Suck or blow pressure in 2x4 tent

[Monkeybigbuds]

So I’ve just set up an air intake just wondering do I set it so the tent walls are sucking in or bulging out
Do I set it so the exhaust fan is a bit stronger than exhaust fan or other way round thanks

 

[budsofgeorgia]

they should not be sucking in and they should not be sucking out , you need as much air going into the tent as you do going out thats what i was told anyway

 

[LoveGrowingIt]

So I’ve just set up an air intake just wondering do I set it so the tent walls are sucking in or bulging out

I have mine pulling air from the top of the tent, where the air is hottest because of the light. I open vents at the bottom for fresh air intake.

Do I set it so the exhaust fan is a bit stronger than exhaust fan or other way round thanks

Do you have two fans? I doubt two are necessary, especially for a 2x4 tent. There's probably less than 50 ft^3 of air in the tent and most fans can easily move that much air.

If you have a controller, perhaps a splitter could be used to run both at the same speed if they're both the same model of fan.

 

[Monkeybigbuds]

I have mine pulling air from the top of the tent, where the air is hottest because of the light. I open vents at the bottom for fresh air intake.


Do you have two fans? I doubt two are necessary, especially for a 2x4 tent. There's probably less than 50 ft^3 of air in the tent and most fans can easily move that much air.

If you have a controller, perhaps a splitter could be used to run both at the same speed if they're both the same model of fan.

Yes there both 4in spider farmer fans one with carbon filter to take the sweet smell away and humidity & temp control will be running 24/7 when flowering and intake fan with speed control I just like the fact there getting fresh air from outside

 

[Newty]

I always run with a little negative pressure, tent walls suck in a little.

I run exhaust and intakes in my 8x4 as well as my 55"x28". I run the intake speed lower to create the negative pressure.

Main reason I run intakes is for greater control. I can put the intake duct closer to humidifiers, heaters, and air conditioners making changes to tent environment faster.
I also have grates in the floor of my house that pass to a downstairs room, with the intakes I can pull air from directly downstairs. I pretty much have 2 different lung rooms because of this and can have both tents feeding from the same lung room or each from a different one if environment needs to be different.

 

[BB22]

I don’t know if it’s the best way. I usually have my tent doors open during the day.

I have to battle heat and humidity. Especially in the summer months.

At night, I zip up the tent and get strong negative pressure. Really strong. I tried using plastic coat hangers wedged in between tent and frame. They all broke quickly.

 

[Novaracer69]

I personally gave up of balancing the airflow fo I purchased the new vivosun 4x4 pro model with the reenforcement bars. My old sypder farmer tent looked like it was going to cave in on itself. I am using a 6" fan

 

[AZreefer]

Ive been reading about this exact same thing as this is my next step for my build. I am installing a 6" intake and putting it on the floor. Im putting an 8in exhaust on the celing and using a controller to keep the intake set lower than the exhaust to keep a negitive pressure through the tent.

 

[DougV]

It depends.
If odor control is essential, negative is your best option as you can easily use an exhaust fan directed into an air filter or even vent it outside.

If you do not need odor control, have environmental control, and have a means of introducing CO2 then you are mainly just turning air over at a slow rate, positive, negative, doors unzipped, fanned by palm tree leaves, no matter.

If you lack environmental control, as in growing in a shed, barn, or garage, then it depends on if heat or cold is a problem. Cold is easiest, so it goes first. Run your lights opposite day/night. Take advantage of their heat when most needed. Use a small electric heater with a remote temperature control.(Propane heaters produce CO2, but even on low settings will probably produce too many BTUs in a small tent to control.) You don’t need to cook the place, shoot for the lowest temp you can. For veg, 70, for flower 60-65. Get a vaporizer type humidifier to control humidity. I use a 1000/750 watt heater on the low setting in an 8x8 tent. Negative or positive pressure isn’t important but you want to minimize outside/inside air exchange to preserve heat, but not so much as to starve plants of CO2.

Finally the worst case, heat. It’s 95 outside, you’re grilling a steak on the sidewalk next to the potatoes baking in the sand. You think, no problem and you go grab a shiny new portable AC and you stick in your tent. Feel the cold, best $400 you’ve spent. You go have a beer to celebrate. Upon your return you find out you have rediscovered how people make water in a desert. That nice cold tent interior met that nice shinny warm tent wall and every bit of moisture condensed out. Your hydroponic system and tent walls are literally dripping wet. If you have negative air pressure nice wet tent walls are pressed up against leaves, humidity is sky high, and as an added bonus your brand new AC cycles on/off continually as thin tent walls offer no insulation. You’re best bet in this case, forget AC, bring positive air pressure coming in from as cool of a place as you can. (Ground level, or air vented in from the outside.) Then use duct work to passively extract heat from lights by ducting air out above the lights, or actively by ducting to an exhaust fan. (An active system is probably the most effective at heat removal.) It will be difficult to control humidity. To minimize mold/fungi turn over air constantly and make sure you keep air moving around your ladies. Plus clean the heck out of tent and everything in it with H2O2, bleach, or antifungal after a grow. Finally remember next year to time your pre-summer grow to end when it starts getting hot.

