Negative pressure vs positive pressure?

What’s the science?

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Negative pressure inside the tent keeps smells in…
Positive pressure does not.


Here is a paper on pressure differential :

Probably not what you meant. :grin: Going from my anecdotal experience, with weed, recently I had to change my exhaust filter so I experienced both conditions.

In a negative pressure environment, everything goes through the exhaust filter, so no smell. With positive pressure in the tent, I could smell it all through the house.

Now, if I was growing mushrooms, or I was growing something to where I was worried about mold, I’d want to intake filtered air, and I’d want positive pressure in the tent to keep spores from getting sucked in.


Your exhaust fan running 24 /7 will create a negative pressure inside tent An control odor. If you choose you can also leave a pass through hole open at the bottom which will pull fresh air in. :v:


I have an intake with hepafilter but if I run ecsaust fan I loose heat. If it’s just for smell I get it. Thought it was something more.


The vacuum effect from the exhaust fan allows me to pull enough fresh air in the bottom of my tent to satisfy the plants. If you don’t need to worry about smell you’re good , open the door for fresh air.

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I had been wondering the same thing. Have a big 6” in-line fan with filer for exhaust, and gonna hook up a 4” small in-line for passive intake so would fluctuate between passive and negative pressure when the big fan kicks on for hi temp or hi RH.

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I’m not sure what your configuration is like, but I sounds similar to mine. Here’s what I do and why:

I’m growing weed, so I have my exhaust fan running 24/7 to deal with the smell. I’m growing in my basement, so my temp never dips lower than 60 all year. That’s just fine for sleepy time when the lights are out. If your environment is cooler than that then my advise probably isn’t the best thing for your situation.

I have an AC infinity intake fan, with the controller, which I use in part to manage the heat with the lights on. On the controller, I program the high temp to the point where I want the fan to come on. That point for me is 74-75 degrees with the lights on.

Below that range, the temperature becomes my limiting factor for growth, and the CO2 boost provided by the intake air isn’t as important as keeping the heat up. So my intake fan turns off below that point.

I do want the intake air to be on normally when the lights are on, so as things have cooled off in the winter, I lower the speed of both my intake and exhaust so I can try to hit that 75 degree mark with the intake running.

My exhaust fan always runs a little faster than my intake, so as to maintain negative pressure in the tent. You can test this by zipping the tent closed. If the sides “suck in,” you have negative pressure. If they bulge out, you have positive.


Ya it’s 28 degrees out side my tent. So I run my 6inch exhaust fan into my 4inch fresh air and end up with positive pressure. I retain some heat and humidity.

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