Negative pressure in grow tent

2.5x2.5x5 grow tent, 80 degrees, 69 humidity, a floor fan running on one plant (the other is a runt) to strengthen. I have an inline fan carbon filter set up but it’s not in use BECAUSE when I turn it on it seems to make negative pressure and sucks in the entire grow tent in and then all the temp and humidity numbers go out of wack. Humidity drops to 39-something%. I attached a pic of my setup but with the door open the temp/humidity went down of course. One plant doing well, the other is Runt.

Do I HAVE to run the inline fan and carbon filter and what to do about the negative pressure please and thank you. Oh and pray for Runt lol.

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Negative pressure is good, means your exhaust fan is working. Open up one of your vents at the bottom, the screen covered holes, so fresh air can come in easily and it won’t suck in the sides of the tent so much.


@Hellraiser I experimented with the different vents and it helped with the negative pressure but dropped the humidity down to maybe mid to high 30%. So then how do I adjust that? Thanks for replying

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Probably really don’t need to filter the air now. Main purpose other than air movement as per Hellraiser is to control odor (do open intake at bottom of tent). I doubt if those girls are emitting much odor yet. Give them another month then you may need it. You can also run some string or wire from post to post about 3’ up. It will help the walls from collapsing in.


Yeah, that’s the conundrum, fresh air or humidity? Fresh air is way more important than keeping humidity high. I do whole grows under 30RH, because fresh air sucks away the humidity. I’ve learned that while nice, higher levels of humidity is not needed. Fresh air and the co2 that comes with and temperature control is much more important.

And as beardless said, no need to filter at this point, I only carbon filter in flowering.


Imo you should try to maintain the room that your tent is in for the best experience. if rh and temp in the room intake air comes from is good, it will be good in your tent.


Your tent is small and that is going to be your biggest hurdle because that makes your indoor environment small. Any changes made will be paramount i.e., exhausting the air, in a 4x4x8 it is a completely different scenario when compared to a 2.5x2.5x5 however the equipment used is basically the same. As mentioned by another poster, opening the bottom flaps will help you with the negative pressure by creating a passive cooling environment. However this also means you will lose humidity faster too. With that area being small and by the looks of the fan (assuming it is on a fan controller of some type) it appears that once the temps come up the fan will go 100%/max power and suck that tent in like a skinny girl on crack. You can counter this with a fan that modulates the voltage (fan builds up speed based on temp) this makes it so the fan doesn’t come on at 100%. Look into AC infinity T4(temp/humidity controlled) or S4 fans (temp controlled). These fans will make it a lot easier for you to dial in your environment.

I used a small tent like yours 3x3x7 for cloning and vegging, but when I used it as my main tent for veg and bloom it was a pita to get dialed in perfectly. You want the humidity, but when the fan comes on all the humidity is gone. You just need to control that exhaust a bit. You will need a humidifier for veg and you will need a dehumidifier for bloom to get max performance.

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@Arborfarms thank you for your reply. I could only get this size tent because we live in an 800 sq ft home lol. There’s just no indoor space. I do have a fan in there now and running 24/7. I am ordering two more for the top of the tent. I have a humidifier. Not sure if I should use the humidifier once I start using that carbon filter and fan in order to help with the drop in humidity when I open those vents. It sure is a balancing act but I’m determined. Runt is coming along far too slow. I started germinating another seed yesterday and I guess I’ll have 3 going in there then. Ut oh!! But really Runt is just taking up space right now. Not thriving…just there…should I off it or let it try to grow? The other Girl is doing well. I’m just on a wing and a prayer that something productive comes of this endeavor.!
These were both put in soil on the same day, 4/1.

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I’d cull that small one, no time or space for slow growing runts, they never catch up.


Totally understand about the space and that is a great sized tent for a few girls.

If it was me, you need one small oscillating fan for the tent and I’d get the AC T4 and stop there. That will get you moving in leaps and bounds in the right direction for control. You want the tent to exhaust about four times per hour.

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Are these autos?

What type of soil are you in?

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@Hellraiser thanks. That was my instinct also. Since I started germinating seed #3 yesterday, I’ll switch it out when #3 is ready to go in the soil. Thanks for the reply.

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@Arborfarms The big girl is in frog … Not sure I’m allowed to say it. Runt is in potting soil with some perlite added. Lessons to be learned I guess. I ordered ocean forest and I’ll put seed #3 in that. Gosh wish me luck. :four_leaf_clover:

Yes sour diesel autos

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@Hellraiser thank you. I need all the help I can get

Forgot to tell you this -

Seedlings need to be in small pots, red solo cups work great, clear solo cups even better so you can watch for the roots. They can be in solo cups for awhile (4th to 5th node)

Also very important, do not put seedlings into soil that has been amended with nutrients (potting soil). It will stunt growth, stress them, and possibly kill them from nutrient burn. Feed seedlings pH’d water only, I like a pH of 6.0

If you are using bottled nutrients the soil I recommend is coco coir, however you need to flush the hell out of it till the runoff is damn near the same ppm/ec as the water going in.

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I just ordered those yesterday!

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@Arborfarms. The Bergman page on these said they can go from gemination to their permanent pot. I did just that with both these. One is doing ok, the other is a goner, two different soils. My bad :disappointed_relieved:

Yea… soil matters.

To piggyback what @dbrn32 said… get a humidifier for your 800sqft. They are like 25 bucks, or you can splurge and get a whole house one for 250 or so. If you gotta go with the cheapo set it very close to that open intake vent, it will help some. You could try putting a small one i. The tent but those small tents are a bitch tat way.

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@Silverback I did end up putting a humidifier in there and it’s helping. Thanks…

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