Positive pressure makes it much more difficult for bugs, pollen, and dust to get in, just prefilter the intake.
Negative pressure will put more stress on tent walls, zippers, frames, and your exhaust fan, reduce interior pressure, and reduce fan efficiency.

Sorry for rambling. I’ve been exploring this exact problem for three years.

 

[RoosterMan]

Had anyone heard of VPD? I'm currently setting up my 3rd tent and am trying to come up with a controller for the fans to keep temp and humidity in check without causing a high VPD, which is not good for the plants.

 

[DougV]

My dogs got me up at 4am, your VPD comment has kept me up. So, before I go on, if lack of sleep makes me grumpy, which then gets my wife mad at me, and in response she burns down my plants, then please realize it’s all your fault.

There are dataloggers to monitor Vapor Pressure Deficit (VPD). With minimal programming skills you could make your own. There are a host of sensors available to measure the three parameters you need, RH, room temp, leaf surface temp. There are computer controlled relays available which allow you to control equipment for humidity and temperature. If hydroponics are used you might even be able to modify leaf temps by altering water temps. You can find charts on VPD that can let you derive control equipment max/min statements. (dimluxlighting.com under knowledge for charts, explanation, and info on their VPD datalogger.)

For this info to be useful, you need total environmental control of your grow area. You’ll need to either be able to respond to changing environment, or more simply force your environment to remain in a desired range. For example a room temp of 68, leaf temp 65, RH 38%, gives you a leaf VPD (LVPD) of 1.22. This is a safe range for flowering. Rather than monitor changing conditions, just force RT and RH to these points. To get this type of control you’ll need a well insulated area, be that growing area or the area the growing area is in. Probably R30 walls, R45 ceiling, R15 floor. To keep ongoing costs, ie electricity, reasonable, you’ll also need to seal seams, cracks, access points, and ducting. Now your system is closed so you’ll need CO2. Bottom line, costs to control this will get high. For commercial operations I’d guess environmental control, in the long run, is the most cost effective. For the home grower I don’t know. How much costs, on a per pound basis, is it worth?

Using myself as an example, I grow three indoor grows a year. If I took the steps necessary to control temperature I’ll have to build a really well insulated and sealed grow room, window AC, AC control unit. At a quick guess, $1500-2K for insulation, $1-2k to build room or to convert a shed. Then equipment costs for AC, humidity control, sensors, and so on. This excludes time and labor. Let’s say $3-5k gets it all done. You get an extra grow, healthier plants, less worry, better control, and possibly a better end product.

Far from intending to discourage you, my intent is to challenge you to go for it. I think for many the complexity and costs make this too daunting. Personally, I’m an idiot control freak. I like the concept of using info, like VPD, to allow me to maintain optimal growing environments. But I’m rebuilding my house so my barn is full of household crap. Soon as I barn room available, a woodshop, a walk in cooler, and an environmentally controlled grow room are on the list. If, a young intrepid intelligent trendsetter, for example, such as yourself, would embark on this journey, while carefully documenting success, failure, costs, changes in energy demand, then an old idiot control freak could save time, money, and effort. A great win for me and a valuable learning experience for you. Such a deal.

 

[Newty]

Had anyone heard of VPD? I'm currently setting up my 3rd tent and am trying to come up with a controller for the fans to keep temp and humidity in check without causing a high VPD, which is not good for the plants.

The AC Infinity Controller 69 has triggers for VPD so you can trigger multiple devices/equipment based on VPD values.
I don't personally use VPD triggers, I use temp and humidity triggers and my VPD stays mostly within +/- .2 from recommended VPD values.

 

[PianoStan]

The research that I did prior to starting my grow said that keeping negative pressure in the tent was necessary to keep an adequate flow of CO2 to the plant.

