Today, it’s relatively easy for cannabis growers to provide consistent levels of light for year-round cultivation using artificial sources. But while artificial illumination can give cannabis plants the light they need in the right spectrum and intensity, grow lights often contribute excessive, unwanted levels of heat to the cultivation environment. And if this heat isn’t extracted or otherwise addressed, the growroom quickly warms up past the optimal range -- hurting growth and even leading to ruined crops in extreme instances.

If money was no object and we didn’t live in a world where cops and thieves were out looking for grows to bust or rip off, we could just open a window or perhaps install an industrial-grade cooling system. However, since this isn’t the case, we must plan a ventilation system carefully for optimal and safe marijuana production year-round.

One size rarely fits all, and that’s especially true when it comes to an efficient ventilation and cooling system for your growroom. In fact, what counts as “efficient” typically changes with the seasons. For example, growers who want to maintain consistent yield levels throughout the year and also keep a sharp eye on their operating costs may opt for in/out ventilation in the winter months; then, with a few quick changes, they are able to switch to air-cooled lighting assisted by air conditioning in the summertime.

In order to get a better handle on ventilation, let’s start with some of the basics and then look at a working model that takes advantage of the several different methods for keeping a growroom cool (and with maximum efficiency) rather than relying on any single one.

Circulate Freely
No matter how you set up your indoor grow, good air circulation is crucial. If you read a lot of cultivation literature, you will see this stated time and time again -- and for a very good reason. Plants release water and other vapors through tiny openings in their leaves called stomata (or stomates, if you prefer). They also take in carbon dioxide, which is an essential building block in the synthesis of glucose -- the net gain or ultimate “energy food” produced by photosynthesis.

Indoors, if your growing area doesn’t have air movement throughout the plant canopy -- from one side of the plant to the other (around all sides, preferably) and all the way through from top to bottom -- the air can stratify. This means that pockets of stale air develop in the plant canopy, limiting growth and inviting problems. Stagnant air in turn creates pockets of moisture on leaves and growroom walls, providing the ideal conditions for plant diseases that can hurt your crop or even wipe you out in extreme cases.

Oscillating fans have long been the best solution for this, although in tighter spaces, having several smaller stationary fans or perforated fan-pressurized duct tubing blowing in several directions can create sufficient turbulence, especially with deflection from the walls and other surfaces.

Another important point to remember is that if your plants can release moisture more freely through their leaves while taking in a steady supply of CO2 (rather than being stifled by stagnant air and excess moisture), they may then absorb more water and nutrients for maximum yield potential.

Stay Cool
Inexperienced indoor or greenhouse growers can easily be caught off guard by just how quickly an enclosed space heats up from high levels of light energy coupled with waste heat from the lighting system. Even if a room is naturally cool to begin with, it can exceed the optimal 75F to 85F range surprisingly fast when the lights are on. Most plants begin to suffer in temperatures above 90F, even if there are higher levels of CO2 (we’ll talk more about that in a bit).

The game also changes seasonally if you rely on outside air to keep your growroom temperatures in check. As a rule of thumb, the outside air you use to ventilate and keep your temperatures optimal should be 10F to 15F cooler coming into the room than the temperature you are trying to maintain with the lights on. Of course, different light sources will cause different spikes in tempera- ture. Usually, this just means a faster air changeover rate is required -- and not necessarily colder temps, although cooler, free-flowing air is always a blessing. The level of insulation in your growing area and the surrounding temps can have a noticeable impact, too. Well-insulated grows are more efficient to cool and don’t hum anywhere near as loudly, assisting in the stealth factor.

In/Out When It’s Chill Out
For the budget-minded grower, in/out ventilation can be a practical and effective solution when the temperatures outside remain below 65F. This means that in addition to internal air-circulation fans, there are ventilation fans (typically furnace types or inline centrifugal style) that serve to move hot, humidified, CO2- depleted air out and away, and smaller-capacity fans that introduce fresh, cooler air into the grow environment.

Optimally, the fans run 24/7 and are plugged into a controller -- for example, a TV-1 or TV-2, which maintains an idle fan speed but increases or decreases it as conditions dictate. You should also securely suspend ventilation fans from bungee cords or EPDM rubber, since this will greatly cut down on the vibrations and rattling noises traveling throughout the rest of the building from your grow. If you’re securing the fans in a grow tent, consider poking a hole in the covering material and securing them to a ceiling joist overhead rather than the frame of the structure -- especially if you intend to use activated carbon filters to remove smells from the air.

While in/out ventilation systems can be cost-effective to install and operate, they do have their limitations -- including the fact that they move the air from your growroom (and its associated smells) outside of the building you’re in. Also, their effectiveness is sensitive to changes in the external environment -- something you have little or no control over.

CEA: Getting Serious About Growing
If you aim to maximize indoor medical production and keep it perfectly consistent from crop-to-crop regardless of outdoor conditions, then CEA (controlled- environment agriculture) is the way to go.

While it’s more costly in terms of initial setup (because a serious air conditioner, controlled CO2 dispersal and dehumidification equipment are required in a room that doesn’t directly vent from or to the outside), a lot of growers who make the switch to CEA never look back, because they’re too busy getting bigger yields of buds.

Essentially, the air is “engineered” in CEA to meet exact optimal conditions for the different phases of growth. A consistent, healthy environment means steady production rates; couple this with the ability to increase CO2 above ambient levels -- for example, to 1,200 ppm (parts per million -- and it also means the ability to get up to 30 percent more production out of the same space. Also, growers using CEA are sometimes able to run more lights safely in the same amount of space because they have ample access to electricity as well as the ability to throw down the level of cold air needed to get the job done, above and beyond what can be accomplished using outside air for cooling.

