Passive or Active Intake

[Tokensmoke10]

My grow cabinet is just over 9 cubic feet and was thinking about setting up a passive intake.

As you can see i took off the bottom board that went down to the floor so now there's an opening about 2 in off the floor that runs the front length of the box. This allows me the option of having a passive intake.

I've only heard good things about passive intakes but I'm unsure about the specifics. I'm going to be using around 250 watts of CFL's. But I still have a few questions.

1.To keep the floor of the cabinet still usable, would it be better to go with several smaller holes opposed to 1 or 2 large ones?
2. How big/how many exhaust fans would I need for a space of 9 cubic feet, I was thinking 3 80mm axial fans or 2 120mm axial fans.

Thanks everyone

 

[Trafic]

I have passive intake in my veg box and it works well. I cut a hole in the top of my box and ran a 6" duct into my flower tent, which has a 165 CFM fan pulling it. I still another fan for circulation around the plants but it stays around 77F.

You may need to tweak with your set up but make sure the intake is low and the exhaust is high and you should be alright.

 

[warfish]

If you have 2 fans you can use both of them for exhaust and have a passive intake, this effectively doubles the amount of air you are exchanging.

Warfish

 

[Tokensmoke10]

Thanks guys! But should I use 1 or 2 big holes for the intake or many smaller ones spread out among the floor board?

 

[PUFF MONKEY]

no need for an active setup for 9 cuft. my veg room is over 250 in it has passive air intake...my flower room is about 400 and it too has a passive intake.

 

[Tokensmoke10]

Did a quick little sketchup of what I have in mind.

What ya think?

 

[kasgrow]

I would spread the holes out to create an inverted shower of fresh air for your plants. That way all of your plants get a good flow of air through them.

 

[DonJones]

Tokensmoke10,

I can't find your sketch, but maybe I'm just missing it. Did it get posted because I can't find it?

Structurally several small holes would probably make more sense than just a couple of large ones. The several small holes will also let you spread the intake air openings around the floor better so that you get more even air flow and the chances of blocking the holes off with pots or something goes down too. A rule of thumb that I see used on passive intake systems is at least twice the square inches for the intake as for the exhaust. Also remember that it takes approximately 2.25 4" round holes to flow as much air as one 6" round hole, or four 1" holes to equal one 2" hole.

Then same thing is true of your exhaust fans. One 120mm should flow approximately 2.25times as much air as one 80 mm assuming that both fans are running at the same efficiency. Actually the difference might even be bigger because the portion of the opening blocked by the motor assembly should be a lower percentage in the 120 mm than in the 80 mm. Two 120 mm fans should move way more air than three 80 mm fans.

To retain the structural strength of the floor, you need to either reinstall the board you removed and cut large holes in it or figure out some way to support the middle of the floor because that front bottom board form one side of the box that supports the floor of the cabinet.

Also, even though it will restrict the airflow slightly, I would recommend installing metal ( it is much more rodent resistant than fiberglass and is only slightly more difficult to work with) window screen over the intake holes to keep critters and bugs out better.

Also, how are you going to seal the crack between the 2 doors to prevent light leakage? One way would be to cut a 1" x 4" piece of lumber, plywood, chip board or what ever you have the same length as the door opening, then attach it to the back side of one door with half of it sticking out past the edge of the door so that when the doors are closed, the board creates an overlapping lip behind the crack. If you are really worried about it then you could also put a strip of foam weather striping on either the board or the backside of the other door.

I think you are probably going to have to use some kind of a jam/batten along the top of the cabinet and down the sides to to seal the light from leaking around the side and top edges of the doors too.

Outside of those issues, I think you're going to have a great grow cabinet.

Great smoking.

 

[Tokensmoke10]

Tokensmoke10,

I can't find your sketch, but maybe I'm just missing it. Did it get posted because I can't find it?

Structurally several small holes would probably make more sense than just a couple of large ones. The several small holes will also let you spread the intake air openings around the floor better so that you get more ven ari flow and the chances of blocking the holes off with pots or something goes down too. A rule of thumb that I see used on passive intake systems is at least twice the square inches for the intake as for the exhaust. Also remember that it takes approximately 2.25 4" round holes to flow as much air as one 6" round hole.

