Calculating By Wattage
Hello there. First off, I'm used to working with Celsius, not Fahrenheit, but I've done my best to provide formulas for both. My method for calculating fan requirements does not cover active cooling with air conditioning systems or cool-tube designs. We're talking about everyday grow chambers here, totally enclosed for airflow control, with no large amounts of radiant heat into or out of the box. Your mileage may vary some for these reasons.
RIGHT THEN, LET'S GET STARTED:
1) Start at the beginning and design this right! Before you even buy or cut anything for your new project, determine the highest temperature that your intake air will ever be when lights run. Call this T (inlet).
2) Use these formulas to determine difference in temperature you can tolerate. 80°F (27°C) is just about the optimal for growing most plants. You can go up to 76°F (30°C) if you have to, but aim for 80°F (27°C).
Tdiff = 27 °C – T (temperature of inlet air)
3) Add up wattage for all power sources in your indoor garden. Lights, pumps, heaters, humidifier, radio, coffee maker, whatever! Add it ALL up and call it Watts. If it is on for more than three minutes and uses more than a watt, add it up. This will make your number worst-case and therefore a conservative value.
4) Compute the absolute minimum fan power you will need using the following formulas. Fan power is measured in the amount of air (cubic feet) shifted per minute. The formula below is the minimum fan rating you must have to achieve your temperature goals. You will have to increase fan power to compensate for duct constriction, small inlets, carbon scrubbers, screens, or other items that block airflow.
CFM = 1.75 x Watts /Tdiff (in Celsius)
If you prefer to work in Fahrenheit, try this formula:
CFM = 3 x Watts / Tdiff (in Fahrenheit)
5) Get at least this fan power or don't come and ask questions! If you are going to have more than one fan, they should be mounted side-by-side
rather than inline if you want to add their different CFM ratings. For inline fans, use the lowest airflow rating of all fans in the path. A fan on the inlet and a fan on the exhaust of the box are considered inline fans. Fans just circulating air inside the indoor garden should not be counted for airflow but must be included in your initial wattage calculations.
OK, TO SEE THESE FORMULAS IN ACTION WE'RE GOING TO HAVE TO DO A LITTLE NUMBER CRUNCHING:
Ok, let's say you have 2000 watts in a 8 foot by 8 foot room with an 8 foot ceiling height.
So what amount of air do I need to move to keep the room at 82°F (28°C)? My incoming air temperatures are 68°F (20°C) during the lights on period.
Tdiff = 28 – 20 = 8°C
For Celsius the formula comes out at:
CFM = 1.75 x 2000 / 8 = 438 CFM
For Fahrenheit we get the following:
Tdiff = 82 – 68 = 14°F CFM=3x2000/14=429 CFM
Remember, Tdiff shows how much your temperatures will rise above your inflow air temperature for a given wattage and air movement.
If you are adding any carbon scrubbers or extensive ductwork, this is where you add to the fan size to account for air pressure losses. You have to move this many CFM, or the numbers don't come out right. Exactly how much these items diminish your airflow depends on your exact configuration and is beyond the scope of this introductory article!
What to do when your outside temperatures are higher than your maximum allowed indoor garden temperatures!
YOU HAVE A FEW CHOICES:
1) Stop growing for a while till things cool off or try running your grow lamps at night when inlet air will be cooler.
2) Reduce your lighting to drop the heat load. Not good if the incoming air is already over critical when it arrives in the box. Might be possible if the inlet air temperature is lower but you are running too many lights to keep up with the cooling.
3) Use active air conditioning.