Good info @DEEPDIVERDAVE, here is a copy of some a tech tip from HTG, hopefully it clearer and less boring than I would be.
Indoor Garden Air Circulation Setup
There are several tools which make it a breeze to monitor and control your humidity and temperature, but the gadgets are only as helpful as the actual sense behind the setup. A fan doesn’t do any good if it’s not pointed at something, right? That’s a fairly silly example, but it illustrates a point: your setup has to be, well, set up. You’ll get decent results if you do a decent job, but to get excellent results you have to think it out. For example, let’s say you need to ventilate the excess humid air from your tent. Opening your vents is a solution, but it’s not the best one. What you really need in this situation is an exhaust system, which is much less expensive or intimidating than it sounds. A simple duct fan is all you really need for smaller spaces like a tent – just put it right in your tent’s vent port (preferably one that’s higher up) and you’re set, it’ll suck unwanted air out of your tent and as long as you leave an open vent on the other side, fresh air will replace it. If you do it this way, just be aware that the open vent should be covered with a filter of some sort to prevent pests and mold from coming in with the fresh air.
If you’re feeling inspired, you can employ a slightly more advanced setup using a second duct fan. In this case, you’ll place the first fan in a higher-up position, either on top or high on the wall. Since hot air rises, this will efficiently pull the hottest air out of the tent. Then you’ll place your second fan in a vent near the floor of the tent, pulling air IN. Most tents can get by with just a 4″ duct fan for this purpose. With this system, fresh air will be sucked into the tent at the bottom and exit at the top, and it’ll collect moisture from the plant and spread CO2 the whole way up. We recommend having a slightly bigger and/or stronger fan blowing OUT than sucking IN, though, just to be safe. This helps to create a negative pressure inside of the space, which helps to maintain proper airflow.
Will from Commerce City says: “In a 9’ x 13’ room, I would have about 10 fans in total. This includes an intake, exhaust, floor, and circulating/wall-mount fans. I made a diagram to show you what I mean – don’t judge my drawing skills!” (We totally judged his drawing skills and remade the picture, sorry Will!)
How to Calculate CFM Requirements
There’s a pretty simple formula to figure out how strong your fan needs to be to optimally circulate your air. We’ll take the width x length x height. (Example: a tent or room that is 12f x 10f x 8f = 960 cubic feet.) Now you need to decide how often you want to exchange the air the room. For this example, we’ll use 4-minute intervals. Dividing the cubic feet (960) by the number of minutes (4) will provide the Cubic Feet Per Minute, or CFM – in this case, 240. So we’re looking for an outtake fan (or a combination of outtake fans) that’s around 240, or even very slightly higher. But wait, hang on, there’s one last factor: our lighting. LED grow lights don’t really contribute to the heat of the tent/room in any substantial way, but if we’re using lights that heat up (like CMH or HPS lights), we’ll need to account for the extra heat they cause. In that situation, we just add 10% to our total for every 1000w of lighting. So if we have four 1000w lights, we add 40%, which brings our total up to 336. Don’t worry, all our duct fans are specifically labeled with their CFM information and if you’re ever not sure about anything, just ask us! Also, it’s important to know that the strength of the fans increases exponentially with their size; the 4-inch GrowBright HV fan is rated at 170 CFM, but the only slightly larger 6-inch unit is more than twice as powerful at 400 CFM (and it’s only about $25 more). If you can’t find a model that offers a CFM directly in line with your calculations, you can always grab a larger fan and use it with a fan speed controller to dial in the optimal airflow. So if you’re doing the math and worrying about how many fans it’ll take to handle the big numbers you’re coming up with, worry no more – chances are, it’s MUCH cheaper and simpler than you think.
Even in a sealed space, we recommend a full air exchange at least twice a day or up to every four hours if a CO2 generating burner is being used, because combustion leads to ethylene buildup. Ethylene buildup can become an issue even before you can visually see the problem because it stunts plant growth, but advanced symptoms will manifest in wilted-looking leaves that have retained their turgidity (water pressure inside the plant). You should swap out air more often in smaller spaces, of course, but people sometimes assume that a large enough room negates the need for ventilation. If you don’t properly supply your plants with the air they need, they won’t grow to their potential.
FACTORING FILTERS INTO YOUR CFM CALCULATIONS
For those of you with more advanced needs, another consideration to account for will be air filtration. We touched on it briefly earlier, but if you have a big/complicated setup and you want to be sure that your plants are protected, HEPA intake filters are the way to go. GrowBright HEPA filters catch 99.97% of airborne particles 3 micrometers or smaller in size, and all you have to do is slip one over your intake fan/port. For further protection (and for odor control purposes), an activated carbon filter for your outtake fan will do wonders. Same principle, just connect it directly to the duct fan that’s blowing air out of your tent and you’re set! Using these filter types is a best practice for indoor growing, but they will lower the airflow rate of their partner fans in the ventilation system. Be sure to factor this into your calculations if applicable to ensure proper air movement is maintained.
We hope we’ve given you something to think about – your air circulation and quality should be just as carefully planned as your lighting and nutrition to ensure a bountiful harvest! Be sure to see below for this weeks coupon code and sale information! As always, drop by your local HTG Supply store if you have any questions, and don’t forget to follow us on social media to be notified whenever we post more articles just like this one (and whenever we have our awesome sales). From all of us here, good luck, stay safe, and Happy Growing!