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.
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.
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 with 6 inch connector, it’s way better to get a fan controller to reduce the 6 inch fan’s output to reach the necessary 175 CFM exhaust level.
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.
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.
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.