The Comprehensive Guide to Grow Room Ventilation

A good dose of sunshine, light breezes, precipitation, evaporation… Such is the life of plants grown outdoors. But since outdoor cultivation is characterized by higher risks of crop loss due to adverse weather conditions, more cultivators are moving indoors, substituting grow lights for sunshine, watering systems for precipitation, and fans for breezes.

Indoor growing means an endless growing season and complete control over growing conditions. This is all great, but too often, it’s easy to forget air circulation — cultivators forget that air circulates more naturally outdoors than in the grow room and this leads to a disregard for grow room ventilation.

While often overlooked, grow room ventilation is a key aspect of running a successful operation. This guide will show you what ventilation is, why it’s important, and why getting grow room ventilation right requires more than a fan in the corner of the grow room.

What is ventilation?

Ventilation refers to how air is exchanged between the outside and inside environment of a space. Ventilation also involves the internal circulation of air within that space.

All technicalities aside, in the grow room, ventilation is essentially simulated wind. Grow room ventilation ensures the continuous flow of air between the indoor grow area and the outdoors.

Definition out of the way, let’s answer an important question…

Why is grow room ventilation important?

It’s simple. Grow room ventilation is a key contributor to accelerated plant growth and optimal grow room conditions. Let’s have a look at some of the main reasons why grow rooms need to be ventilated.

Photosynthesis and respiration

Ventilation is responsible for maintaining three critical elements in the grow space — temperature, humidity and carbon dioxide (CO2). All three elements directly affect photosynthesis, and therefore, how plants develop. Without fresh air, your crop will literally starve to death.

Besides photosynthesis, grow room ventilation is vital for plant respiration, which plants need to avoid suffocating. Put another way, ventilation promotes the healthy circulation of both carbon dioxide and oxygen. The former is required for photosynthesis while the latter is essential for respiration.

Grow space humidity control

Humidity is one of the main elements of an indoor grow’s environment. Get it out of balance and your grow room will become a hotbed for pests and diseases. In extreme cases, your plants will die, and with that, your goals of running a profitable operation.

As plants transpire throughout the day in an indoor grow, humidity increases. Without a good ventilation system, uncontrolled humidity is a huge possibility. High humidity will trick plants into halting transpiration or the uptake of water and nutrients from the roots.

Enter ventilation.

Good grow room ventilation will wick away some of the water released during transpiration. Doing this will allow plants to absorb water and pull up nutrients from the roots, resulting in better plant growth and development.

Elimination of excess heat

Grow lights are heat emitters and they increase grow room temperatures. If the temperature gets too high, it will stress your plants or even kill them. As such, the existence of grow light in the grow space warrants ventilation.

You may be thinking that you’re exempt because you don’t have MH or HPS lights, and you use lower-heat LEDs. Well, you’re not. Even those LEDs produce heat that can damage your plants if you don’t have good grow room ventilation. Good ventilation will pull heat from the grow room and help keep your plants cool — it can define the line between a healthy yield and no yield at all.

CO2 control

Outside, plants absorb CO2 from the air as part of their nutrient cycle. In a grow room setup, air doesn’t move as it does outside, and CO2 levels gradually decrease, limiting a crucial element of plant growth. Ventilation removes stale air from the grow space and brings in fresh CO2.

Mold, pests and pathogens prevention

Without proper grow room ventilation, warm, stale air builds up in the space, creating the perfect hot and humid environment for mold, pests and pathogens to thrive. Mold, mildew, fungus gnats, spider mites, unwanted microorganisms; you name it, they all love humid conditions. Good ventilation helps control humidity and makes it harder for all these elements to establish themselves in your grow room.

Improved ability of plants to deal with wind stress

Fruit-bearing plants must be healthy and rigid, otherwise, they can break under the weight of their own fruit. Part of this health and rigidity stems from being able to deal with wind stress.

Plants growing outdoors are constantly buffeted by the wind, but those indoors need ventilation to help toughen them up. Ventilating the grow space will ensure that plants are strong and able to produce good yields. Do note, however, that you should avoid having too much air movement near the plants as this can end up damaging them.

Odor control

Combining a good ventilation system with carbon filters and other indoor air quality (IAQ) products can help you mitigate odor in the grow space. This does not apply to all grow rooms, but it certainly applies to cannabis grow rooms.

