I remember my first time setting up a grow tent and I took one look at a ventilation diagram. My heart sunk, my head hurt, and I felt incapable of setting up a ventilation system.
Guess what? It’s super easy — it just looks way more complex than it is. Honestly, I felt a bit dumb after it took me less than 5 minutes to set it all up.
Great ventilation does five vital things that are crucial to growing healthy plants.
- Lowers temperature — Removes the build up heat from your lights.
- Lowers humidity — Removes the moisture your plants and grow media produce.
- Provides fresh air (CO2) — Plants use CO2 for energy during photosynthesis.
- Creates air circulation — Prevents pests, mold, etc. from resting on the plants.
- Moves air through the plants — This creates stronger stems for bearing the weight of heavy fruits and flowers.
Passive or Active Air Flow
There are two main types of ventilation systems you can set up in your grow tent: Passive Intake vs. Active Intake.
For the majority of grow tent growers, a passive intake system works perfectly.
Passive intake works by placing an inline fan at the top of your tent. The inline or exhaust fan will suck the air out of your tent creating a vacuum that pulls in fresh air from the intake ports at the bottom of your tent.
It’s the same principle as sucking air through a straw. As long as you have negative pressure inside the tent you will have a passive system — negative pressure means your grow tent has less air pressure in it than the environment it’s sitting in.
With an active intake system, you will place an additional fan at the bottom which will actively pull in fresh air.
An active intake system usually isn’t needed unless you have a very large tent, struggle with high temperatures, or your intake ports at the bottom are smaller than your exhaust ports.
Most tents have larger intake ports at the bottom so this isn’t much of a concern, but if you plan on running a passive system and haven’t bought a tent yet, make sure to keep an eye out.
Warning: Both HID lighting and hydro Deep Water Culture (DWC) systems are extremely common in indoor gardens, but this combination can spell disaster for passive intake systems. HIDs generate more heat than any other lighting system, but DWC systems require the water in the reservoir to never go over 70℉ to prevent algae. An active system should be enough to keep temperatures in your grow tent down, but you may want to think about ducting in cold air if you run into issues. You can also use a hydroponic water chiller.
Grow tent ventilation system
A ventilation system is usually composed of four components:
- An inline fan to exhaust air out of the grow tent.
- A carbon scrubber to eliminate odor if it’s a concern.
- Ducting for connecting the components.
- (Optional) A fan to move air directly through the plants — clip-on fans work well.
- (Optional) Intake fan for active systems.
Setting Up Your Ventilation System
There are several ways you can connect the exhaust port to the inline fan to the carbon filter. How you set it up exactly will depend on how your lights are hanging and the type of lighting system you’re using — does it use a reflector or not. When your ventilation components and tent are physically in front of you, setup and configuration makes sense, so don’t worry.
For LED, CMH and Fluorescent users, you won’t have a reflector you’ll have to connect to, to adequately remove heat. Setup couldn’t be easier for you guys.
- Step 1. Attach your inline fan to one of the extra ceiling bars that your grow tent most likely came with — adjustable rope clip handles work great as you fan will hang only a few inches from the ceiling.
- Step 2. Either slide the inline fan over to fit into an exhaust hole or even easier connect your inline fan to the ducting which can be run through and out an exhaust hole.
- Step 3. Hang your carbon scrubber — it most likely will come with ropes for hanging.
- Step 4. Attach the carbon scrubber to the inline fan so that the fan is pulling air through the scrubber — you can use ducting or simply connect them if they are the same size.
- Step 5. Confirm your components are the in proper order: Outside air is passively pulled in through the bottom intake ports up into the filter (passing air through the lights), which then moves the air through the inline fan and out the exhaust holes.
For those using a reflector as would be needed for HID, your set up will be slightly more complex as it’s recommended to have direct airflow through the reflector, cooling the bulb.
Your setup will probably look something like this: carbon scrubber -> ducting -> reflector hood -> ducting -> inflan fan -> exhaust hole.
