Negative pressure in tent

So im using a vivosun 4x4 and while in veg i kept the bottom flaps open to help with air flow but now that I’m in flowering i can no longer do that but when i close them up i get negative pressure and i also cant turn down the 6’inch inline fan because it helps to keep humidity out… does anyone have any tips to help out

Well u need some kind of passive airflow…can you not use the flaps bc of light pollution? If so use some ducting and put in a couple of bends on it, a 5 ft piece out of both lower duct ports. Negative pressure is a good thing but you have to have some kind of passive airflow or your going to burn your fan out.


@MrRobot are you trying to create negative pressure in the tent to hide the smell? I also use a Vivosun 4x4 tent and pretty much eliminated the odor issue this way. I ran the 6” exhaust to a double fan window unit. 1 fan works with the tent fan to pull from the tent and the other fan pulls the room itself. With the rooms door closed you’d have no idea I’m in full flower mode today.


An exhaust fan and an intake fan with speed controls on both. Is going to be one of your best options.


I read online to much negative pressure makes it hard for the plants to breathe thats why i thought it was bad…but right im currently running a 4 inch inline passive fan

No im not trying to hide the smell…the setup hasn’t given me that issue

I use this 6” fan to pull in fresh air and blow it over this waterwick bucket. My ladies seem pretty happy. I was confused about what you were seeking. My mistake, take care and good growing!!

In my opinion a slight negative pressure is fine. But I think the plants take a deeper negative pressure as a sign of bad weather is coming, and slows down growth somewhat. But I don’t think this has ever been proven with any research.
If smell is not a concern a little positive pressure can be a good thing. You wouldn’t have to deal with the tent sides sucking in. You can get a cleaner tent by filtering the intake air and maintaining a slight positive pressure.

Hadn’t heard that, but you know adding an intake fan is an option, I use them. I want maximum air exchange but also still a bit of negative pressure to contain the smell. So the intake always runs a little slower than the exhaust. It also helps me dial in my temp and RH to some degree, if I run them faster it gets cooler and drier, a little bit.


Agree with @CurrDogg420, my speed controls make it so easy to set a negative pressure, neutral pressure, or positive pressure.

Running the exhaust on the lowest speed is still creating a good amount of negative pressure i tried to run the passive fan at full speed but it doesnt help… the tent walls suck in a lot but when the light switches on i will try the duct idea

Just an another tip here. 4 inch flex duct ready kills air flow, and you do not want to run more than about 10 foot lengths. A 6 inch duct will work much better for a long run, or a passive cooling duct.
I really think you will end up with an intake fan to get things balanced out. But I can’t see first hand what your site situation is, so don’t let me discourage you from trying your ideas.

Added in a 4inch vent helped but will change to a 6inch…my current setup


What’s in the third picture? Is that what you are calling a passive fan?
If so do you have a 6in exhaust w carbon filter and a 4in intake fan w a filter?

Yea to the 6inch exhaust and 4inch intake…this is first grow so pls let me know if im doing something dumb

OK, it sounds like the exhaust is oversized to the intake. If you are using something like a HEPA filter on the intake it is really going to cut down on what the fan can deliver. You may want to use an end of duct type filter on the intake to increase air flow. A filter that breaths easier on the intake matched with the larger exhaust with the carbon filter will help. If this doesn’t get you where you want to be. You can add a 6in passive intake duct, (no fan) just a duct with a end of duct filter on it. That should get rid of most of the negative pressure.

Yea adding 2 6inch ducts fixed it

Nice set up, and your plants are looking good to me.

Thanks…hopefully everything goes smoothly


What strains are you doing?