What is the benefit of "negative pressure" in a tent?

Humanrob

Humanrob

Well-Known Member
As we're going into fall, I had been having trouble keeping the tent as warm as I want it, so I tried something, not sure if there is a reason I should not have done this...

My exhaust fan is on top of the tent, so it had been pulling cool air from the floor level past the plants, and pulling the warmth from the light up and away from them. I disconnected it and reversed its direction -- now it's pulling warm air from 7' off the floor, and pushing it down through the tent past the warm light onto the plants and out the passive bottom vents.

That adjustment changed the steady state temp above the canopy from around 73-74º, to 78-79º, which is more where I want it. People talk about "negative pressure" in the tent being important, but honestly I don't know why -- is "positive pressure" a problem?
 
Yes and no but honestly it sounds like you are in same state as before and have only swapped vent areas what you described is a passive air system you have an inlet and exhaust. A positive pressure system will inflat the tent and push air out of every crevasse in the tent preventing any bugs or foreign debri from entering. a negative pressure tent would be like having a exhaust with no intake. And would pull stuff and cool air into the tent through the same crevasses. Have a neutral system that pulls air in on a restricted intake and allows you to control the rate at which you move air from the tent is what you want not necessarily negative and not necessarily positive. Neutral or passive air flow.
 
Negative pressure indicates good air exchange. It's actively "sucking" the air out of the tent space and exhausting it elsewhere.

Postive pressure, when your tent is "ballooning" out, means more air is coming in than is getting sucked out. Not a huge problem IMO, but, the air in the tentspace could get stale in regards to not having sufficient cO2, and the humidity could creep up on you.
 
Thanks everyone, I couldn't imagine how it could be a problem, but vaguely remembered discussions around this, and thought I should check in and make sure I wasn't missing anything.

The tent does "balloon" a little, but I can feel plenty of air exiting the lower vent openings, so I know I have good air movement. The ballooning just means there's more room for the plants! LOL, I'm going to need it, I'm really packing them in this run.
 
Humanrob said:
Thanks everyone, I couldn't imagine how it could be a problem, but vaguely remembered discussions around this, and thought I should check in and make sure I wasn't missing anything.

The tent does "balloon" a little, but I can feel plenty of air exiting the lower vent openings, so I know I have good air movement. The ballooning just means there's more room for the plants! LOL, I'm going to need it, I'm really packing them in this run.
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I run positive pressure in my 2x4 tent. Gives me more room in there. Never had an issue. I think negative pressure is more for controlling smell if you have a filter.
 
PopAndSonGrows said:
Negative pressure indicates good air exchange. It's actively "sucking" the air out of the tent space and exhausting it elsewhere.
Click to expand...
One of the benefits of neg pressure and increased air exchange is it promotes evaporation because of the increase in free electrons. Evaporation is obviously good when you want your pots to dry out, or to prevent mold that thrive in damp.
 
It makes sense that if you're trying to control odor with a filter and want to be sure no air/smell is leaving the tent anywhere but through the filter, then negative pressure would ensure that. I've never had to use a filter, so I guess that's why I didn't know that. Around here, we love when the house fills up with the smell of an upcoming harvest.
:weed:
 
Humanrob said:
It makes sense that if you're trying to control odor with a filter and want to be sure no air/smell is leaving the tent anywhere but through the filter, then negative pressure would ensure that. I've never had to use a filter, so I guess that's why I didn't know that. Around here, we love when the house fills up with the smell of an upcoming harvest.
:weed:
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I do too. But we gotta be careful sometimes. We found out the hard way that the smell can be absorbed into all kinds of stuff. My wife works at a hospital and people kept smelling weed around her one time. We finally figured out it was her leather purse, :bigjoint:
 
Yeah but positive pressure will really just make the lung room stink. If you exhaust your air elsewhere & it's filtered, negative pressure or not, that air shouldn't stink.

But you guys still raise a great point in regards to containing that odor
 
Temps are also a consideration. Typically a fan will have air exchange near the top of the tent. If it's sucking air out, it will often be pulling in relatively cool air from the floor vents. If it blows air into the tent, it will likely be warmer air near the ceiling of the lung room.
 
PadawanWarrior said:
I do too. But we gotta be careful sometimes. We found out the hard way that the smell can be absorbed into all kinds of stuff. My wife works at a hospital and people kept smelling weed around her one time. We finally figured out it was her leather purse, :bigjoint:
Click to expand...
Ever wonder why shes in the tent with her purse....? :wink:
 
HydoDan said:
Run a filter without negative pressure??
Click to expand...

Exactly. Positive pressure pushes air out every seam, the zipper, any places the tent has any kind of opening. If your plants are flowering and you have positive pressure, your whole house smells. Negative pressure has air flowing in the same places, but it's all being sucked in and passed out through the filter & exhaust fan.
 
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