Why is it necessary to ventilate my garden?
There are two main reasons to keep fresh air flowing through your garden:
1) to evacuate excess heat generated by lamps, pumps, etc.
2) to provide fresh air (i.e. oxygen and carbon dioxide) to your plants while removing the excess humidity they generate.
A high-wattage lamp in a sealed, unventilated room will quickly raise the ambient temperature to 100+ degrees Fahrenheit. At these temperatures, plants grow slowly (if at all) and can succumb to heat stress, or may simply die. Barring issues caused by temperature, without proper ventilation, your plants will eventually use up the available carbon dioxide and oxygen, and increase the humidity in the garden (via transpiration) to unacceptable levels, leading to suffocation. A single properly placed and connected ventilation fan can easily prevent these issues.
How much ventilation do I need?
For our purposes, we’ll consider two main variables: 1) the size of your garden and 2) the amount of power drawn in the garden (e.g., lights, pumps, etc.). Since most fans sold in the U.S. are rated in CFM (cubic feet per minute), we’ll stick with feet as our units. There are a couple methods for determining how many CFM are needed to ventilate a given garden.
Estimate using the volume of your garden:
One method bases ventilation needs on the cubic footage of your garden. Depending on who you ask, the recommended minimum rate of air replacement for an indoor garden (the amount of time taken to evacuate the total volume of your garden) is anywhere from 1 to 5 minutes. That is, the air in your garden should be completely replaced once every 1 to 5 minutes. First, calculate the size of your room in cubic feet – length x width x height. For example, a room 12 feet wide, by 10 feet deep, by 8 feet high: 12 x 10 x 8 = 960 cubic feet. Volume (in cubic feet) / time (in minutes) = CFM. If we use five minutes as our target for complete air replacement: 960 cubic feet / 5 min. = 192 CFM. If our target is 1 minute, the math gets even easier: 960 cubic feet / 1 min. = 960 CFM. So the absolute smallest fan we might get away with must pull 190+ CFM. Remember that lamps and other electrical equipment generate heat and increase the amount of ventilation required. A good rule-of-thumb is to add ~10% per lamp. So in this same room, running 4, 1,000-watt lamps, we’d have to add ~40% to the above totals: (190 CFM x 1.4) = 266 CFM minimum, up to (960 CFM x 1.4) = 1344 CFM. Hot and humid ambient conditions require you to aim for the high estimate; if it is downright cold, aim for the lower estimate. Most gardens will require significantly more air movement than the stated minimum.
Estimate using the combined wattage of your garden:
Another method derives required CFM from electricity (in watts) used, along with the desired difference in temperature between your garden and the ambient air. First, we must determine the highest inlet temperature for your garden – the maximum temperature of the air that will be flowing into the garden. We’ll call this number T(inlet). The next number we need is the desired air temperature for the garden itself. Most plants can handle 80 degrees Fahrenheit or so without any problem. Let’s call this number T(target). With T(inlet) and T(target), we can calculate T(diff): T(target) – T(inlet) = T(diff). Let’s assume our garden is in a house that stays around 76 degrees fairly constantly – this is now our T(inlet). So: 80 – 73 = 7; T(diff) = 7. How many CFM are needed with a T(diff) of 7? Here’s the formula: CFM = 3W / T(diff), with W being the combined wattage of everything in the garden (lights, fans, pumps, the radio, etc.).
Let’s work out an example using the CFM = 3W / T(diff) formula. Let’s say our garden uses 1,200w of lighting and 200w of pumps, fans, etc., for a total of 1,400w. Plugging in the values: CFM = 3(1400) / 7; CFM = 4200 / 7; CFM = 600.
Keep in mind, these figures are rough estimates. There are many variables that influence the amount of ventilation your particular situation requires, beyond the scope of this FAQ. CFM ratings for fans are calculated without any attached ducting; any and all filters and runs of ducting will add friction and/or resistance, thereby inhibiting airflow through the system. Undersized ducting (e.g., a 6” fan reducing to 4” ducting) will choke off airflow. When in doubt, it is far better to overestimate your needs than to have a hot, humid, inadequately ventilated garden.
But math isn’t any fun…
No problem! Here are some simple pairings for fans and lamp wattages/room size:
A 4” fan (~190 CFM) is adequate for a garden using 400w of light or less. A carbon filter may be added at the expense of some airflow, but this is generally not a problem for an area of 8 or 9 square feet.
A 6” fan (~420 CFM) is adequate for gardens up to 1,000w in a 5’ x 5’ tent or other enclosed space. Depending on the ambient temperature, a 1,200w (2 x 600w) garden could be ventilated with this size fan as well. Again, at this scale, a carbon filter may be added to the exhaust system, so only one fan is necessary.
For gardens using two 1,000w lamps or more, it is usually necessary to cool the lights and ventilate the garden separately, with two fans. A 6” fan can cool multiple tube-style reflectors using 1,000w lamps; an 8” fan is usually necessary for multiple box-style vented hoods, or tube setups with more than 4 lamps. To ventilate the garden itself, an additional 6” fan is usually enough for areas up to 8’ x 8’, with the option of adding a carbon filter.
What type of fan should I use?
Centrifugal (squirrel cage) fans
Centrifugal fans, also known as squirrel cage fans, use a turbine to force radial airflow. That is, the exhaust side of the fan is typically oriented 90 degrees from the intake. In addition, the exhaust flange is usually square or rectangular, so accommodations (flange or adapter) must be made when connecting into runs of ducting.
Axial (in-line) fans
Axial, or inline, fans push air perpendicular to the fan wheel (think of a jet engine turbine). With a compact, somewhat cylindrical housing, these fans are designed to be placed in straight runs of ducting, since they do not change the direction of airflow as centrifugal fans do. Axial fans are ideally suited for pulling air through a carbon filter, as they are designed for higher resistances upstream from (behind) the fan. This trait makes axial fans the gold standard for ventilating a garden.
In-line duct booster fans also qualify as axial fans. They are easy to distinguish from the heavier-duty inline fans. Booster fans move air at far lower pressure and volume, with minimalistic housing and fan impellers. As their name implies, booster fans are designed to be placed in long runs of ducting to compensate for friction losses and keep air moving. On its own, a booster fan lacks the power necessary to effectively move air through a carbon filter or other source of resistance.
In addition to exhaust fans, it is wise to place circulation fans in your garden to disperse heat and humidity around the garden canopy. Growers typically employ one circulation fan (oscillating floor fans are perfectly fine) per lamp.
In tight spaces, wall-mounted oscillating fans (above) can be great as well.
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