Air-Cooled Lighting for Efficiency
You can’t discuss efficient growroom ventilation without discussing air-cooled lighting. While these systems can be laborious to set up and maintain, they definitely remove heat at its source -- i.e., high-intensity discharge (HID) lamps -- reducing your interior cooling needs by half or even more versus using lights with- out enclosed air-cooled lamp reflectors.

The larger-diameter straight-through types tend to have more cooling power, but the engineered lenses can spread light better, so look for one that covers both bases well. Vertical air-cooled lamp reflectors are also available, but they can be tricky to install neatly.

Funk Fighters
Growroom air can get funky, whether you vent to the outside or recirculate the air in a CEA setup. To keep the air fresh for any passersby as well as to keep it free of spores, pollen and other contaminants, an activated carbon scrubber is a great choice. However, these scrubbers are only reliable when you install and operate them as directed, with the proper fan sizing and rated maximum humidity levels.

Control Your Destiny
Of course, you will need some controls to run your environment. These can range from simple and inexpensive cooling thermostats, to slightly more sophisticated fan-speed controllers, to total solutions like an NGS (networked growing system), which allows Internet- or network-accessible interfaced controls for all aspects of the grow environment, including temperature, humidity and CO2 levels. A high-temperature relay switch that shuts off the lights if things get too warm is always recommended; it can prevent crop loss or worse.

Putting It All Together
So the promise at the beginning of this article was that we would illustrate how all of these methods can be put together to allow a grower to operate using either in/out ventilation or CEA, with little in the way of changes to the setup required -- thereby reducing cooling costs when the outdoor temperatures are favorable, while achieving consistent yields of high-quality medical nuggetry. And since there are plenty of combinations and possibilities for putting together a good working ventilation system (including the “switchable” type we’re about to discuss), this should provide a decent background to create the system that works best for your garden.

First, install or connect a minimum six-inch diameter duct port to bring fresh, cool air in, and the same (or, preferably, a slightly larger diameter duct port) to move hot, depleted air out. If you run lots of lights and want to run in/out ventilation as the seasons permit, you will need to go larger than this -- but for most folks, this standard should work. (Also, if you are cutting into walls, etc., make sure you know what’s behind them first -- i.e., anything like plumbing pipes or electrical wiring.)

Second, size up the correct volume of air movement you will need from your fans. Usually, a good rule of thumb is a rated 250 cfm (cubic feet per minute) of air movement for every 1,000 watts of lighting. This will keep things optimal when the outside temps allow for in/out ventilation -- and when you run CEA (i.e., a sealed room), these fans and duct ports will handle your air-cooled lighting to maximize cooling efficiency. Naturally, you’ll be connecting your exhaust and intake fans to a controller. Backdraft dampers may be required in some installations.

Third, install an activated carbon filter, firmly anchored into secure ceiling studs, above the plant canopy. It cannot be overemphasized how important it is to have the right fan-to-filter sizing with regard to the fan’s rated cfm capacity. (Your supplier can help you make the right choice if you’re not certain.) You will place the filter before your exhaust fan when running in/out, but disconnect it from the ducting -- and use a separate fan to run as a recirculating scrubber -- when the garden is operating in CEA mode.

Fourth, install dehumidification equipment if necessary in your climate area; alternatively, install humidifiers and controls. Note that with the right kind of gear, you can collect the condensate and feed it to your plants—pure, living water from the garden back into the garden.

Fifth, install your air conditioning. Water-cooled units are the stealthiest and can be installed just about anywhere. They require about 1.5 gpm (gallons per minute) of water flow when operating, so having an abundant and inexpensive source of water may be necessary, though you can run RO (reverse osmosis) wastewater for cooling purposes in some instances from waste-collection tanks. Air-to-air split AC units can also be quite stealthy (and don’t require all that water), but they do need the heat exchanger to be installed outside of the building or in a specially built shed. There are models that are available as “triple-split” units that house the noisy compressor elsewhere, so it runs whisper-quiet outside.

Sixth, install your CO2 system. Gas-fired, water-cooled models are best in the long run for anything bigger than a few 1,000-watt lamps. This system may or may not operate when running in/out, but it will be necessary for CEA mode. As always, follow the safety guidelines religiously with any gas-fired appliance.

Seventh, install horizontal air-cooled lamp reflectors that can be operated with or without ducting and forced fan cooling (i.e., large-diameter duct ports). This way, when you’re using the duct ports for the growroom air when it’s cold outside, you can operate your reflectors without the ducting. Alternatively, if you can pop the tempered glass lens out (while being able to replace it later), that can work too. When it’s hot outside and you want to run CEA, connect the ducting (and the lenses, if applicable) to the air-cooled lighting system on both the intake and exhaust fan systems; your room will now be running “sealed” because the AC, dehumidifier/humidifier, CO2 system and controllers will be engineering all of the air, with no outside exchange required.

In Conclusion
There are plenty of ways to apply the principles discussed here to keep your marijuana crop healthy and productive, regardless of what the world outside might be like. But you should never underestimate the value of a good ventilation system when budgeting the construction of your room. A lot of growers tend to blow their load on lighting, only to realize too late that they have dealt with just a part of the equation for achieving a healthy grow.

As a final note, it’s typically better to slightly oversize your ventilation system, because there is little in the way of machinery that ever operates at 100% efficiency. Keep in mind that it’s always easier to slow a fan down rather than be forced to find another way to help keep things cool because you’ve sud- denly discovered that your system can’t handle the cooling load.