Nov 09, 2018

HVAC Like A Pro

How Proper Ventilation Can Impact Your Grow

Do you know that awful feeling of being stuck inside a car with the windows rolled up for more than just a few minutes? Your skin gets clammy, the air feels heavy with stillness and it feels difficult to breathe. Your plants experience the same feeling when your grow space cannot circulate and exchange fresh air. Ensuring proper air circulation within your grow system is integral to achieving an optimal grow space for your plants. Many issues can arise due to lack of proper ventilation such as heat regulation, humidity levels, stagnant air, dust, mold mitigation, and odor reduction.

What does ventilation do for my plants?

Oftentimes, grow rooms struggle with staying cool. This is caused in large part by heat generated by the grow lights. If CO2 burners are used, they will also contribute to heat within indoor grows. In these situations, it is super important to have systems in place to circulate fresh, cool air. Just as plants thrive in the fresh air outside, it is crucial for their health indoors, as well.

Plants need CO2 to continue conducting the metabolic processes that help them grow and produce. During daylight hours, plants deplete the air of CO2 during these processes. When you introduce fresh air to the atmosphere, fresh CO2 is also introduced. Plants only require CO2 during photosynthesis. Increasing CO2 during nighttime hours is a waste of your money and is not recommended. Plant roots use the nighttime hours to respirate. If the CO2 levels are high, then oxygen is low. If oxygen is low then roots do not have what they need to do their job optimally. The moral of the story is to make sure you exhaust your daytime CO2-rich air for lights off.

On a micro level, the air directly surrounding the leaves of your plants creates their microclimate. These microclimates have varying humidity levels, temperature readings, and atmospheric concentrations that differ from the grow room as a whole. The metabolic processes taking place, as well as their byproducts, are responsible for these microclimates. The water vapor and oxygen emitted by the plant have a difficult time moving away from the surface of the leaves without adequate airflow. Air movement is necessary to reduce both the size of these microclimates, as well as the length of time the plants are exposed to these microclimates.

Circulation versus Air Exchange

Closed Loop Ventilation System

A closed ventilation system operates with the air, humidity, and molecules that reside within the four walls of your grow. It recirculates the air and does not take in the outside environment. Due to the air not being replaced, the room can experience a lack of atmospheric elements, such as carbon dioxide and oxygen. These elements need to be in balance to ensure proper and vigorous growth. Supplemental CO2 is needed in a closed-loop system. Otherwise, the only CO2 being introduced into the environment is from root respiration.

In a closed-loop system, temperature regulation can be tricky because of the need to supplement atmospheric elements (i.e. burning CO2, which raises temperatures). Thermal layering can also be a battle in a closed system. Without proper air circulation, heat will rise and reside in the canopy, slowly cooking your plants. Placing an oscillating fan near the floor to help circulate the cooler air upwards will help disperse the thermal layers that tend to occur.

Open Ventilation System

A system equipped with an air exchange system (an open system), replaces the air within the grow space with air from an outside environment. As it pulls air in, the air circulates within the growing unit. It actively blends the air to prevent thermal layering, aids in temperature adjustment, and helps to regulate humidity levels. In a 10’ x 10’ x 10’ grow room air should be removed at about 200 feet per minute, exchanging the air in the entire room once every five minutes. One downfall to running an open system is you run the risk of making your grow op vulnerable to pests and outside bacteria.

Extraction Ventilation System

If you are pulling air from your grow room you will need an exit strategy. If possible, release your grow room’s extracted air directly outdoors. If that is not a possibility, try to exhaust into another room or space that is larger than the growing area. If you live in a neighborhood or close to anyone that is not a grower, you must attach a carbon filter to your exhaust fan to reduce any odors that your garden may emit. We recommend using Can Fans and Filters for smell mitigation. They are professionally trusted to eliminate the garden funkiness that will be exiting your grow.

To pull the cooler air from below the plants through the canopy, you will need to place your exhaust fan near the top of the grow space. First-grade science lesson here. Heat rises, and it will collect at the highest points of your grow. It minimizes energy use to pull from the hottest part of the room. Find these “hot spots” strategically by placing thermometers around your grow, wherever you think there may be a potential heat dump. By pulling heat from the hottest part of the room, you also pull cool air up from the bottom. This air current is a good thing for your grow.

HVAC Intake

HVAC HEPA filter diagram

It is best to pull air from indoors (i.e. an adjacent room) to avoid having to combat extreme temperature changes caused by intaking from the outdoors. It puts an unnecessary strain on your temperature controls. Equip your intake ducting and fans with both a bug blocker and a HEPA filter to prevent pests, eggs, dust, and spores that may attempt to enter your system. The goal is to replenish the fresh air within your grow system, not to take on other combatants to fight to produce a quality product. The intake air duct should be placed near the floor pulling air in, and moving it towards the ceiling where it will be exhausted after it has been heated beyond value.


