No announcement yet.

"Let them Breathe" by DJ Short

  • Filter
  • Time
  • Show
Clear All
new posts

    "Let them Breathe" by DJ Short

    Hello all. I was searching around for some ventilation info and came across this great article by DJ. After searching through ICMAG, I did not find this on the site, so I decided to post it up for all to see. Hopefully posting it here will help some growers out.

    Let them breathe!

    Article Taken From: Cannabis Culture (Issue #13)
    Written By: DJ Short

    Proper ventilation and air circulation are essential to growing healthy, happy plants.

    An important aspect to consider when growing plants indoors is proper ventilation, air circulation and temperature control. This practice becomes especially necessary when working with lights over 400 watts, in very small spaces, any time the temperature exceeds 32°C (90°F), or if the humidity gets too high.

    The Perfect Temperature

    Though sense of feel is adequate to gauge the "perfect climate" for a given plant, there is no real substitute for a thermometer and humidity gauge.

    Thermometers are cheap and accurate enough for our purposes. I usually employ several thermometers in different areas in and around the grow room. Somewhere between 32-35°C (90-95°F) is the absolute highest room temperature your plants would care to tolerate. The perfect temperature would be somewhere between 24-29°C (75-85°F). Peaks of 38°C (100°F) are allowable for most strains, but not for any longer than a half hour or so. And only above the root level.

    Roots and Aeration

    The main area of concern involving temperature are the roots of the plants. Ideally, the roots should be kept at as constant a temperature below 21°C and above 10°C (70-50°F) as possible. The fact that warm air rises and cool air sinks works to our advantage in this case. Also, the plants end up under the larger lights by the flowering cycle, and so they're usually large enough to help shade and cool their root areas.

    Still, some rooms build up sufficient heat to require a separate circulating fan, or fans, focused specifically on the root systems. A soil thermometer may be a wise investment.

    Proper aeration of organic based soils is crucial in high temp/humidity areas. Perlite and vermiculite, are the tips here – add more to the soil if need be. In hydroponic systems make sure that the nutrient water temp is below 21°C (70°F). If necessary, store the reservoir outside of or below the grow room.

    Squirrel Cage and House Fans

    There are many different types of fans and air movers available on the market. Most fans can be purchased at the average home improvement store. Proper research and smart shopping will net the best purchases. Careful planning will help avoid costly mistakes. Using the "hot air rises, cool air drops" rule, one can figure out the right solution.

    The two most common types of fan are the squirrel-cage and what I call the "common house fan" (box or oscillating fans). Both come in a seeming endless variety of shapes and sizes. Generally speaking, squirrel-cage fans move air either in and/or out of the room, while common house fans move the air up, down and around the room. There are also neat little "muffin" fans that can be used for many things such as light-hood venting and passive ventilation systems.

    A passive ventilation system is one that moves air either in out of room (not both). The room is not sealed and so air exchange is allowed free movement from inside and outside the room.

    Squirrel Cages

    The squirrel-cage fan is the most popular fan for moving large volumes of air into or out of a room or rooms. A common placement for this fan is inside of the room, up high, blowing out. This will help move the hot air out. This method is what is used to stimulate the passive intake of cool air with vent holes cut in the floor or lower walls to access the cooler areas outside of the room.

    Other hardware such as dryer-vent tubing or muffin fans may be used to best access the cool, dry air outside of the grow room. It is a simple step further in this type of system to add an oscillating fan or two on the floor, pointing at any angle up, to help circulate the cooler air up and around the plants. This is the simplest of vent systems and works quite well. Choosing the correct squirrel-cage fan is part of the trick to success.

    Measuring Air Movement

    Squrrel cage fans are rated by their volume of air movement in cubic feet per minute or CFM. A fan with a rating of 100 CFM is able to move 100 cubic feet of air per minute. A room that is eight by ten feet and eight feet tall holds 8 X 10 X 8, or 640 cubic feet of air. Therefore, it would take an optimally running 100 CFM fan 6.4 minutes to fully circulate the air in that room.

    Generally speaking, most fans move a little less than their rated CFM due to intake resistance or a dirty fan cage. Bigger fans usually will work more efficiently. Potentiometers, or a "volume control", could be installed in the power line of the larger fans to adjust the fan speed. This would give further aid in the specific control of air volume and ventilation.


    The ideal ventilation system utilizes automation in the form of thermostats and regulators. A thermostat, as with the common household thermostat, would cause the fans to turn on at a certain temp, and turn off at another. That is, a sensor would turn on the fans on at around 30°C (86°F), and turn them off if the temperature dropped below 21°C (70°F). A well-stocked, high-tech grow shop will have several types of thermostats available in a variety of systems.

    Box and Oscillating

    Common summer house fans also come in a wide array of types and sizes. The most common being the box and the oscillating. Box fans are self explanatory. They can be used in a variety of ways, depending on the innovation and imagination of the user. Experimentation will yield the most efficient uses for these devices.