How to set up a good grow room ventilation system

A hobby indoor grower can get away with a ventilation system that consists of a little more than a couple of exhaust fans. However, advanced grows require more than that.

The following are the common grown room ventilation setup options you can adopt.

Grow room fans

Growers can use motorized fans to achieve the environmental conditions most conducive for plant development and growth. With the grow room fan setup, the three common components you’ll likely require are an oscillating fan or an exhaust fan, an extractor fan and a carbon filter.

Oscillating fans

The goal of these fans is to encourage efficient airflow in the grow room. The gentle breeze provided by an oscillating fan will help strengthen the plants’ stems and it will also prevent the build-up of stagnant air in the grow room, thereby reducing the risk of pests and pathogens in the space.

A word of caution! Make sure not to aim the fans directly at the plants as this will cause “wind burn”, which is the unhealthy drying of plant tissue that causes plant stress and damage.

The number of fans you need will depend on the size of your grow room and the type of fan you need will depend on the Cubic Feet per Minute (CFM) or flow rate of your grow space. But before you can choose the right CFM level, you’ll have to determine the volume of your grow room.

For example, a 5 x 5 x 8 ft. grow room will have a volume of 200ft3. This means that your oscillator fan should have at least 200 CFM. Note that this calculation assumes air will be replaced every minute. If you want to replace air, every three minutes, for example, you’ll need to incorporate this into the calculation. Let’s say your grow room has a volume of 720 ft3, you’ll need a fan that’s at least 240 CFM (720 divided by 3).

Do remember that as a general rule, you should opt for a fan that has a higher CFM value than your calculations. This will ensure that the fan can completely replace the air in the grow room when you account for factors such as temperature, humidity, grow lights and the number of plants in the room. However, make sure that the CFM is not a huge amount higher than required to avoid wind burn.

Exhaust fans

Some growers will use exhaust fans instead of oscillating fans. These fans suck hot, humid air from the grow space and return it outside. Similar to oscillating fans, you have to calculate the required CFM before getting an exhaust fan.

Once you have your oscillating (or exhaust) fan, it’s time for the other half of the puzzle — the extractor fan.

Extractor fans

These fans pull old air out of the grow space, creating a vacuum pressure that pulls in fresh air into the space. Since hot air naturally rises, you need to install extractor fans at the top of the grow room. You’ll also need to match your fans to the size of your grow room.

Some tips:

Set up your fans before adding plants to the grow room. This way, you won’t have to work around them or risk damaging them.
Automate your fans to provide more consistent climate control. This can be done using ventilation controllers. New to ventilation controllers? Don’t worry, we’ve got your back. Here’s an ultimate guide to grow room controllers.

Now that the fans are sorted, there’s just one more thing you’ll need to ventilate your grow room…

Carbon filters

Carbon filters will help you eliminate contaminants from the air. When attached to an exhaust system, carbon filters will absorb the terpenes given off by plants as well. This action cleanses the air you extract from the grow space. In cannabis grow rooms, activated carbon filters also play the important role of mitigating odors in the grow room.

Note: Depending on your ventilation system setup, you may also require additional humidifiers and dehumidifiers (in addition to the fans and carbon filters) to quickly and efficiently control grow room humidity levels.

With the fans and filters you need for grow room ventilation in place, it’s time to put them to work. But first, you need to determine how you will bring air into the grow room. The two common methods for this are passive intake and active intake.

Passive vs. active intake

Passive intake relies on natural airflow. The air flows through holes or vents in the grow room. Oscillator and exhaust fans make up passive intake. They remove air from the grow space, creating creates a vacuum that pulls air in through the holes or vents. As a general rule, the holes or vents should be situated at the bottom of the grow room, preferably opposite the exhaust or oscillator fans, to create a good cross-flow of air.

Active intake, on the other hand, works by pulling air into the grow space using an intake fan. Active intake ensures much higher air circulation, making it a more effective way to ventilate the grow room compared to passive intake. Generally, the CFM of the intake fan should be slightly lower than that of the exhaust to ensure negative air pressure in the grow space.

The other way: Grow room HVAC

As mentioned before, if you’re running a big grow op, fans just won’t suffice. You will likely need to consider HVAC for your grow room ventilation. HVAC will give you the most control compared to fans — it’s more efficient at getting rid of excess heat and maintaining the optimal humidity and temperature in the grow room.

