Passive Intake Question

New grower, and I’m almost done setting up my 4’x4’x73" grow tent (pictured below). I’m using a AC Infinity CLOUDLINE T6 exhaust fan, and 2x HLG 260W XL V2 Rspec Kit. I’m planning on just doing a passive intake with the leftover 6" flexible duct that I have left, but had two questions.

  1. Does the length of the passive intake duct matter?
  2. Should I have more than one passive intake duct?


I don’t follow your question

It looks good - obviously just the filter when your in flower and an intake fan , you need air coming in at the same speed your taking air out , simply that

You will get various feedback. I have one open intake for my 3x3x7. However don’t use an intake fan on it, and it’s about 3 feet away from tent, and I put an S shape on it, to keep out light, am in veg stage. Hope this helps

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Just so long as it’s long enough that it blocks the light, and not too long that it restricts the airflow. The longer it is, the less air you’ll be able to pull through it with the same exhaust settings. You’ll get a negative pressure in the tent, causing the walls to suck in, the longer the intake is, un"boosted".

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Just like any ventilation system you need a straight passage of air flow to promote a better circulation , you don’t need two In this set up

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It will depend what speed you run it at. If that 6" Infinity is running on speed 10, one 6" passive opening will not keep up and your tent will look like it’s getting sucked in. Try for at least two. I used to use flexible duct, and make several S turns to block the light. I even used black duct tape inside the 90’s to darken it up. I don’t do that anymore. Now I use dark colored towels, and strong little rare earth magnets from Amazon. I open the 6" socks on the tent, roll them up like a shirt sleeve so it’s just a 6" opening. Hold a dark towel over the opening either inside or outside the tent. Starting at one corner of the towel, hold a magnet on the outside of the tent, and place another magnet against it inside the tent. I put magnets every 3 or 4 inches or as needed around the towel, and I do all the available socks on the lower half of the tent for intake. For me it works better than the flexible duct lines. I wash the towels after each run, but also I’ll change them out if I notice the airflow getting restricted.
Looks like you’re set up nicely. Do you have a filter to go with that fan?


@Cap_Ron, I learn so much from this site everyday. I have been running the process of ensuring I have 100% blackout in my tent through my brain, and the magnet process makes so much sense and reason. Great idea!

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I just use the lower vent slots and open them…one or both depending on what needed based on temps and humidity and exhaust fan speed etc.
I don’t worry about light leaks as the tents are in basement and windows are covered so when light are out it’s dark
I have a 4” in line and have hooked it up but really didn’t need it nor the extra electricity used.
Everyone’s environment is different so you’ll need to play around with things and adjust as needed.


I have a peace of 6 inch Flex about 5 ft long in the bottom pass through hole laying on the floor with a slight Bend in the flex.


I hear you @Zee, I live in a climate that’s really freezing in the winter and pretty darn hot and humid in the summer. So I’ve had to play with ventilation and air conditioning issues a lot.
These are the magnets I use. I get the 30 packs because I find uses for them ALL over my grow rooms.

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Thank you all for the suggestions. @Cap_Ron that’s an interesting idea with the towels, might have to try that. Are the towels porous enough to allow air in? Yes I have a 6" Vivosun filter

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Hey @BeerGeekGamer, I originally tried it with a towel on the outside and another on the inside because I didn’t know if one would block enough light. The two towels together slowed down the air flow a little too much, but just one on the outside or inside blocks enough light (in my situation) and allows enough air flow. The towels do trap a lot of dust (and cat hair in my place, lol.) so I make sure to wash them after each grow or as needed. Also, good to note, just because you can see some light leaking out of your tent when the grow light is on and the room is dark, doesn’t necessarily mean that light from your room will be strong enough to leak into the tent. Grow lights are typically much brighter and stronger than our room lighting, so I learned to make sure it’s a problem before you spend time and money trying to fix it. Just climb in your tent with every light in the room on and your grow light off. Have someone zip you up inside the tent and look for leaks. Even have them shine a flashlight at the vulnerable spots like zippers and socks. Then you’ll know for sure.

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@Cap_Ron thanks for all the info. One other question. I haven’t really looked into yet, but was planning on doing negative pressure in my tent. I saw you mentioned about the tent sides being sucked in. We’re you trying to say, that, that’s something you don’t want to see? Thought that was an indication that you have negative pressure setup “correctly”.

I try to keep my tents as evenly pressured as possible @BeerGeekGamer, but if I lean either way I’d rather have a bit of negative pressure. My exhaust fans are running the majority of the time, it’s rare that they’re off. During lights on the fans are exhausting lots of heat, and during lights out there’s usually humidity to deal with. I keep all the available 6" socks (ports) open (with my towels) or I find the negative pressure gets too strong. If we’re in a dry or cooler period and my fans aren’t running as much, sometimes I’ll start smelling my plants. This is due to a lack of negative pressure, and I close up a port or two temporarily.
One more thing to note. The tent being sucked in is a sign of good negative pressure like you said, but it also takes space from your plants, and you don’t want the tent squeezing in and pressing on your leaves. To get around this, I cut a length of bamboo the width I want the tent to stay during negative pressure. I wrap a piece of tissue over each end and rubber band it so it doesn’t rip or damage the tent material, and I put it in place as I’m zipping up the door. I’ll include a pic of my tent bulging out in the middle even though it has negative pressure, and a piece of the bamboo I use for it.


So here’s what I did. Took some of the 6" flex duct and sandwich one of these round black carbon filters I found, in between some mesh screen, and strapped it on the end with a worm clamp.

  1. Increasing the length of a duct increases static pressure, which basically decreases the efficiency of your fan.

  2. I would open multiple ports to get more flow, but I’d also place a fine screen over all intakes. Hosiery would work, although maybe people are still making leggings into masks.

The cool thing is that small increases in diameter significantly increase surface area. So instead of using 6” intake duct, try 8-10”. Maybe you can repurpose a tubular container instead of buying a length of duct.

I use a 4" AC Infinity in myb3x3 just keep the passive air flaps open on bottom and ya be just fine :slight_smile: