negative pressure vs. Positive pressure

2

2feather18

Well-Known Member
I recently just setup my inline fan and carbon filter also have small oscillating fan my grow tent is sucking in somebody said that's a good thing but then somebody said that's a bad thing..? Which is true..? also I see inline fans being set up outside the grow tent and then inside the grow tent I don't know which one to do so I just put it outside
 
Johiem

Johiem

Well-Known Member
I've had the same question, so I'm testing opposing pressures. First run, neutral pressure 1 plant, less than 1/4 oz. Harvest. I also screwed that one up. Second trial(current) 4 plants negative pressure all plants >100cm, harvest is still a week off though, next run I'll put my fan on one of the other vents and just the tube with filter on the exhaust. I'll post my results as I get them.
 
Amnesia420

Amnesia420

Member
Filter, extractor in the tent for me, I think some people may have it outside if they don’t have Much room but I’m not 100% sure. Yes that’s right negative pressure your tent should be sucking in. Some people will have an intake fan on the outside maybe that’s what you’re seeing.
 
2

2feather18

Well-Known Member
Amnesia420 said:
Filter, extractor in the tent for me, I think some people may have it outside if they don’t have Much room but I’m not 100% sure. Yes that’s right negative pressure your tent should be sucking in. Some people will have an intake fan on the outside maybe that’s what you’re seeing.
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so what I did was open both fence on the bottom and that would relieve some of the negative pressure however I just learned that negative pressure is better than positive pressure
 
2

2feather18

Well-Known Member
Johiem said:
I've had the same question, so I'm testing opposing pressures. First run, neutral pressure 1 plant, less than 1/4 oz. Harvest. I also screwed that one up. Second trial(current) 4 plants negative pressure all plants >100cm, harvest is still a week off though, next run I'll put my fan on one of the other vents and just the tube with filter on the exhaust. I'll post my results as I get them.
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I think I'm going to have to get an exhaust fan and put it in the rule for something I've been looking
 
R

r1291

Active Member
I’ve dealt with both. At first, I only had an 8” intake connected to phresh filter to supply the room with conditioned air from different room thinking that it would maintain desired temps since I was running leds. At max Fan speed, the room would bow out and temps were unstable. After further research I learned it’s more efficient to exhaust out rather than pump air in so a proper exhaust was installed later when flowering was going to start since that’s when you really want shit dialed in. Plants don’t give shit about pressure (okay maybe they do) but more importantly is the quality of air circulation & maintaining environmental factors such as Temp & Humidity. You definitely want to err on side of negative pressure since that’s the only route of keeping the smell Late in flower under control.
 
Gardenator

Gardenator

Well-Known Member
Try for negative pressure, your exhaust should always over compenste your intake and negative pressure will be achieved... i cannot quote all the scientific information or calculations on here but most often people are shooting for negative pressure... just think if air is being forced into the tent at a higher rate then it was being exhausted you would be left with a portion of dead stagnent air that would be of no good use to your plants and if you were a breeder you woulndt want that pesky pollen blasting out everytime you opened the door to your room or tent cross pollenating all of your years and years of hard work as a breeder would you? Also with negative pressure you continuously cycle fresh air into your space that your plants will transpire. Your air quality and your room temps will be improved, exhaust is for heat dissapation as well not just air exchange, way more important than airflow or just as important at the very least, no air exchange is bad plants wont breath or transpire rather... air needs to be exchanged every 3-5 minutes for fresh air to be present, so therefore you want an intake that can move that volume of air and over comp ur exhaust fan by 30% more volume rating then your intake or even 50% more volume rating (cfm rating- cibic feet per minute). In short this more then garantees ur negative pressure and air exchange. Good luck
 
2

2feather18

Well-Known Member
Gardenator said:
Try for negative pressure, your exhaust should always over compenste your intake and negative pressure will be achieved... i cannot quote all the scientific information or calculations on here but most often people are shooting for negative pressure... just think if air is being forced into the tent at a higher rate then it was being exhausted you would be left with a portion of dead stagnent air that would be of no good use to your plants and if you were a breeder you woulndt want that pesky pollen blasting out everytime you opened the door to your room or tent cross pollenating all of your years and years of hard work as a breeder would you? Also with negative pressure you continuously cycle fresh air into your space that your plants will transpire. Your air quality and your room temps will be improved, exhaust is for heat dissapation as well not just air exchange, way more important than airflow or just as important at the very least, no air exchange is bad plants wont breath or transpire rather... air needs to be exchanged every 3-5 minutes for fresh air to be present, so therefore you want an intake that can move that volume of air and over comp ur exhaust fan by 30% more volume rating then your intake or even 50% more volume rating (cfm rating- cibic feet per minute). In short this more then garantees ur negative pressure and air exchange. Good luck
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yes I'm going to do an exhaust fan and by opening the flaps on the bottom it relieve some of the negative pressure the only thing about that when I do that is the humidity level drops down where I don't like it
 
2

2feather18

Well-Known Member
r1291 said:
I’ve dealt with both. At first, I only had an 8” intake connected to phresh filter to supply the room with conditioned air from different room thinking that it would maintain desired temps since I was running leds. At max Fan speed, the room would bow out and temps were unstable. After further research I learned it’s more efficient to exhaust out rather than pump air in so a proper exhaust was installed later when flowering was going to start since that’s when you really want shit dialed in. Plants don’t give shit about pressure (okay maybe they do) but more importantly is the quality of air circulation & maintaining environmental factors such as Temp & Humidity. You definitely want to err on side of negative pressure since that’s the only route of keeping the smell Late in flower under control.
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I'm yes I'm fixing the problem now by getting exhaust fan also Letten the bottom flaps open to vent that relieves some of the negative pressure however when I do that the humidity temperatures drop to where I don't want to be that low
 
Boatguy

Boatguy

Well-Known Member
Put the exhaust fan on a plug in thermostat. They are pretty cheap, and your humidity will be at a higher level than running the fan 24/7

I use one of the cheap inkbird controllers that does humidity or temp. I use temp for the grow, and the humidity side for drying with a humidifier
 
2

2feather18

Well-Known Member
Boatguy said:
Put the exhaust fan on a plug in thermostat. They are pretty cheap, and your humidity will be at a higher level than running the fan 24/7

I use one of the cheap inkbird controllers that does humidity or temp. I use temp for the grow, and the humidity side for drying with a humidifier
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thank you seems like the only time I can raise the humidity level as when the tent is sucking in when I vented Stabilizes out that's when my humidity level drops to 45 I will just get a humidifier.. also what do you mean put the exhaust fan on a plug thermostat
 
f series

f series

Well-Known Member
I wonder how many people have problems with "dead stagnant" air, while running exhaust and intake fans in a grow tent lol.
 
Gardenator

Gardenator

Well-Known Member
f series said:
I wonder how many people have problems with "dead stagnant" air, while running exhaust and intake fans in a grow tent lol.
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Yeah not a one thats the point right? If air isnt exchanged it depletes in either oxygen or Co2 depending on if your breathing it in or plants are transpiring it therefore needs to be exchanged- as far as weathingrizing a house for cold climate a certain standard has to be met to allow for a certain amount of air exchange to occur or the air becomes "stagnant"... same thing happens to your plants air in a sealed grow tent hence why i suggested using intake and exhaust fans to avoid this issue... plants need to breathe or "transpire" fresh air or they wont grow properly or worse they will die... lol?
 
2

2feather18

Well-Known Member
Gardenator said:
Yeah not a one thats the point right? If air isnt exchanged it depletes in either oxygen or Co2 depending on if your breathing it in or plants are transpiring it therefore needs to be exchanged- as far as weathingrizing a house for cold climate a certain standard has to be met to allow for a certain amount of air exchange to occur or the air becomes "stagnant"... same thing happens to your plants air in a sealed grow tent hence why i suggested using intake and exhaust fans to avoid this issue... plants need to breathe or "transpire" fresh air or they wont grow properly or worse they will die... lol?
Click to expand...
 
Boatguy

Boatguy

Well-Known Member
Gardenator said:
same thing happens to your plants air in a sealed grow tent hence why i suggested using intake and exhaust fans to avoid this issue... plants need to breathe or "transpire" fresh air or they wont grow properly or worse they will die... lol?
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Do you really think an intake fan and exhaust are necessary? I've never ran an intake fan, and havent had a problem.
I find it hard to believe air in the tent goes stagnant in 3 to 5 min. If it does, it doesnt seem to have much effect on my plants. My exhaust kicks on every 30 min or so to keep temps around 80
 
f series

f series

Well-Known Member
Gardenator said:
Yeah not a one thats the point right? If air isnt exchanged it depletes in either oxygen or Co2 depending on if your breathing it in or plants are transpiring it therefore needs to be exchanged- as far as weathingrizing a house for cold climate a certain standard has to be met to allow for a certain amount of air exchange to occur or the air becomes "stagnant"... same thing happens to your plants air in a sealed grow tent hence why i suggested using intake and exhaust fans to avoid this issue... plants need to breathe or "transpire" fresh air or they wont grow properly or worse they will die... lol?
Click to expand...
We're talking about tent growing, which was my point.if you've got circulation inside the tent and an exhaust, you're plants will not die from "dead stagnant" air.
I doubt they would even be adversely affected, at all.

But yes, of course more than adequate air exchange is better
 
AquaTerra

AquaTerra

Well-Known Member
Boatguy said:
Do you really think an intake fan and exhaust are necessary? I've never ran an intake fan, and havent had a problem.
I find it hard to believe air in the tent goes stagnant in 3 to 5 min. If it does, it doesnt seem to have much effect on my plants. My exhaust kicks on every 30 min or so to keep temps around 80
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Actually that would be a good video to make, demo of intake and exhaust fan vs just exhaust fan vs temp changes and or Co2. The industry standard was twice the cfm out then coming in to create negative pressure. Now the negative pressure idea is to keep the smell from escaping from any nukes and carnies. You want to move all the air in the room in about two minutes.
 
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