However, there are many makes of HVAC systems on the market, so you will need to determine the system that works well for your grow. One of the biggest things to consider when choosing a system is size. For this, you will need to calculate all the contributing factors that create excess heat in your grow space.

Sizing an HVAC system for an indoor grow

The five major factors that will influence the size of your HVAC system are the size of the grow space that needs to be cooled, the lighting, the ballasts of the lighting equipment, other electrical accessories in the grow space, and any CO2 equipment you have in the space.

Space size

The size of the space to be cooled will determine the minimum British Thermal Unit (BTU) requirement for the HVAC system, i.e. it will help you determine the best HVAC system size for your grow room.

One BTU is the amount of heat/energy required to increase the temperature of one pound of water by one degree Fahrenheit. To avoid getting too caught up in the definition, it helps to just think of BTUs as a way of quantifying cooling and heating equipment.

Grow lighting

Lighting is responsible for emitting lots of heat in grow rooms. This is why it’s important to consider it when figuring out the required cooling load. After determining the required BTU based on the size of the grow space to be cooled, you’ll have to determine the additional BTUs required to offset the heat created by grow lighting.

A good rule of thumb for finding this value is multiplying the total grow room lighting wattage by 3.5. For instance, if your grow lighting has a total of 12,000 watts, your HVAC system will need an additional 42,000 BTUs (12,000 x 3.5) on top of the BTUs required for the grow space size.

Do note that different lighting types and wattages emit different levels of heat. For example, a grow space with some LED lamps, fluorescent lamps, or a few induction lights, plus good air volume relative to the number of lights, won’t need as intensive a ventilation system as a space with, for example, HID lamps.

Ballasts for grow lighting

After the grow lighting, you’ll also need to consider the ballasts used in the lighting system. Ballasts regulate the voltage each grow light receives. Without them, grow lighting would draw too much current and voltage all at once, leading to overheating and burning out within seconds.

If your grow lights have ballasts, the ballasts will have to be added when determining the required BTUs for cooling. The general rule of thumb is to multiply the total grow lighting wattage by 2.5. For instance, if you are using ballasts in a grow space with 12,000 watts of lighting, you will need an additional 30,000 BTUS (12,000 X 2.5) of cooling. This number will be added to the required BTUs for the grow space size and those for the grow lighting.

Additional electrical equipment in the grow room

Besides the lighting and ballasts, other electrical equipment in the grow space adds some heat to the environment. This equipment includes components such as grow room controllers, air pumps, extra humidifiers and dehumidifiers, and fans.

Although the excess heat emitted by this equipment will likely be negligible, it’s a good idea to include it in the calculations for the BTUs for cooling. That’s because it’s better to go with a slightly higher BTU than a lower one. This way, you won’t have to worry about any additional equipment throwing your system off balance.

CO2 burners in the grow room

If you use CO2 burners to enrich the grow space with CO2, you will need to consider them in your BTU calculation. The BTU output of burners in a grow room will depend on the size of each burner and how many burners the room has. But do note that most CO2 burners don’t operate continuously, so the BTU rating on a burner only applies when the burner is operating.

The basic rule of thumb when calculating BTUs for CO2 burners is to add 10,000 BTUs of cooling for each burner unit with two burners.

By considering these five elements, you will be able to more accurately determine the size of the HVAC system that will provide the optimal ventilation in the grow space. At the end of the day, remember the function of the ventilation system — to provide the optimal temperature, humidity and CO2 levels.

Whether it’s fans or an HVAC system, the ventilation system you choose for your grow should depend on your grow room setup, size, budget and business goals. Nonetheless, it’s crucial to remember that while hobby growers can usually get away with a couple of fans, commercial growing requires a robust HVAC system. And in most cases, a successful grow requires much more than any generic HVAC system; it will likely need a specialized HVAC system. Such a system integrates cooling and dehumidification into one unit, providing the tightest and best indoor grow ventilation.

Find out why a specialized HVAC system may be the best solution for your grow.

Conclusion: Grow room ventilation the right way

Never underestimate the importance of a good grow room ventilation system. Like other key contributors to grow room success, a good and consistent ventilation system is vital for your operation’s sustainability.