Negative Tent Pressure In Relation To Temperature - Is This Ok?

[Perception]

Just set up a new grow tent. I've been playing with my light and temperature control. Maybe I'm crazy, but it seems to me that the more negative pressure I have in tent, the better the cooling. On that note, my tent walls are taught like a drum skin when they get sucked in - like... no ripple effect when you tap them.

-LEC 315 light with exposed bulb
-4x4x6.5 tent
-6", 450cfm exhaust fan mounted inside tent
-2x, 6" passive intake ducts (No filter yet)
- Room temp (outside temp) = 71 deg F
- Tent temp (inside) = 77 deg F

If I open up all the passive intake vents in the tent, and there is LESS negative pressure (can tap tent walls and create waves), my temperature difference is 8 degrees. With HIGH negative pressure, my temperature difference is 6 degrees.

My question is:
Can TOO MUCH negative pressure be detrimental to the plants? I'm going to need all the cooling I can get in the summer, and more negative pressure seems to cool better for me.

 

[GT21]

Just set up a new grow tent. I've been playing with my light and temperature control. Maybe I'm crazy, but it seems to me that the more negative pressure I have in tent, the better the cooling. On that note, my tent walls are taught like a drum skin when they get sucked in - like... no ripple effect when you tap them.

-LEC 315 light with exposed bulb
-4x4x6.5 tent
-6", 450cfm exhaust fan mounted inside tent
-2x, 6" passive intake ducts (No filter yet)
- Room temp (outside temp) = 71 deg F
- Tent temp (inside) = 77 deg F

If I open up all the passive intake vents in the tent, and there is LESS negative pressure (can tap tent walls and create waves), my temperature difference is 8 degrees. With HIGH negative pressure, my temperature difference is 6 degrees.

My question is:
Can TOO MUCH negative pressure be detrimental to the plants? I'm going to need all the cooling I can get in the summer, and more negative pressure seems to cool better for me.

Run as much air flow as possible...never too much

 

[Evil Monkey]

Install a inline damper so u can control room pressure.

 

[Perception]

Run as much air flow as possible...never too much

I agree, and if things get hotter in summer, I might even up my CFM. I'm just concerned about how much negative pressure I had in the tent.

Install a inline damper so u can control room pressure.

I'm assuming that the damper would go on the exhaust ducting? At this point, I don't want to reduce airflow/exchange, as I feel like my CFM is "just" enough to keep the tent cool. I tried a smaller fan and it didn't make the cut. Does it hurt anything to have a lot of negative pressure?

Thanks!

 

[Junk]

I'm just concerned about how much negative pressure I had in the tent.

Why?

 

[GT21]

I agree, and if things get hotter in summer, I might even up my CFM. I'm just concerned about how much negative pressure I had in the tent.



I'm assuming that the damper would go on the exhaust ducting? At this point, I don't want to reduce airflow/exchange, as I feel like my CFM is "just" enough to keep the tent cool. I tried a smaller fan and it didn't make the cut. Does it hurt anything to have a lot of negative pressure?

Thanks!

Run it till the tent poles bend... they love it... less chance of mold, less heat, more co2

 

[Perception]

Ok, sounds good! Thank you.

@Junk I had been googling earlier and saw people posting stuff like, "you want the tent walls sucked in, but not to taught", so I didn't know if it was a real concern or anything. Sounds like it's a non-issue though. Thanks all

 

[jumpincactus]

I agree, and if things get hotter in summer, I might even up my CFM. I'm just concerned about how much negative pressure I had in the tent.



I'm assuming that the damper would go on the exhaust ducting? At this point, I don't want to reduce airflow/exchange, as I feel like my CFM is "just" enough to keep the tent cool. I tried a smaller fan and it didn't make the cut. Does it hurt anything to have a lot of negative pressure?

Thanks!

the only downside is with a lot of neg pressure your fan is working harder to exchange the air. Which technically could shorten it's service life. Question is by how much? You need to allow more air to be pulled from the tent with your passive ports. The quicker your fan can pull hot air out and bring cooler air in the better. But keep in mind as summer comes your cooling will only be as good as the ambient air temps are from the room that the tent sits in.

