Air pressure in a tent

[Tonyveitch]

Hay guys. I have a debate between my brother and I about air flow in a tent. We are at a stake mate about negative or positive air pressure. So I figured I’d ask the pro’s. Negative or positive and why? And help to shed some light on the situation would be greatly appreciated

[OzBud]

Negative, more air going out than coming in means all air is taken out through the filter eliminating smell. the sides of the tent will suck inwards

Positive pressure - If you have more air coming in than going out (if you had a bigger intake fan than exhaust) then air will find a way to get out through stitching, zippers etc and the smell will get out of the tent unfiltered

[Budler]

Negative.
For example the smell. If the pressure is negative in the tent, than all the air leaving the tent is going through your carbon filter. If it’s positive and your tent is not sealed properly...

[Colombo]

As a Retired FF I use positive pressure as it moves a higher cfm of air compared to negative pressure. Positive pressure pushes air into my tent and a vent allows it to leave. For proper air flow the vent needs to be needs to be the same size as the intake or greater. you can change air flow by adjusting the vent. More pressure in the tent adjust vent down, less pressure ,open the vent. I have positive and neg pressure set up so I have complete unobstructed air flow through the tent. (Two 6" inline fans) Push air in, pull air out! The only thing in my tent are 2 300 watt led lights and the plants. No fans. The air movement through the tent creates a slight breeze over the plants and they love it. It also helps control the temps. Mine maintains 72*73* consistently and 38-42% humidity. I would say its all about preference and your type of grow. Hope the info helps! Good Growing!

[Toker1]

How does positive air pressure move more CFM, please explain??? Through out my entire career as a mechanical engineer, this is the first time I have ever heard this. Would love to hear more about your thought process.

[Colombo]

I am not a mechanical engineer and can only base this on my experience in the fire service with its use. If I have a house full of smoke and I use positive pressure I can clear the home twice as fast as neg pressure. Putting a ventilation fan at a distance to encapsulate the opening of the front door charges the building(positive Pressure) and allows a larger CFM of air to move through and clear the building. By opening or closing any openings, interior or exterior, you can control air movement to the exterior changing the CFM. The more openings the more air movement as long as the air coming in does not drop to an equal pressure. (Too many openings). If I put a fan in a window and pull, suck, create a vacuum air out, you can only move what the fan will allow through that window. If you have to many openings you lose neg pressure and air flow reduces. If you close more openings the neg pressure increases but will still only move what the fan allows through that opening. Negative pressure on a fan reduces its efficiency until you allow some type of air flow in to it. Working at a refinery the buildings are positive pressurized so nothing from the outside gets inside. This is not about air flow it is strictly about maintaining a positive pressure for health reasons. Air movement is minimal until you have an opening, then it increases such as opening a door. Science may say different but from my experience positive pressure moves more CFM than Negative, its all how you use them. As I said, I have no degree, only what I have learned from my experience JMO.

[Toker1]

Yeah...I understand what you are saying. Let me chime in there and say when dispersing smoke... you are looking to lower the PPM of particles in the air. Your CFM can be exactly the same in both cases...but positive turbulent air pressure will lower ppm more quickly. Heat does not work the same way (as it forms a gradient and acts differently than individual particles do). With that being said... positive air pressure allows the lower ppm air to flow into a high ppm air zone. This effectively lowers ppm and scatters particles around which even further lowers ppm. When sucking (negative air pressure) you are removing zones of low and high ppm... so your ppm will go down much slower because you are also sucking out low ppm air with high ppm air with out any discretion. in short, your CFM is not increasing...but positive air pressure is more efficient at lowering ppm when it comes to introducing low ppm air into smoke and high ppm air quality.

[Toker1]

Negative air pressure... for a few reasons. 1. Sucking air creates a more laminar flow... where blowing air creates a more turbulent flow. Laminar flow results in larger CFM movement through the ducting.
2. Smell protection. Positive air pressure allows smelly air to leak outside of the tent. Negative air pressure creates a vacuum effect, pulling all smelly air through your carbon filter. That is the only way to remain discrete about the stink.
3. Heat removing capability, even when experiencing faulty ducting. (Specifically speaking about air cooled hoods and the ducting to attach them.)
Pulling air through the hoods (negative air pressure) allows the best heat removal. Not to mention, if your ducting gets any holes in it... you are not leaking out hot air into your garden. Reverseing the flow (blowing air across the air cooled hood causing positive air pressure) can create hot air leaks if everything is not 100% sealed up tight.
Always pull air (negative air pressure) for best results in a tent.
Also, make sure you have your screens or filters set up and functional to block out pulling in any undesirables into the grow room.

[Toker1]

So the oscillating fans only slightly increase pressure inside the tent. That effect is practically negligible. What you need to look for is air puffing up the tent, inflating it slightly and pushing air out of any orifice it can? If so..you have positive air pressure.
If not, does the air suck in the tent, deflating it slightly ...where air is rushing inside from every orifice? If so, you have negative pressure.

[Colombo]

My tent has a slight balloon pressure which is positive at all times. I can change this by opening or closing the exhaust fan shoot opening. At no time do my walls suck in. If I shut down my intake fan and close the port, then and only then does it become neg pressure. (walls suck in) I have no fans inside the tent, only exterior mounted intake and exhaust. Air movement inside is suttle and consistent With temps always at 72* 73*. as stated above. I know the concept, I've used it for 38 years. I just might not explain it in the right terms. Thanks for answering!

[Toker1]

Yes, understood. If you are growing some great buds... keep on keeping on.
I was strictly talking from an efficiency stand point. Imo there is no wrong way to grow fantastic herbs. The plants don’t lie.

[Tonyveitch]

Hay guys thank you all for the great info.

[OkieSmokey]

I went negative for smell and heat reasons. The only advantage I see to a positive flow is that all intake air can be filtered if things like dust, pest, bacteria, mold are important to you, but I doubt many have that level of environment control. If you are growing mushrooms or sometging that ls a big deal but I think negative better for bud

[Toker1]

I have screens and filters on all my intakes and still rock the negative pressure in all my tents plus the room the tents are contained in. You don’t need positive pressure to filter the air. If anything positive pressure blows more dust around your grow room balecause of the turbulent flow it creates.