Fans and air filters are an important aspect of any successful marijuana grow for a number of reasons. First and foremost is the issue of legality, and since cannabis cultivation is still illegal in most jurisdictions that means cannabis cultivators must do something to contain or eliminate the distinct odor that cannabis produces when growing. (Here it is important to note that it is not recommended to try to cover up the smell with perfume, incense, or spray atomizers such as Lysol, Glade, Febreeze, etc.)
The lights used in cannabis cultivation (particularly HID bulbs such as the high-pressure sodium and metal halide) can also produce quite a bit heat, which means that your grow room can quickly get too hot for your plants. Where space is not an issue and the heat can freely dissipate, minimal ventilation may be required; but in the case of most small grows and stealth closet grows additional ventilation will be necessary.
Before we get into additional detail about your ventilation however, I’d like to include a note about the merits of a simple oscillating fan in your grow space. An oscillating fan, run on low power and aimed at the stalks of your plants, will simulate a natural breeze, which in turn encourages your plants to develop strong and robust stalks. A plant with a stronger main stalk is healthier and capable of supporting a much heavier load, i.e. more bud.
Back to ventilation however, it’s important to remember that what comes in must go out. So you can’t simply open a window or draw fresh air into your room as an adequate means cooling, you must also expel the hot air from your grow room. With that hot air, of course, goes the distinct odor of your vegetating or budding crop, so unless you wish for your neighbors and/or local police to know about your grow, a little bit of air purification goes a long way.
When it comes to proper ventilation for you cannabis grow there are a few key things to consider. First and foremost is to consider how large your grow space is. Small grow areas, like a 2’ x 2’ or a 4’ x 4’ grow space, won’t need nearly as much ventilation as larger areas, like an 8’ x 8’ or 10’ x 10’ grow space. Calculate the volume of air in your grow space by multiplying the length x width x height of your grow area (so a 4’ x 4’ x 8’ grow space has 128 cubic feet).
You’ll want to choose a fan that is capable of exchanging the air in your grow room roughly once every 3 to 5 minutes when it’s in operation. So in the case of a 4’ x 4’ x 8’ grow room with 128 ft.³ of air you would need a fan rated to move 42.6 CFMs (cubic feet per minute) or more. Most growers couple there extraction fan with a suitable carbon air filter, so for instance of four-inch extraction fan would be coupled with a 4 inch carbon filter.
For the best results, it’s recommended that your carbon filter have a CFM rating that is two times or more the CFM rating of your extraction fan. This is because if the air passes through your filter too fast, the filter will not be able to scrub the scent of your cannabis from the air. If scent is escaping your grow room you may need replace or increase the size of your carbon filter.