Negative pressure 3x3 grow tent

First indoor setup. AC infinity inline fan T4. I turn it on and it sucks the walls in. Opening the intake flaps at the bottom creates light leak. How do y’alls remedy this situation? I appreciate it! Happy growing!!

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The only way to mitigate negative pressure is to have as much or more air going in than coming out. Having negative pressure isn’t a bad thing. Many of us do it to contain smell.


I have a 6" intake hose going out of the back of the tent to allow air to enter. The intake hose has a dust cap on it that also blocks out light. I still get negative pressure like @MidwestGuy said it is not a bad thing, but the air intake allows the exhaust fan to work less hard.


I do the same. I run both intake and output ducts. I have a 5x5 and run 8" AC Infinity both in and out.


I keep my side vents that are not on a wall closed. I keep negative in my 3x3 with just a little opening of unused circular vents. These tents are not airtight and can get exchange with all vents closed actually. More negative but the small space will do a total air exchange in a minute completely zipped up.

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I use bamboo from corner to corner to hold the tent out. Negative pressure sucks! My 4x4 tent was reduced to about 3’6"x3’6" when the fan kicked on, but putting 2 braces on each wall only sucked an inch or so.

In this pic, some had fallen off and I haven’t re-taped them to the corners. For the door, I use 2 bamboo and make an “X”. I tape the bottom corners only so that it can swing upwards where I have some wire to hold it next to the pole while I’m in the tent, when done it swings down back in place and is locked in by the fabric holding it to the pole. Ya, rubs the material a bit and stretches it a bit, but minimal light leak.

With negative pressure, you’ll always get some bowing inwards on your tent, but 2 angled braces a few feet apart keeps the middle from bowing in. Not too concerned with the bottom suckage since the plant is narrow there, but the middle and top gained about 4" back.


Damn, that pretty good. Ill keep that in mind when i try the grow. Just chopped my two outies like an hour ago and gonna try to dry low and slow in it. Fairly warm in Michigan these last few days so the garage is a little high on temp.

Also run an intake fan. A slight positive pressure is better for the plants, just run an appropriate sized carbon filter.

I keep the vent panel open on my 3 x 3. Also keep the 3" cable port open. There 2.5"-3.0" away from a wall with no light leak issues at all. Have a 6" AC-Infinity Exhaust fan that is on auto and is capped at #5 speed setting with three 6" circ fans in the tent. Two in the grow area, one above the light all on the lowest speed setting. No need to have hurricane force winds in your tent and a mini system as complex as a 50 story building. I also keep my exhaust fan and carbon filter external on the top of the tent with one end of the pre-filter sewed shut and used as a bag filter inside the carbon filter with the carbon filter shoved on the discharge side or the exhaust fan so the fan can move air without the stupid pressure drop of the flex duct, pre-filter and carbon filter destroying fan performance. And wasting space in the top of my tent.

Cut some 3/4 pvc and add it along the inside of the tent.

I have never had a negative pressure situation. I did have my tent blow up like a balloon when I had the exhaust fan plumbed backwards.


Hands down, the best way to go is with an exhaust fan and an intake fan, and put both on speed controls. There are many advantages to this! The only disadvantages that I have found is the cost, and the cheap speed controls can give the fan motors a high pitched noise.

Can you post a pic of the dust cap you use? I would be interested in seeing trying something like that to eliminate light leaks, but still get good air flow.

In one of my tents there is t5 side lights. After hanging them, I used some string tied across the front of them, and pulled the lights outward against the tent walls. This ended up with little or no reduction in area, and gained the benefits of side light.
The side lights do make a difference, but not sure if they worth it or not?
In my other tent of the same size. I have stepped up too a 300 watt led. Time will tell if the single led can match the production of the first tent with the side lights. The best that I have done with the side lights was 2 ounces per square foot, 12 ounces.

Since they are not presently needed the tubes are removed from the lights for cleaning.

I did mine on top externally too. Not sure if it produces much heat, but takes up some real estate. Got prolly another 8" of vertical space.

an easy fix is to run a length of flex duct through the intake port and bend so light won’t enter

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As a old HVAC guy with a 30 year back ground in very large scale engineering, building/processes, pumps, boilers, chillers, air handlers, ventilation systems one would think some cannabis growers are using there tents, enclosures or rooms as autopsy/morgue operations with all the air they think they need to move. Not to be mean, but it’s almost comical…

Just a simple light trap. People worry about light leaks like it’s some horrible monster. If you close yourself up in a tent lights off and you can’t read a large type kids book then your good to go. A zipper leak, pin holes, a led or other light pollution will not cause your plants to demand you use there choice of pronouns, stunt or any number of issues people think are going to happen. It’s shocking how many really don’t know the difference between a herm and pollination or how many grow out a genetic failure of a plant or a runt and it in there minds was caused by something else they had control over or should have had control over. When in fact it’s just a genetic misfire from all the fufu, candy ass bred up cannabis we have now days. Same with pest issues. Pest protection has all been bred out of modern cannabis…

@PhotoFinisH you helped me with this a few months ago with the cross sectional area. Maybe you could help here

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My partitioned 8x4 (i.e. 2x 4x4). Shared air /w separate light cycles.


AC provides temp-regulated inflow (venting exhaust outside), fan 2 moves air from flower to veg tent, fan 3 exhausts out the top through a filter.

The veg tent balloons from positive pressure, so I leave the zipper open a couple inches.