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Ventilation

Edited by St0ney

All plants intake carbon dioxide through their leaves and, as a byproduct of photosynthesis, exhaust oxygen. So it stands as no surprise that indoor cultivated cannabis plants require a constant supply of fresh air. If the air in a grow room is not exchanged regularly, it becomes an oxygen rich(and carbon dioxide poor) environment; which results in the slow asphyxiation (strangulation) of the plants. Thankfully, circulating the air from within your house is sufficient enough to promote vigorous growth. Drawing in air from the exterior is ideal, but not always practical due to the grow room location(within the house) and outside air temperatures.

The primary reason to vent is not for fresh air, but heat. HID lamps emit large amounts of heat, and without proper ventilation, that heat builds up rapidly. Exhausting heat is the primary goal of venting, the fresh air supply for the plants is simply a fringe benefit; "killing two birds with one stone" some may say.

Requirements

As a general rule, you should "replace" the air in your grow space four to six times every minute to control the heat and supply fresh air. If you're growing in a small space(EX: cabinet or closet), you must ensure that there is at least three times the passive intake area as there is for exhaust area. For example, if you are venting a 25 cubic foot cabinet with a 100 CFM 4 inch round inline fan, you must have three 4 inch diameter holes for passive intake(or an equivalent area). A sealed space(door and windows shut in a room, closet doors shut) will not allow enough air in or out to ensure a fresh supply of cool air for your plants.
Depending on the cubic footage, and assuming that the surrounding area is at room temperature (roughly 72 degrees F), use the graph below to help determine your venting needs. To calculate the cubic footage of your grow space, multiply the length, width, and height together; the result will be your cubic footage for that specific grow space). After calculating the cubic footage, compare it to the graph below to find the minimum amount of CFM extraction for your grow space. Once again, this graph is assuming your are not exceeding my recommended light (and therefore heat output) levels.

When venting grow rooms over 200 cubic feet (up to 800 cubic feet), the volume of air involved changes things slightly. It requires less CFM of exhaust to maintain a proper temperature in the grow space of large cubic footage(250 cubic feet +). So, beyond 200 cubic feet, you need no additional venting up to 800 cubic feet; and naturally, the plants still get all the fresh air that is required. When the light is not in operation, the grow room no longer requires all of that air movement for cooling; so, most growers reduce their exhaust to half strength during the down time.

Exhaust/extraction fans can be bought at many hardware stores or parts supply stores, and can be purchased in a variety of sizes. The smallest commonly available extraction fan is the "bathroom exhaust" fan, it comes in sizes from 50 CFM up to 250+ CFM. Beyond 250 cubic feet per minute, we are primarily limited to furnace type "blower" fans, and these fans range anywhere from 200 CFM to 3000+ CFM. As a general rule, growers should buy the fan (more often than not, it is a combination of fans) that best fits the needs of the intended grow space.

 

Forced Intake vs. Forced Output

Most grow rooms "force out" the warm air and allow cool air to be passively drawn into the room by the extraction fans. But some growers argue that forced input (and therefore passive output) is the more efficient way to exchange air. The forced input argument relies heavily on "positive pressure". To put it simply, positive pressure is what happens inside a fully inflated balloon after it has been expanded by our breath, the air inside is wanting out and putting positive pressure on the inside walls of the balloon. Imagine the same environment in your grow room. The cool air being forced in by the input fans is heated by the light and begins to rise. That warm air is then allowed to seep out. The positive pressure inside your grow space ensures that the only matter (air included) entering your grow area enters through a place you have designated. This gives you the ability to filter the incoming air for any unwanted debris or insects and direct the cool, fresh air at the plants more precisely. As already stated, three times the area of forced intake is the minimum required for passive output in smaller grow spaces.

On the flip side, forced output allows you to control the outgoing, warm air; and, if desired, still filter the incoming air. It is obviously better to mount extraction fans as high as possible, it ensures that the only air being removed is hot air. I prefer the forced output method of ventilation, primarily due to safety concerns for the plants. If there are two extraction fans running and one fails, the remaining extraction fan can keep the temperatures at reasonable and bearable levels for the plants. If there are two intake fans running and one of them fails, it will not be able to keep the temperature from soaring to unmanageable levels (in plant terms). Back to the balloon metaphor. Remember the fully inflated balloon from that was mentioned earlier? Imagine that same balloon with only half of the air in it. Obviously, the positive pressure is far less than it was before. That one intake fan cannot supply enough positive pressure to passively "push" out enough of the heat.

Both forced intake and output can be done at the same time, much akin to HVAC. Because air, in its nature, is a fluid, CFM does not add up in the standard linear fashion. If you have a 64 cubic foot grow space, it needs to have at least 192 CFM of extraction. The simple solution (for the combined venting approach) appears to be two 100 CFM fans; one for intake and for exhaust. But that would be incorrect. If there is only 100 CFM entering the space, then, according to fluid dynamics, there can only be 100 CFM leaving the same space. In essence, you need to double the number of fans required to do the same job. Combining both forced intake and forced output definitely provides better overall temperature control.

 

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