Oscillating fans (not intake or exhaust) within the room are beneficial. You can get either standing or wall-mounted. Another option would be Wind King floor fans. You can use these floor fans to ping-pong your low cool air. The cool air ping-ponging below your canopy will be pulled up through your canopy and pushed out the exhaust above. Keep in mind, that oscillating fans are not intended for cooling purposes. They aid in circulating air within the room and mimic the breeze of the outdoors. Having to resist the pressure of the wind strengthens the stem of the plant and creates a more durable stalk. Don’t direct the fans at the plant's point-blank—a light breeze is the goal, not hail force winds. Try adding an 18” or 16” Hurricane fan to your grow.

Oscillating fans typically move in a 180-degree motion and reach many of the plants in a short amount of time. It is wise to strategically place your fans to allow for air movement between individual plants within your grow space. If stagnant air is surrounding a single plant at any time, it has the possibility of overheating and becoming surrounded by overly rich CO2 air. When purchasing oscillating fans for your grow, invest in fans with adjustable speeds. In the wintertime, most grow rooms can reduce the speed of their fans by about a third. Reduced fan speed lets the plants bask in the warmer air for a longer period during the colder months. Your tools and systems are intended to make the growing process easier on you. Make sure you are getting the most out of your products.

Tubing and Ducting

Covert Growers Choice 630 Watt CMH mounted hydroponic grow light and ventilation system

Most fans will require ducting (metal tubing that directs the inflow or outflow of air) to create a seamless connection from the fans to the outside area of the growing space. Try to make your ducting system as straight, short, and simple as possible to avoid inefficiencies. An incredibly efficient method of moving air within your grow space is to run perforated polyethylene tubing between the plants. The tiny holes will release lower pressurized air coming from your intake fan. When the air intake is lower pressure than the exhaust rate, it creates a negative pressure. This short period that fresh air is allowed to reside within the room before being moved within the system is beneficial to the growth of the plants.


When evaluating your need to control humidity levels, keep in mind that 60-70% humidity is ideal during the vegetative phase. During flowering, you want the humidity level closer to 50-60%. Grow room location, the different varieties of plants being grown, and the use of CO2 all affect the humidity levels in your grow.

Adding Humidity

Grow rooms that are located in colder climates and areas (i.e. basement or cement warehouse) will require a heater to bring temperatures up to the 70-degree range. With the implementation of heaters, moisture evaporates and leaves the plants lacking adequate moisture. It is similar to how we feel at the beginning of winter when we begin to use heaters and experience dry skin and chapped lips. Adding a humidifier to your room during times of applying heat will allow you to provide the moisture needed for your plants. Depending on the size of your grow, it may be necessary to add more than one humidifier to not strain a single machine.

Removing Humidity

Excess humidity creates a moist environment prone to mold and bacteria growth. It can also interfere with plant transpiration by disrupting proper “breathing”. High moisture can make the leaves of your plant feel as if they are drowning if exposed to too much airborne moisture. There are many different types of dehumidifiers available to accommodate any size of grow space. We like the integrity that GE provides with their dehumidifiers.

Bacteria and Pest Control

Powdery mildew infection on green leaf

Humidity offers a prime breeding ground for spores and bacteria. When free moisture is available on the surface of the plants, it provides a surface for fungi to spread. Bacteria is not as common as fungi, but due to its airborne nature, fresh air replenishment helps remove potential pathogens. Fungi and molds such as powdery mildew, botrytis, and black spot mold are undetectable by the naked eye and can quickly ruin a crop when overlooked. Anyone dealing with a mold, fungi, or bacteria infection may want to consider implementing an air purification device, such as a KFI carbon filter, to catch harmful pathogens.

When attempting to create a highly efficient ventilation system, you may be welcoming unwanted pests. Small bugs such as root aphids and spider mites, will jump on the opportunity to enter your system if given the chance. By placing a high-quality filter or large particulate screen over the intake vent you will create a barrier for the unwanted guests. For another economical option, you can use Hydrofarm’s Bug Shields. You can pick them up for a couple of bucks. They are compatible with inline fans and have an “insect screen with active charcoal inset”.

The more you know, the better you grow!

When initially setting up your ventilation system, or simply taking a closer look at improving your current system, there are a few things to remember. Most importantly investing in a proper ventilation system will help to mitigate future issues.

By installing filters and fans, you will prevent bugs, pests, bacteria, and fungi from having free reign and ease of access to your precious plants. Your grow will thrive when given in an area with adequate airflow and mindful temperature management. The strategic placement of intake and exhaust fans will create a prime flow of air to keep your grow nice and breezy, creating the optimum environment for productive, high-quality plants and yield.