The majority of people that use grow tents will have their carbon filter and inline fan hanging at the top, so air circulation moves completely throughout the tent.
However, some people will run either the inline fan, and sometimes the filter, outside of the tent and this is fine as long as you're creating adequate negative pressure inside the tent.
Matching Fans and Filter to Grow Tent
Before you purchase your fan and filter, we need to make sure they fit together and are powerful enough to replace all the air in your grow tent about every five minutes.
Matching Fan to Filter
To match fan and filter just make sure the flange size matches — a 4-inch fan connects to a 4-inch filter, etc. As well, you’ll want your ducting to be the same size. If you’re connecting to a lighting hood/reflector you need to take it into account as well — many hoods have a 6-inch diameter opening.
As long as you can fit your ducting through an exhaust hole and create a seal around it, you don’t need to get hung up on matching sizes there. I run a 4-inch ducting through an 8-inch exhaust port with no problem. Most grow tent ports can be self-closed creating a seal around ducting.
Matching Fan to Grow Tent
When matching your ventilation components to grow tent size, you need to figure out your tent’s total volume by multiplying length x width x height. Inline fans are rated based on how much air they can remove in one minute called CFM (cubic feet per minute). If your fan is rated 100 CFM, that means it can remove all the air in a 4 x 4 x 6 grow tent in less than one minute.
When purchasing an inline fan, I recommend to aim for removing all the air in one minute as opposed to five because when you attach your fan to a filter you’ll lose fan strength. Ducting will also weaken the strength of your fan’s pull — the more ducting you use and the more you bend it, the stronger your fan needs to be.
The general rule of thumb for figuring out what CFM rating you need, is to double your grow tent’s volume when using minimum/short ducting and a filter. A 4 x 4 x 6 = 96 cubic feet, so get a fan rated at a minimum of 200 CFM. HID growers may want to triple their tent’s volume because the bulbs produce so much heat. If you have concerns about heat, play it safe and triple — make sure your inline fan has a speed controller if the fan is ever too powerful for the tent.
You can check this by monitoring the walls of your tent — your tent’s walls should be slightly pulled in due to the negative pressure. If they are sticking or have no give reduce your fan’s speed.
Helpful tip: Don’t forget those clip-on fans. Even a passive system removing all the air in a tent within just a few minutes, won’t be able to provide enough of a breeze to strengthen stems or remove hot spots that build on your plants’ canopy.
If you want to grow the strongest plants and maximize your yields, supplementing CO2 is a must — gardeners can see a 20% increase in growth and yields. One of the reasons we want to frequently introduce new air into our tents is because it brings in fresh CO2, which the plants turn into energy.
CO2 injections will grow bigger plants, but you’ll want to make sure you have your other conditions under control first. If you’re struggling with your lights, nutrients & watering, or temp & humidity, fix those first then come back as they will lead to bigger yields for less money compared to supplementing in CO2.
If you want to supplement CO2 you will have to seal off the room. First, because the plants need very high concentration levels and it’s easy for CO2 to escape out the tent. Second, the high level of CO2 your plants required is dangerous to your health. Natural concentrations of CO2 are around 400 ppm, when supplementing we want levels between 1200 - 1500 ppm
You can only inject CO2 into your grow tent if you’re using high-intensity lighting: strong LEDs, HIDs, and CMH all work great. You should only use CO2 when grow lights are on. A CO2 controller with photocell makes this easy.
Don’t worry: With CO2 your grow tent’s environment is going to change. Your temperatures will rise, and you can increase the intensity of your lights as the extra CO2 gives plants the ability to handle higher temp and brighter lights.
Want to learn more about using CO2 in your grow tent? Check out our beginners guide to using CO2 in your grow room or tent.
More on grow tent ventilation and environment
This is a very basic understanding of setting up a ventilation system in your grow tent. If you want to learn more about how your ventilation system interacts with your environment, read our definitive guide on grow room atmosphere and ventilation.