Calculating Your Airflow

Why is important that I keep fresh air flowing through 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 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, your plants will eventually use up the available carbon dioxide and oxygen, and increase the humidity in the garden (via transpiration) to unacceptable levels. One properly placed and connected ventilation fan can easily prevent these issues.

How much ventilation do I need?

There are a couple different methods for determining how many CFM are needed to ventilate a given garden.

There are two main variables: 1) the size of your garden, and 2) the amount of power drawn in the garden (e.g., lights, pumps, etc.).

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 one to five minutes. That is, the air in your garden should be completely replaced once every one to five minutes.

First, calculate the size of your room in cubic feet – length x width x height. For example, a room twelve feet wide, by ten feet deep, by eight 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 divided by 5 min. = 192 CFM. If our target is one 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, 1000w 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 1200w of lighting and 200w of pumps, fans, etc., for a total of 1400w. 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 which 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.

Here are some 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 1000w in a 5’ x 5’ tent or other enclosed space. Depending on the ambient temperature, a 1200w (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 2, 1000w 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 1000w 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.

Still Not Sure? Then why don’t you pop down to one of our stores where one of our experienced staff members will be more than happy to explain it in laymen’s terms…

To Purchase any of our products please contact us on 020 8644 3780 for our Sutton offices or 01293 535 157 for our Crawley offices.