What I have done is taken the tent room that no one lives in and I am running a duct
from my AC vent register that will blow the cooled air into the tent during the hottest part of the day when the ac is needed. This will help keep the tent interior cooler.

 

[Junk]

Ok, sounds good! Thank you.

@Junk I had been googling earlier and saw people posting stuff like, "you want the tent walls sucked in, but not to taught", so I didn't know if it was a real concern or anything. Sounds like it's a non-issue though. Thanks all

I'm not omnipotent, but I can't see how that would even make someone think it's an issue. If the airflow is constant, causing a "vacuum" type effect, it's changing the air pressure relative to the outside space. Not the availability of important gases.

E.g When you get to Denver type altitudes, the air pressure decreases. The amount of oxygen et al in the air is the same. People often say, "There is less oxygen/co2 et al, at altitude." Technically, the percentages are the same, the air pressure is different. The outside air is not exerting as much of a force to enter your lungs (simple explanation). It's there, but you have to breath harder to get it. Compared that to sea level, sea level is practically pressurized.

So, air pressure can make a difference. But you are at the same altitude. The gas concentrations and percentages will be almost the same. The airflow can create a +/- effect, but it's only because it's relative to the non-pressurized area surrounding the tent.

Short answer, worrying about air pressure in the fashion you are talking about is ridiculous. The gas concentrations are the same. And their relative pressure is dictated by sea level, not air flow that creates a slight vacuum.

 

[Junk]

the only downside is with a lot of neg pressure your fan is working harder to exchange the air. Which technically could shorten it's service life. Question is by how much? You need to allow more air to be pulled from the tent with your passive ports. The quicker your fan can pull hot air out and bring cooler air in the better. But keep in mind as summer comes your cooling will only be as good as the ambient air temps are from the room that the tent sits in.

That makes sense, if it's a mechanical issue. And I see the mechanical issue that this could cause. @jumpincactus is exactly right.

As far as your plants, unless you get into scientifically controlled air pressure experiments, it will make no difference.

 

[GT21]

the only downside is with a lot of neg pressure your fan is working harder to exchange the air. Which technically could shorten it's service life. Question is by how much? You need to allow more air to be pulled from the tent with your passive ports. The quicker your fan can pull hot air out and bring cooler air in the better. But keep in mind as summer comes your cooling will only be as good as the ambient air temps are from the room that the tent sits in.

What I have done is taken the tent room that no one lives in and I am running a duct
from my AC vent register that will blow the cooled air into the tent during the hottest part of the day when the ac is needed. This will help keep the tent interior cooler.

I have been able to abuse fans for about 5 years then they crap out... 3 years is a good estimate with any fan and thats runnin 24/7 365

 

[Perception]

I had 2x 6" passive intakes, and I just bought 8" ducting to make 2x 8" passive intakes. Should reduce load a little bit on the fan.

I'm considering using my 90 cfm inline fan or 180 cfm centrifugal (both 4") at night for exhaust (and killing the large fan) to save a little electricity, reduce noise, and slow evaporation from the soil.

Thanks everyone for the comments.

 

[Leew421]

You have negative pressure cause you're exhausting too hard for the rate of air in. Tweak the inlet higher and the exhaust slower to even it out. Consider going co2 to help with higher Temps and to slow the air exchange rate dramatically since you are injecting co2

 

[rmoltis]

If you want to gain extra space.
Take some bamboo stakes cut to fit between your tent poles.

Put one on each tent wall midway up fixed to the poles on each side.
It will keep the tent from sucking in as much.

The new tents have these features.
Like @jumpincactus tent has this feature.

 

[Leew421]

Gorillas version

https://www.gorillagrowtent.com/col...es/products/ggt-indoor-grow-room-high-cfm-kit

 

[Perception]

You have negative pressure cause you're exhausting too hard for the rate of air in. Tweak the inlet higher and the exhaust slower to even it out. Consider going co2 to help with higher Temps and to slow the air exchange rate dramatically since you are injecting co2

Correct - Although I'm restricting my passive intake intentionally. I've found that more negative pressure keeps my interior temps lower. When I open the passive intake screens and the negative pressure lessens, the interior temp goes up another 2 deg F.

