Marijuana Horticulture: The Indoor/Outdoor Medical Grower's Bible

transpiration increases, stomata function properly, and growth rebounds. A vent fan that extracts air from the grow room is the perfect solution to remove this humid, stale air. Fresh air flows in through an intake vent or with the help of an intake fan.
    Ventilation is as important as water, light, heat, and fertilizer. In many cases, fresh air is even more important. Greenhouses use large ventilation fans. Grow rooms are very similar to greenhouses and should follow their example. Most grow rooms have an easy-to-use opening, such as a window in which to mount a fan, but security or room location may render it unusable. If no vent opening is available, one will have to be created.
    All grow rooms require ventilation. This system could be as simple as an open door or window that supplies and circulates fresh air throughout the room, but open doors and windows can be inconvenient and problematic. Most growerselect to install a vent fan. Some growers need to install an entire ventilation system including ductwork and several fans.

For indoor rooms, inline fans are one of growers’ favorites. These fans are quiet and efficiently move large volumes of air.
    A vent fan pulls air out of a room four times more efficiently than a fan is able to push it out. Vent fans are rated by the amount of air, measured in cubic feet per minute (CFM) or (cubic meters per hour [m3/h]) they can move. The fan should be able to replace the air volume (length × width × height = total volume in cubic feet or meters) of the grow room in less than 5 minutes. Once evacuated, new air is immediately drawn in through an intake vent or an intake fan. (Covering the intake vent with fine mesh silkscreen will help exclude pests.) An intake fan might be necessary to bring an adequate volume of fresh air into the room quickly. Some rooms have so many little cracks for air to drift in that they do not need an intake vent.
    Do not set up a circulation fan in the room and expect it to vent the area by pushing air out a distant vent. The circulation fan must be very large to adequately increase air pressure and push enough air out a vent to create an exchange of air. A vent fan, on the other hand, is able to change the pressure and exchange the air quickly and efficiently.
    Squirrel cage blowers are efficient at moving air but are very loud. Blowers with a balanced, well-oiled wheel run most quietly. Felt or rubber grommets below each foot of the fan will reduce noise caused by vibrations. Run motor at a low RPM to lessen noise.
    Inline fans are designed to fit into a duct pipe. The propellers are mounted to increase the airflow quickly, effortlessly, and as quietly as possible. Inline fans are available in quiet, high-quality models that run smoothly.
    Propeller or muffin fans with large fan blades expel air through a large opening, and are most efficient and quiet when operated at low RPM. A slow-moving propeller fan on the ceiling of a grow room will quietly and efficiently move the air.

Straight with no bend is the most efficient

Thirty degree curve cuts up to 20 percent of air transmission.

Forty-five degree curve cuts up to 40 percent of air transmission.

Ninety degree curve cuts up to 60 percent of air transmission.
    Hot air rises. Adept growers locate air exit vents in the hottest peak of the room for passive, silent air venting. The larger the diameter of the exhaust ducts, the more air that can travelthrough them. By installing a big, slow-moving vent fan in this vent, hot stale air is quietly and efficiently evacuated. A fan running at 50 RPM is quieter than one running at 200 RPM. Smart growers install 12-inch ducting and inline fans.

Intake air ducting has holes covered with duct tape. Air flow is directed by removing pieces of tape.

Fresh air in this room is piped in directly where plants need it.

To control the air temperature, hang the thermostat near the canopy of the garden.
    Most often, the vent fan is attached to ducting that directs air out of the grow room. Flexible ducting is easier to use than rigid ducting. To install, run the duct the shortest possible distance, and keep curves to a minimum. When turned at more than 30°, much of the air that enters a duct will not exit the other end. Keep the ducting straight and short.
Intake Air
    Many rooms have enough fresh air coming in via cracks and holes. But other grow rooms are tightly sealed and require fresh air to be ushered in with the help of an intake fan. An intake fan is