Keep Odours from Getting Out
Grow lights produce heat, and plants use up the carbon dioxide (CO2) in the air.
For that reason, the air inside a grow tent or grow room should be changed out by an extraction system. Doing so draws in fresh air with normal levels of CO2, from the outside. To assist with the replacement of the air, some growers also use an intake fan, this intake fan must be pulling in less air than the extractor fan to assure negative pressure.
It is very important that there is always negative air pressure in a grow tent or grow space. This is where the air pressure inside is lower than the air pressure outside.
This ensures that all of the air leaving the tent or room does so via the extraction system with a carbon filter connected. The air is cleaned, and odours are removed before it exits the grow space. As long as there is negative air pressure inside the grow space, none of the air (probably smelling of plants) will exit through the doors or ducting socks. If this happened, it could cause problems with neighbours or passers-by!
How to ensure you have negative air pressure
The best thing to do is to use a smaller fan on the intake than on the extraction system. For example, if you have an 6” fan on the extraction then only use a 5” fan on the intake,
Another thing you can do is use a fan speed controller which connects to something called a balancer which allows the user to alter the balance between the 2 fans.