    Oscillating fans are perhaps the most efficient devices for circulating air in a room. The gentle back and forth sway of the fan is very beneficial for the developing plants. These fans tend to keep anaerobic molds down by constantly freshening any potentially stagnant air. There are wall-mounted styles available as well. Home improvement centres carry a large array of various types and styles of air-moving fans, some relatively inexpensive.

    A warning needs to be expressed concerning the cheaper, discount-store, oscillating fans (or any cheap fan for that matter) that have a tendency to burn out after a period of time. Some of these products are potentially dangerous if left plugged in and turned on after they burn out.

    Therefore, it is a wise idea to check one's fans (and all electric devices and equipment for that matter) on a regular basis as often as possible.

    Noise Concerns

    Another fan consideration is noise. Some fans, especially the squirrel cage, may be a bit too noisy for a given situation. There are higher quality fans available that do run more quietly – expect to pay more, of course. It also helps to mount the fan directly to a main stud or support, by at least two of its support holes, and preferably more.

    Rubber dampers and gaskets can be easily made and used on the support holes or around the overall mounting surface. Keep the fan's bearings sufficiently lubricated as well.

    High Humidity

    Humidity is another factor that influences the overall quality and quantity of a crop. Generally speaking, high humidity (over 80 or 90%) is bad. It inhibits plant transpiration and ultimately stunts growth. Mold and fungus love high humidity as well. Note that warmer air holds more moisture than cooler air.

    There are a few simple practices to help reduce humidity. First and foremost, keep the room as dry as possible. When watering, use just as much as the plants need. Pump, siphon or mop up any remaining water and remove it from the room.

    Keeping the room clean also helps. Moisture likes to hide and store itself in material such as dead leaves, spilled dirt or any garbage. Therefore, keeping the room clean and free of debris will help keep moisture and organisms such as mold, fungus and bacteria down.

    Temperature and moisture levels directly affect the plant's ability to metabolize nutrients and supplements such as fertilizer and carbon dioxide.

    If these practices fail to lower humidity enough, the only solution may be a de-humidifier. However, de-humidifiers are expensive, consume a large amount of electricity and produce heat. These factors will need to be considered in choosing whether or not to employ one.

    Ventilation and Circulation are Essential

    Proper ventilation and air circulation are essential to maintaining a healthy indoor grow environment. The basic rule of thumb is to move the warm, moist air out and to move the cool, dry air in and around the plants and their roots. Many various types of fans and devices are available to achieve this goal.

    Careful planning, basic research and smart shopping will acquire all that one needs to keep it cool and dry, and experimentation will fine tune the system to provide the most perfect indoor environment possible.

    Thanks DJ for another informative read
    Last edited by guerilla1018; 08-31-2006, 07:56.

    great info


      nice find
      Man made booze God did not
      God made pot man did not
      The Old Farmers Almanac
      U.S. State By State Laws on our hobby

      Beneficial Insects
      sick plant ??? <-- mynamestitch's work

      Informative threads by I.M. Boggled

      PotHeads BunKer 2011


        helpfull, thanks for the info!
        Murphys 400 Watt CMH Cabinet Grow DJ Shorts Grape Krush & Mandala Safari Mix

        The Warden is pulling the plug.....going out as big as I can .


          Originally posted by guerilla1018
          Thermometers are cheap and accurate enough for our purposes. I usually employ several thermometers in different areas in and around the grow room. Somewhere between 32-35°C (90-95°F) is the absolute highest room temperature your plants would care to tolerate. The perfect temperature would be somewhere between 24-29°C (75-85°F). Peaks of 38°C (100°F) are allowable for most strains, but not for any longer than a half hour or so. And only above the root level.

          Does this mean even with co2, the perfect temp is still between 75 and 85?


            I would imagine that the temps would be different with a co2 system, but I am not sure. I would still imagine that 75-80 degrees fahrenheit would be fine even with the co2 though.

            I am glad that some people have found this informative and helpful. All praise to DJ for another helpful piece.



              great info. just what i was looking for.
              "Each betrayal begins with trust, every man returns to dust"
              East Coast Croppers




                  One question -

                  I know that a system is only as strong as it's weakest component....

                  But in the event of having to chose between either cold temps at night (but recieving air circulation) -- or cutting the time the fans come on from every 5-15 min to every half hour or so for just enough time to cycle new air whatever is drafted in from the constantly ON outlet fan.

                  Which would be more beneficial for the plants? Cause if i do one, then i risk the cold stunting them - or I could just cut back the air intake during the time lights are off (the lights make the room perfect temp) and let the temps be less extreme.
                  &quot;Federal scientists state that 10,000 acres of hemp, maintained year after year, will yield as much pulp as 40,500 acres of average wood pulp land&quot;

                  My Sweet Tooth HEMPY's

                  Granny Storm Crows medicinal cannabis use list!

                  Gray Wolfs has his oils down.


                    I'd be concerned that during your off cycle that RH will spike, without adequate venting. Cold temps are not good either, I use a space heater during cold months to keep lights off temps up..
                    "Live your life not celebrating victories, but overcoming defeats."