So I want to be running negative pressure to make my cooling more efficient. I was double checking to see if this could negatively impact plants. Sounds like the only potential problem is stressing out the exhaust fan though! :)

 

[Perception]

Thanks for links to tent support braces! That may be in my near future. I'm definitely losing a little interior space from the walls bowing in.

 

[Junk]

So I want to be running negative pressure to make my cooling more efficient. I was double checking to see if this could negatively impact plants.

I get what you are saying. I think it's me getting stuck on that word...but the only thing you care about is temps/rh etc. The negative pressure you are talking about is relative. This...

-LEC 315 light with exposed bulb
-4x4x6.5 tent
-6", 450cfm exhaust fan mounted inside tent
-2x, 6" passive intake ducts (No filter yet)
- Room temp (outside temp) = 71 deg F
- Tent temp (inside) = 77 deg F

Just for illustrative purposes, if you eliminated one of your passive intake ducts, and put your fan on the other one....blowing into the tent, you will have positive pressure. The tent walls will bulge out. So, negative pressure, positive pressure, it all depends on how the air flow is designed. But I wouldn't be designing it to have negative pressure, any more than I would be designing it to have positive pressure. You get the environment right and whatever happens, happens. You didn't list your rh, but your temps are good. You can play with them too. Some plants like 75, some like 79. You'll see it in the leaf structure, when they are happy.

So I want to be running negative pressure to make my cooling more efficient.

This is my point, and I realize it's a silly thing to get hung up on, but the above is backwards. You want your cooling to be more efficient. The side effect for your design, is negative pressure.

What I wanted to make clear to everyone, is that all one cares about is temps/rh. The pressure thing, I understand what you guys are talking about now. But it would just be a consequence of your cooling. You are aiming for the right environment, you really could give a damn about the pressure. It's semantics, I know, I just don't want noobies leaving this thread, thinking they need negative pressure.

Also, for your intake, you don't need a charcoal filter (I'm not sure if that's what you meant, or just so birds don't get in). The charcoal filter would need to be in the tent, or after it in the chain. For your design, it sounds like a cap filter in the tent with the fan pulling air through, and pushing it out of the tent.

In your case, you would likely pull more air through if you blocked one intake enirely, instead of both, mildly. You want a drinking straw type effect, not one straw, that is connected to two straws. It would probably make your pressure even more negative though. But that is what is happening. Your temps go up bc the exhaust fan can't draw as efficiently. But you seem to be figuring that out. One can actually try it with straws. Take a swig with one straw. Then add another, and take a swig, and you will see you don't pull the same amount through with the same effort. What I would do in your case is get 2 cheap, 6", inline fans, on both of the intakes (you have to mess with it). You will have even greater control over your temps and rh, and your $$ fan won't be straining as hard. Just remember to turn them on, or u are adding additional strain to your main fan. I've done that before.

I haven't seen them in a while though, 6" plastic, inline fans, 110-150 cfm-ish. I can't remember what I paid for them, but I remember they were shockingly cheap. $13-ish? And I use them all the time.

So, I would be making these adjustments, and checking the hygro...not the tent pressure.

 

[Perception]

@Junk thanks so much for the input. It seems simple, but balancing things just right takes a lot of fine tuning.

I bought these passive intake filters (they had a tiny bit of charcoal fluff filling), and they blocked airflow significantly. I'm going to go with panty hose. I know I'm not really filtering the air much, but I'm ok with that.

The best combo I've found so far is:
-6" exhaust ducting
-6" 400 cfm exhaust fan inside tent
-2x, 8" passive inkate ducts

I might even open up the passive intake Velcro duct (gorilla light line tent) on the back of tent too. Just need to build a light box for it. Without the back side velcro passive intake duct open, it feels like a bit of a wind tunnel in the tent. With it open, there is a less aggressive breeze. I live in a very dry climate, so I'm battling evaporation.

I just popped some seeds a weeks ago, and the tent will be going in to service soon. I'll be curious to see how things pan out.

 

[Junk]

@Junk thanks so much for the input. It seems simple, but balancing things just right takes a lot of fine tuning.

It does. Your temps are amazing though, so you are doing something right.