Passive exhaust ventilation allows you to use just one inline fan to cool your lights while you exhaust air within grow tents or grow rooms. The exhausted air is pushed out which creates a vacuum effect that introduces fresh air through vent holes.
You’ll need to exhaust your grow area to remove excess heat and moisture so you can maintain the best temperature and humidity levels for your grow room. Exhaust will also ensure an incoming supply of carbon dioxide for your plants. Typically exhaust systems will use a carbon filter can to remove odors from the exhausted air.
Passive ventilation in grow tents differs from active ventilation (where a small intake fan is used to introduce fresh air). Both methods can work well in smaller tents, but in larger tents you should consider using the intake fan since using just an exhaust fan will create negative pressure that will suck in the sides of the tent unless you have a very large intake area.
Passive exhaust is also easier in the vegetation state since you don’t have to worry about 12 hours of total darkness. Many grow tents have small panels at the bottom you can use to let fresh air in. Be careful using these in flowering though since they’ll allow any light in the room to penetrate the grow tent which will cause problems in flowering.
Setting up a passive exhaust ventilation system for marijuana
Setting up a passive exhaust system to cool your grow lights is very easy. You’ll need an inline fan (commonly found in 4″, 6″ and 8″ sizes), a carbon canister filter, circular aluminum ducting, aluminum tape to seal the seams, and an air cooled light hood or cool tube. Venting outside directly through a window or a wall will also require a vent l’ouvre.
The carbon filer can be set up in your grow tent or room, suspended from the ceiling using bungee cords. It is connected next to the inline fan. In a grow tent, the inline fan can be placed just outside the tent to save on space, but in a larger grow room it can be kept inside.
Carbon filters are available in a lot of sizes. The larger the length of the filter, the more air that gets filtered. Eventually the active carbon will stop working and it will need to get replaced. For most small tent or grow room setups the smaller filter lengths will suffice, but larger grow areas with powerful exhaust fans and lots of air to scrub will benefit from the “big boy” filters.
Carbon filter setup
When setting up a carbon filter, the easiest way is to first secure the filter in place and then connect the exhaust using aluminum tape. While you probably have a roll of duct tape available, be aware the duct tape is never used by professional HVAC technicians to connect ductwork. Use aluminum tape instead.
Inline exhaust fans
The inline fan pulls air through the carbon filter and then pushes air through the lighting hood which is vented to the outside world. While using a fan to pull the air through the hood would be more effective than pushing it, but it also might pull unfiltered air through small cracks or gaps in the light fixture which would mean the exhausted air would smell like marijuana.
Cool tube lights are usually air-tight so this is less of a consideration. But unless your fan is undersized/underpowered to do the job, always push air through the lighting fixture for best results.
How to choose the correct inline fan size for your grow tent/room
Fans are rated by the amount of air they remove. This is measured in CFM’s, or cubic feet of air removed per minute. The bigger the fan, generally the more air it can circulate.
It’s always a good idea to buy a fan that is bigger than your needs and then use a fan controller to reduce the fan speed. This results in a much quieter fan, and there is much less vibration. Plus if you decide to upgrade your grow tent or room you won’t need to buy a larger fan. Learn how to reduce noise from inline fans.
To calculate the amount of air to remove, determine the amount of cubic feet in your grow area by multiplying the height, width, and length. Try to have the right fan setup to completely replace the air in your tent every one minute, this ensures you will introduce lots of fresh carbon dioxide while controlling moisture and temperature. So if you use a grow tent that is 5x5x7 feet, you’d need to remove 175 CFM to achieve this.
One additional consideration is the efficiency of the exhaust venting. The venting setup can in some cases greatly reduce the actual amount of air removed. Short, straight runs are much more efficient than long, curved runs. Plan to multiply your grow rooms volume in cubic feet by 2 to 3 times to reach the final number you need. 2x for short, straight runs, and 3x for longer or frequently angled runs.
Cooling grow lights with passive exhaust systems
Most air-cooled grow light hoods attach to either 6 inch or 8 inch fan ducts. Don’t run a 4 inch fan at full speed with 6 inch connector, it’s much quieter to get a fan controller to reduce the 6 inch fan’s output to reach the necessary 175 CFM exhaust level. If you only have a 4 inch fan and want to connect it to a 6 inch air cooled fixture, learn how here.
If you’re looking to get a fan controller, read more about how to select a fan controller.
How much air should you exhaust from your grow tent/room?
There are times when you’ll need to lower or raise the amount of air being exhausted. If you grow in a really hot environment (for example, in the summertime) then you may not achieve enough air movement to properly cool the heat from the light.
Use a digital thermometer to monitor this, preferably one that tracks high and low temperatures so you can see how things are going. Remember that marijuana plants prefer temperatures that are below 80-85 degrees unless you can introduce carbon dioxide, so it’s important that you ensure the right temperatures within your grow area.
Learn more about how much ventilation is needed for grow tents here.
Intake fans for grow tents
Most small tents and grow areas will do fine if you just allow for light-proof openings where fresh air can enter. Grow tents will typically “suck in” once zipped up because the amount of air entering doesn’t match the amount getting exhausted.
You can buy small fans that attach to ducting to increase air intake, but in small tents or rooms this usually isn’t required as long as you allow for adequate air intake through an opening at the bottom of the tent.
How to keep light from intake holes from entering your tent during flowering
If your tent has small vent panels at the bottom to allow air intake, it’s very hard to make these light proof. Your best bet is to tape the outside of the panels (which often are velcro) to prevent light from entering during flowering. Gorilla tape is black and thick, and is also very sticky so it does this job nicely.
If your tent has an opening at the bottom designed to allow 6″ tubing (usually an option on better quality tents) you can easily rig up a light proof duct. Take about 8-10 feet of flexible aluminum duct and run it through the lower vent hole. Tape a cloth filter (the kind used on AC vents, cut into a small circle) on the outside opening to prevent dust and bugs from getting pulled into the tent.
To make it lightproof, leave about 7 feet of that tubing outside the tent, looping it loosely a few times or tying it in a very loose knot. Be aware that since it’s shiny and silver, light can bounce around the inside of the tubing, so once it’s set up then look inside your dark tent while the lights are on in your room to make sure it’s completely dark.
Setting up a passive exhaust ventilation system in marijuana grow tents provides the necessary heat, humidity, and fresh air control needed for most small and medium tent grows.
Passive exhaust can also be used to cool lighting with a single inline fan, reducing the amount of equipment you need to purchase. Adding a carbon filter to reduce odors will ensure that your exhausted air won’t stink up your neighborhood and alert others to your grow.