Getting Positive Results with Negative Pressure
By Rich Hamilton from Maxmium Yield 2 June 2018
We often use many scientific methods for indoor gardening, often without even knowing it. It may not occur to us we are using and learning not only about biology, but chemistry and physics as well. With indoor growing, the aim is to mimic the environment the plant would naturally grow in, while also trying to optimize that environment so the plant achieves its maximum potential. The environmental controls we use are also intended to make the process as easy for us as growers as possible, allowing us to get great yields easily and with the greatest convenience and minimum fuss. Given that, it’s interesting to take the idea of negative pressure in the growroom, look at how to achieve it, and some of the science behind it.
Understanding Negative Pressure
Never heard of negative pressure? Could you explain it to someone if you had to? Pressure is defined as an area of force. Positive pressure is when an enclosed area has higher pressure than the area around it, so the gas or liquid inside of it is pushing to get out. A balloon filled with air or water is a good example. Air wants to and will escape from the balloon if you let it go as you are blowing it up. The force of the air escaping will send it flying off round the room. Similarly, if it is pierced it will make a loud bang as all the air rushes to escape through the tiny hole. If you pierce a water balloon, the liquid will squirt out like a jet from the positive pressure force trying to escape the balloon.
Negative pressure is simply the opposite, so an enclosed area has a lower pressure than the area surrounding it. A good example is a simple drinking straw. When you suck on the straw you create a vacuum or area of negative pressure by removing the air from the straw. To relieve or fill that negative pressure, fluid (your drink) or more air rushes into the straw, just like fluid rushes out of the balloon when you puncture it.
Negative Pressure in the Growroom
So how does this apply to your growing regime? Your grow room is a perfect example of an enclosed environment and your ventilation system is the air source being sucked in and expelled out. An area of negative pressure is created inside the grow room by the rate at which air enters and leaves the environment. If more air is leaving the grow room (outtake) than entering (intake), you have achieved negative pressure. You will know this is the case by looking at the outside of your tent. The vertical sides should be slightly concave (pulled in) in appearance. This is because you have created a vacuum inside. Keep your tent in negative pressure for the entirety of your grow for best results. Negative pressure is beneficial to your plants as it makes it easier to control and maintain temperature, humidity, CO2 levels, and other contaminants because the hot, humid air is extracted from the tent quickly and constantly at the same rate. This allows for a stable environment within your grow room and avoids any risk of mold or mildew forming on plants.
Other Benefits of Negative Pressure
Negative pressure makes life easier in the growroom because you need only make small adjustments during the different stages of the plants’ life cycle and can otherwise be assured atmospheric conditions are stable within the grow area. The other benefit is it controls unwanted odors.
Most gardens produce odor, whether it's the type of plant being grown or just a moist soil-type smell. With an indoor garden, especially if it’s in your home, you may not want to be constantly exposed to the smell of your plants. You also may not want to bother neighbors with the smell or attract attention to your garden. Negative pressure helps eliminate odors by pulling out all the odor along with the unwanted heat, humidity, and CO2. This stops the smell from lingering in the tent, growing stronger, and seeping out into your home and surrounding area. However, some plants have stronger smells than others, so by fitting a carbon filter to the end of your extraction ventilation, you can ensure all odor leaving the growroom is "scrubbed" clean and undetected.
How to make Negative Pressure
What is the best way to create negative pressure? Effective ventilation is key. You’ll need two fans: An intake fan that brings cool clean air into the grow room and an extraction fan which removes warm, odor-filled air. The size of your fans depends on the size of your grow room and how much air you need to move. Your extraction fan should always be bigger or more powerful than your intake fan (maybe even use two smaller fans for outtake if you are worried about noise), so you can guarantee more air is leaving the growroom than entering. It is always a good idea to buy a bigger/more powerful extraction fan than you need because you can run it on a lower, quieter setting and still have enough power to create negative pressure in normal conditions, but also have that extra boost of power if needed on extra hot or humid days. Another effective way to save on energy and equipment costs is to have a simple intake air vent that is twice the size of your outtake outlet and just run one fan for extraction.
Use Acoustic Ducting
Make sure your ducting is efficient too. Buy acoustic ducting to keep airflow noise levels to a minimum and try to keep it as short and straight as possible since your fans’ performance can be reduced by two per cent for every meter of ducting used. Any change of direction in ducting can slow airflow down and cause condensation. In terms of eradicating odor completely, it’s essential you match up the correct size of carbon filter to your exhaust fan, so you can achieve a 0.1 second contact time to efficiently scrub the air passing through the filter.
Hopefully this gives you a better understanding of negative pressure, why we use it, and how it helps plants and growers alike. On top of that, you've just taken a physics class and probably didn't even notice.