Active vs Passive Air Flow

Hey Guy’s,
I’m looking to upgrade my setup this summer before I start another grow closer to the fall. I’m looking for some advice as I battled high temps this spring. So I have a 2x4x5 grow tent in a closet for both stealth and convenience. I currently have a 4" fan with carbon filter pumping outside through a window. I have a hole knocked out to pull air from another room through the tent. I guess my question is has anyone used an active setup? Is it worth the extra noise and utility costs? I’d love to grow year round but I don’t see a way to keep my temps in the high 70s with the summer sun either. Thanks for any suggestions and happy growing!

Hi Chris, Not sure what you mean by ‘‘active’’ , but 4’’ might be a bit small for that size tent, i have a high output 4’’ for a 2x2 & its barely enough when it gets warm outside…

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I have an active intake in my 2.3 x 2.3 and this is what I’ve found so far.

I started with just a 4inch in-line fan attached to a 4 inch carbon filter to exhaust the air in the grow tent but even at full blast and vents all the way open in my tent, my LED grow lights were constantly trying to push the tents temps over the 80-85f mark.

So I decided to upgrade to a 6 inch fan to help increase the amount of air exchange that was happening inside my tent and hopefully that would bring the temps down.

While this did lower my temps dramatically, (about 10 degrees), the negative pressure inside my tent was so high my humidifier couldn’t keep up with the air exchange and my RH started to drop dramatically.

So I took my old 4 inch fan and carbon filter and created an active air intake. While this helped a little bit, it still wasn’t enough even with the 4 inch fan on full blast and the 6 inch fan on minimum, and I still had to create some screened air intakes with my left over 4 inch hose.

As long as there is a 180 degree curve in your tubing (a U shape) you won’t have any light leaks and the screens keep the bugs out.

In conclusion I would just upgrade to a six inch in-line fan and filter and just create passive screened air intakes to feed air in from that other room, and help dial in your negative pressure inside the tent.

At this point the 6 inch exhaust fan is the one really doing all the work for me and my 4 inch active intake fan/carbon filter combo are basically just moving air from the bottom of my tent to the top for better air exchange, while also working as a back up carbon filter for any smells that happen to escape the 6 inch carbon filter attached to my exhaust. I’ll include some pics for reference. Hope that helps!

Picture of both fans at work. The fan and filter on the floor ante my active intake. The fan on top is my exhaust.

Picture of a screened air intake

Picture of how I ran the active air intake to blow air from the bottom of my tent to the top.

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Man thank you! That has been the best explanation I’ve found. I really appreciate it, I think I’m gonna grab a 6" fan. How did you know you had too much negative pressure? I’m curious what to look for when I get setup.

Thanks Mike! By active I meant an inline fan pushing air in to my tent. It’s a process i was reading about but don’t know all the details.

got it, if you can pull air from a cool space, it would really help, a good 6’’ exhaust fan will pull in the same cfm as its blowing out, as long as it does not create a negative pressure, ultimately you need the same available intake as you are trying to exhaust & if your exhausting to the outside, then you should also pull the air from outside, so you just have a steady air exchange without positive or negative pressures.

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I offered 90, then 95 and got this on ebay
vivo 6"

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Thanks Mike,
Other than the sides sucking in is there a way to tell if you have too much negative pressure

not without airflow meters, but if the sides are sucking in, you just need more cubic inches of intake opening, whenever you add a filter it will restrict the airflow, so if you are forcing air out with a 6’’ fan, you may need 2 or more 6’’ filtered openings to let air in… its just a matter of balancing the intake to exhaust… i hope this helps… im a beginner indoor grower, but i spent many years working in the hvac industry…

I adjust (the vivo has a speed adjustment) the air volume to keep the Neg at a minimum. When it gets hotter, I open MORE of the LOWER vents (I want air intake from the bottom (fresh CO2) and expels from the top. My filter resides outside my tent…I want as much height space as possible. You can install intake fans (even the small 4" at any hardware store - $30) if needed