 

[DougV]

I’ve seen the same thing. It makes no sense. CO2 is a component of the air we breathe. Exchanging air is automatically replenishing CO2, whether done by pushing air in, or pulling air out. From FloraFlex a list of the benefits of negative pressure:

The Importance of Negative Pressure​

Negative pressure plays a crucial role in creating a controlled and efficient growing environment. Here's why it is important:

1. Air Circulation and Ventilation: Negative pressure helps facilitate proper air circulation and ventilation within the grow tent. When the pressure inside the tent is lower, fresh air is drawn in through intake vents, while stale air and excess humidity are expelled through exhaust vents. This continuous exchange of air helps maintain optimal temperature and humidity levels for plant growth.
2. Prevention of Odor Leakage: By maintaining negative pressure, you can minimize the risk of odor leakage from your grow tent. As the air is being pulled into the tent, it creates a vacuum effect that helps contain any strong odors within the enclosed space. This is particularly important if you are growing aromatic plants, such as cannabis.
3. Pest and Disease Prevention: Negative pressure can act as a natural barrier against pests and diseases. When the air is being drawn into the grow tent, it creates a gentle inward airflow. This airflow can help deter insects, as they find it more difficult to navigate against the current. It also reduces the likelihood of airborne pathogens entering the tent, reducing the risk of plant diseases.
4. Temperature Regulation: Negative pressure assists in regulating the temperature within the grow tent. By creating a constant flow of fresh air, it helps dissipate excess heat generated by grow lights and other equipment. This prevents the buildup of hot spots and promotes a more even distribution of temperature throughout the growing space.

#1. True but same is true of positive pressure, just air is forced in by a fan and exits through the same vents/air leaks.
#2. True. If you need to cover up smell, negative pressure is the easiest option.
#3. Not true. The air in the tent is negative. It pulls things into the tent through every crack, vent, open zipper, and so on. When you enter the tent every bug, spore, and dust particle around is pulled into the tent. Positive air pressure does exactly what this claims negative does. It forces flying insects to navigate air currents pushing them out, not assisting their entrance.
#4. Like #1, true, but the same is true of positive pressure and active systems exchange air even more effectively.

Negative pressure is far superior to no ventilation at all. Excepting odor removal, positive pressure does the same things. Using a hybrid system with passive ducting above lights allows positive pressure to force hot air out, or place the same passive ducting to bring in cooler air allows negative pressure intake above lights to pull heat out. Active systems allow you be either negative or positive depending on what works best at a given point, or even allow you to match atmospheric pressure and still move tons of air.

In my limited experience, I found active, with cool air coming in low, and hot air being forcibly extracted from above, to be by far the best method for removing heat. Active systems have one addition advantage that I’ve never seen mentioned, redundancy. If, and when, one fan fails, the other fan/s still allow for forced air movement. That might be the difference that saves a crop.

 

[Mikedin]

I run full negative pressure on all my tents, all bottom flaps open and exhaust and filter at the top,

My tents are always pulled in even with all flaps open, in my 4x8 I have 4 flaps and I recently used one of the tent inlets to add in another piece of 6” flex ducting just to let a bit more air as well

The 4x4 has 1 flap so I added 2 of the 6” ducts as inlets at the bottom in there as well

As long as you don’t restrict the inlet you’ll have plenty of air exchange to remove heat and humidity as needed if you think it’s still too much negative pressure just throw in a 6” duct on a bottom tent inlet (I cover my ducts with panty-hoes so no bugs can get in through the ducting, at least that’s my method lol)

 

[PianoStan]

I’ve seen the same thing. It makes no sense. CO2 is a component of the air we breathe. Exchanging air is automatically replenishing CO2, whether done by pushing air in, or pulling air out. From FloraFlex a list of the benefits of negative pressure:

The Importance of Negative Pressure​

Negative pressure plays a crucial role in creating a controlled and efficient growing environment. Here's why it is important:

1. Air Circulation and Ventilation: Negative pressure helps facilitate proper air circulation and ventilation within the grow tent. When the pressure inside the tent is lower, fresh air is drawn in through intake vents, while stale air and excess humidity are expelled through exhaust vents. This continuous exchange of air helps maintain optimal temperature and humidity levels for plant growth.
2. Prevention of Odor Leakage: By maintaining negative pressure, you can minimize the risk of odor leakage from your grow tent. As the air is being pulled into the tent, it creates a vacuum effect that helps contain any strong odors within the enclosed space. This is particularly important if you are growing aromatic plants, such as cannabis.
3. Pest and Disease Prevention: Negative pressure can act as a natural barrier against pests and diseases. When the air is being drawn into the grow tent, it creates a gentle inward airflow. This airflow can help deter insects, as they find it more difficult to navigate against the current. It also reduces the likelihood of airborne pathogens entering the tent, reducing the risk of plant diseases.
4. Temperature Regulation: Negative pressure assists in regulating the temperature within the grow tent. By creating a constant flow of fresh air, it helps dissipate excess heat generated by grow lights and other equipment. This prevents the buildup of hot spots and promotes a more even distribution of temperature throughout the growing space.