Great smoking.

To retain the structural strength of the floor, you need to either reinstall the board you removed and cut large holes in it or figure out some way to support the middle of the floor because that front bottom board form one side of the box that supports the floor of the cabinet.

Also, even though it will restrict the airflow slightly, I owuld recommentd installing metal window screen over the intake holes ot keep critters and bugs out better.

Also, how are you going to seal the crack between the 2 doors to prevent light leakage? One way would be to cut a 1" x 4" piece of lumber, plywood, chip board or what ever you have the same length as the door opening, then attach it ot the back side of one door with half of it sticking out past the edge of the door so that when the doors are closed, the board creates an overlapping lip behind the crack. If you are really worried about it then you could also put a strip of foam weather striping on either the board or the backside of the other door.

I think you are probably going to have to use some kind of a jam/batten along the top of the cabinet and down the sides to to seal the light from leaking around the side and top edges of the doors too.

Outside of those issues, I think you're going ot have a great grow cabinet.

The image had the wrong format but I changed it, you should be able to see it now.

I think I'm sold on many small holes spread out as opposed to a couple large ones. But the board that I took out isn't what you think. It's purpose was mainly to just extend the cabinet down to the floor. It's still screwed in by the edges to the other sides of the cabinet, I think it'll hold up just fine

Great minds think alike though cause I was thinking the same thing you were about putting some kind of mesh/chicken wire over the holes!

As for light leaking, I was thinking about finding some sticky strip foam and attach it where there's a gap and see how it holds up. But what you said about weather stripping would work too!

Thank you for your input!

 

[PUFF MONKEY]

i'd try to put the intake ports at the bottom and clear it through the top...cool air in , hot air out.

 

[DonJones]

I see the sketch now. I would suggest moving your bottom holes towards the front more and maybe making 2 or even 3 rows so that he air flow come sup through the plants a little more evenly. You definitely want to get them away from the back wall or the fresh air will basically rise right up the back wall and out the exhaust fans without flowing through the plants.

Ideally the intake should be diagonally across from the exhaust, that is intake bottom front and exhaust top back in your case. That way the fresh air travels through the plants rather than just along the back wall.

I think the sticky foam you mentioned is the same kind of weatherstripping I was talking about. It is available in several different thicknesses and widths. It is a lot more effective if it is squeezed between 2 surfaces rather than with the door just closing against the side of it.

If you want me too, I'll try to sketch what I'm talking about but I'll warn you, my drafting ability leaves much to be desired.

Great smoking.

 

[Tokensmoke10]

I'll take whatever advice or sketches ya throw at me ha

and thanks for the help guys

 

[Tokensmoke10]

So something more along the lines of...

 

[LEFTHAND]

instead of putting the holes in the floor.. where it may reduce the air flow.. just put a few holes on either side near the front of the box this will allow a non restricted air flow.. unless you planned on raising your box a tad..
2 holes at the front of either side is what i would do man..
LH

 

[Tokensmoke10]

Well its hard to make in the sketch program I have but theres about 2 in of empty space beneath the holes. So it's pretty much hollow underneath the floor board.

 

[Trafic]

The only problem with several smaller holes is keeping the light out when you switch to 12/12. With one hole you can use pipe to block the light from getting in. With several smaller holes that would be a PITA.

 

[LEFTHAND]

The only problem with several smaller holes is keeping the light out when you switch to 12/12. With one hole you can use pipe to block the light from getting in. With several smaller holes that would be a PITA.

thats why i usually have possitve air coming in.. along with the exhaust.. in a small box its just easier to have forced air blowing over your plants insted of trying to pull it over. IMO.. comp fans work great for this prosess.
LH

 

[Tokensmoke10]

What ya mean by positive air lefthand?

 

[dragracer]

Positve air means using a fan or other source on your intake vent instead of just drawing it through with your exhaust fan...

 

[Tokensmoke10]

So lefthand doesn't think I should use passive intake?