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

the same as an exhaust fan, except it blows fresh air into the room. The ratio of 1 to 4 (100 CFM [m 3 /h] incoming and 400 CFM [m 3 /h] outgoing) should give the room a little negative pressure. Delivering fresh air to plants ensures they will have adequate CO 2 to continue rapid growth. One of the best ways to deliver air directly to plants is to pipe it in via flexible ducting. Ingenious growers cut holes in the intake ducting to direct air where it is needed. The air is dispersed evenly throughout the room.
    Always make sure fresh air is neither too hot nor too cold. For example, one friend that lives in a hot, arid climate brings cool fresh air in from under the house, where the air is a few degrees cooler than ambient air.
Security
    When installing a vent fan, security concerns dictate that no light or odor escape from the exterior vent while allowing ample air release. This can be accomplished in several ways. Baffle or turn the light around a corner to subdue brilliance. Many chimneys in British Columbia, Canada, shoot light out like a spotlight against the low cloud cover. You can walk down city streets where half the chimneys in the neighborhood are beacons of bright light. A 4-inch (10.16 cm) flexible dryer hose will subdue light in smaller grow rooms. Larger 8, 10, and 12-inch (20, 25, 30 cm) heat duct pipe is ideal for moving larger volumes of air.
    Place one end of the duct outdoors. It should be high enough, preferably over 12 feet (3.6 m), so the odor disperses above most people’s heads. One of the best vents is the chimney. The outlet may be camouflaged by using a dryer hose wall outlet attached to a vent fan. The vent fan is then placed near the ceiling so it vents hot, humid air. Check for light and air leaks. Set up the fan and go outdoors after dark to inspect for light leaks. See “Setting Up the Vent Fan” and “Odor” in this chapter.
    Greenhouse fans are equipped with louvers (flaps or baffles) to prevent backdrafts. During cold and hot weather, undesirable backdrafts could alter the climate in the room and usher in a menagerie of pests and diseases. Installing a vent fan with louvers eliminates backdrafts, but may present a security risk if it attracts the attention of the wrong crowd.
Temperature

A thermometer that measures in Fahrenheit and Celsius will come in very handy.

Thermometer Hygrometer
    An accurate thermometer is essential to measure temperature in all grow rooms. Mercury or liquid thermometers are typically more accurate than spring or dial types, but ecologically unsound. An inexpensive thermometer will collect basic information, but the ideal thermometer is a day-night or maximum/minimum type that measures how low the temperature drops at night and how high it reaches during the day. The maximum and minimum temperatures in a grow room are important for the reasons explained below.
    Under normal conditions, the ideal temperature range for indoor growth is 72-76°F (22-24°C). At night, the temperature can drop 5-10°F (2-5°C) with little noticeable effect on growth rate. The temperature should not drop more than 15°F (8°C), or excessive humidity and mold might become problems. Daytime temperatures above 85°F (29°C) or below 55°F (13°C) will slow/stop growth. Maintaining the proper, constant temperature in the grow room promotes strong, even, healthy growth. Make sure plants are not too close to a heat source, like a ballast or heat vent, or they may dry out, maybe even get heat scorch. Cold intake air will also stunt plant growth.

Check the temperature and humidity in several places in the grow room. Lights really heat a room!

This combination thermometer/hygrometer is fairly accurate and easy to mount.

Knowing Celsius and Fahrenheit equivalents makes grow room calculations more precise.

Fill barrels with water and let the sun heat them several days. Water stays warm all night to heat the greenhouse!

This simple humidistat/thermostat controller is just one of many different time/controllers found at www.greenair.com . It is very easy to use and foolproof.
    Cannabis regulates its oxygen uptake in relation to the ambient air temperature rather than the amount of available oxygen. Plants use a lot of oxygen; in fact, a plant cell uses as much oxygen as a human cell. The air must contain at least 20 percent oxygen for plants to thrive. Leaves are not able to make oxygen at night, but roots still need oxygen to grow. A plant’s respiration rate approximately