#1. True but same is true of positive pressure, just air is forced in by a fan and exits through the same vents/air leaks.
#2. True. If you need to cover up smell, negative pressure is the easiest option.
#3. Not true. The air in the tent is negative. It pulls things into the tent through every crack, vent, open zipper, and so on. When you enter the tent every bug, spore, and dust particle around is pulled into the tent. Positive air pressure does exactly what this claims negative does. It forces flying insects to navigate air currents pushing them out, not assisting their entrance.
#4. Like #1, true, but the same is true of positive pressure and active systems exchange air even more effectively.

Negative pressure is far superior to no ventilation at all. Excepting odor removal, positive pressure does the same things. Using a hybrid system with passive ducting above lights allows positive pressure to force hot air out, or place the same passive ducting to bring in cooler air allows negative pressure intake above lights to pull heat out. Active systems allow you be either negative or positive depending on what works best at a given point, or even allow you to match atmospheric pressure and still move tons of air.

In my limited experience, I found active, with cool air coming in low, and hot air being forcibly extracted from above, to be by far the best method for removing heat. Active systems have one addition advantage that I’ve never seen mentioned, redundancy. If, and when, one fan fails, the other fan/s still allow for forced air movement. That might be the difference that saves a crop.

Purely from the standpoint of physics your post is completely true. I've been a nurse for over 30 years, and we use negative pressure rooms when we want to keep the infectious things inside the room. Positive pressure is used to keep infectious things away from the patient.

Even building PC's keeping positive pressure in the case helps to keep dust and debris away from your sensitive components.

Being this is my first ever grow, I wasn't sure if there was some obscure reason for the use of negative pressure that eluded me. So I just did what I was told, and it has worked out. But, as you say, it almost certainly would have worked out either way.

When you say that you use an active system, do you have different sized fans? Like say a 6" for intake, and a 4" for exhaust so that you maintain positive pressure?

 

[PianoStan]

I run full negative pressure on all my tents, all bottom flaps open and exhaust and filter at the top,

My tents are always pulled in even with all flaps open, in my 4x8 I have 4 flaps and I recently used one of the tent inlets to add in another piece of 6” flex ducting just to let a bit more air as well

The 4x4 has 1 flap so I added 2 of the 6” ducts as inlets at the bottom in there as well

As long as you don’t restrict the inlet you’ll have plenty of air exchange to remove heat and humidity as needed if you think it’s still too much negative pressure just throw in a 6” duct on a bottom tent inlet (I cover my ducts with panty-hoes so no bugs can get in through the ducting, at least that’s my method lol)

The panty hose is a good idea. I live in an old house, and it is always somewhat dusty. I never thought about using nylons as a pre-filter.

 

[Mikedin]

Purely from the standpoint of physics your post is completely true. I've been a nurse for over 30 years, and we use negative pressure rooms when we want to keep the infectious things inside the room. Positive pressure is used to keep infectious things away from the patient.

Even building PC's keeping positive pressure in the case helps to keep dust and debris away from your sensitive components.

Being this is my first ever grow, I wasn't sure if there was some obscure reason for the use of negative pressure that eluded me. So I just did what I was told, and it has worked out. But, as you say, it almost certainly would have worked out either way.

When you say that you use an active system, do you have different sized fans? Like say a 6" for intake, and a 4" for exhaust so that you maintain positive pressure?

I’ve also heard negative pressure stresses the plants more than positive but I’ve never tested it myself I’ve always ran negative, tried with an inlet fan but it seemed to be causing more problems than it was solving so I just went back to the passive intake method

 

[PianoStan]

I’ve also heard negative pressure stresses the plants more than positive but I’ve never tested it myself I’ve always ran negative, tried with an inlet fan but it seemed to be causing more problems than it was solving so I just went back to the passive intake method

I'm not dissatisfied with how things have gone with my negative pressure, and I have no plans to change it. Other than to steal your pantyhose trick. But it is a subject that vexed me from the start. I mean either way you are taking CO2 laden air and placing it in the tent. And if you use a CO2 tank to bolster the level around the plant(s) positive pressure might be better since negative pressure would just exhaust more CO2 into the room and away from the plant(s).

 

[Mikedin]

The panty hose is a good idea. I live in an old house, and it is always somewhat dusty. I never thought about using nylons as a pre-filter.

It’s just something to cover it, and damn do they get dusty, and my wife is a CLEAN FREAK she vacs the house twice a day, sanitizes every counter etc, and those nylons still dust up but we live out in the country so between everyone rippin 4 wheelers etc it builds up, I haven’t done a full tear down in like a year, really need to do one eventually lol, just been bleaching the inside while wearing a full face respirator between runs