, or should I be able to get good air distribution throughout simply by smart intake hole placement?
 
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