Having proper circulation in your grow tent is essential to preventing mold and powdery mildew from growing on your cannabis. I’m going to lay out everything you need to know about selecting an inline fan for your grow tent, and getting it set up properly.
I’ll help you determine the proper grow tent fan size and power rating, where to place the fan, how to set up your grow tent fan, and the other accessories you’ll need.
Selecting the proper grow tent fan size and power:
The first questions you’re probably asking are “What’s the proper grow tent fan size?” and “What’s the proper grow tent fan power?”
When you’re replacing air in a grow space, the general rule of thumb is being able to replace all of the air once per minute. That’s the CFM rating you’ll learn about below.
The specific fan diameter – four, six, or eight inches, will be determined by the CFM (cubic feet per minute) rating you need to keep your grow tent cool. So worry about getting the proper CFM rating. Once you know you need 200 CFM inline fan, you’ll find out that most 200 CFM inline fans are 4 inches in diameter.
Then you need to make a few additional considerations, such as 25% fan power reduction if you add a carbon filter, and 30% power reduction for each 90 degree bend in your ducting.
The proper size fan for a 4×4 grow tent:
Let me give you an example calculation for a 4×4 grow tent, which is a pretty common size among indoor growers:
Most 4×4 tents measure 48in x 48 in x 80 in. So the total volume of air in a 4×4 grow tent is 4ft (length) x 4ft (width) x 6.667ft (height) = 106.67 cubic feet. But that doesn’t mean you need a 106 cfm fan.
When you consider a carbon filter reducing your fan efficiency by 25%, and one 90 degree duct bend reducing another 30%, a fan with 215cfm will get reduced to ~112cfm (200cfm x 0.7 x 0.75).
So for a 4×4 grow tent with an area of ~106 cubic feet, a grow tent fan (inline fan) rated for ~215cfm would get the job done.
What size fan is needed for other grow tent sizes?
Note: Grow tent inline fans aren’t manufactured to the exact CFM output you need, so you may have to go over by 50-100cfm when picking a fan. I recommend a fan with adjustable speed settings like the AC Infinity S or T Series so you can adjust the CFM to your specific requirement.
The AC Infinity fans mentioned below come with 10 speed settings, so if you only require 100cfm, you can buy the smallest model – a 205cfm fan, and run it on the middle setting for 100cfm.
Grow tent fan placement:
There are two options for placement of your grow tent fan. Either inside or outside the grow tent will work. What’s the difference? If you choose outside, the fan will pull air from the inside of your tent to the outside, via a duct on the top corner of your tent.
If you choose inside, you’ll want the fan and filter (if you use a filter) hanging in the top area of the tent where the hottest air naturally flows to. You’ll need these cheap hangers to hang your fan and filter from the inside ceiling of your grow tent. Not all fans include hangers, so make sure you’ve got them.
If you ask me I think hanging inside the tent is easiest. If you put the fan outside, you’ll need to find a way to make it rest in a good position.
The bottom of most grow tents has passive air intakes – netted windows allowing new air to flow into the tent. The new air gets pulled in naturally as the fan creates a low pressure environment inside the tent as air gets pumped out.
Grow tent fan setup:
Does the grow tent fan and duct have to be the same size / diameter?
Yes, your inline fan and duct will need to be the same diameter at the connection point.
Passive or Active Air Flow For Your Grow Tent?
There are two types of fan setups you can use in your grow tent or growing space – passive and active. The difference is whether you want air to re-enter your grow tent passively or actively.
Passive grow tent air circulation uses only one inline fan to exhaust hot air from your grow space. When the air exits your enclosed grow space, the pressure in the room goes negative, meaning air that’s outside the tent or room should naturally be pulled in through any openings to replace the air that was removed.
A majority of people choose the passive method, which works fine. You would choose active grow tent air replacement if you want to ensure the air coming back into the tent is being filtered, or maybe it’s not coming back in quickly enough.
The setup for both methods is fairly simple provided you have the necessary parts. Just make sure you have the proper size ducting (same diameter as your fan), duct clamps, hangers for hanging, and zip ties (make sure zip ties are long enough to go around the diameter of your carbon filter).
The carbon filter is optional, but if you want to prevent odors escaping your grow space, I’d highly recommend one.
To set up the filter, simply connect ducting to the side of your fan that sucks in air. Connect the other side of that duct to your filter. Secure both sides with a clamp (usually provided).
Once you have the parts, follow the installation steps below for each fan you have. If you’re running an active air flow setup, I’d recommend running both fans at the same speed so the air exiting the grow tent is equal to the amount of air re-entering.
How to install a grow tent fan & carbon filter inside your tent:
- Attach & adjust hangers:
Attach the hangers along the support bars of your grow tent ceiling. You’ll need enough for the fan and ducting. Adjust them to the length you’ll be hanging the fan. Remember hot air stays at the top of the tent so don’t hang too low.
2. Attach zip ties:
Place zip ties or metal brackets on the attachment area of your grow tent fan. Connect zip ties to hangers.
3. Connect ducting:
Connect ducting to the output side of your grow tent fan, wrap zip ties or metal brackets around ducting and connect to hangers.
4. Connect carbon filter:
Connect your carbon filter to the intake side of your grow tent fan (the side that sucks air in). Use provided clamp or zip tie. You can connect it directly to the fan, or use ducting in between.
5. Run ducting through tent exhaust opening:
Run the ducting through the opening in the top of your grow tent. Secure the grow tent duct sleeve tightly around the fan ducting.
Other Inline Fan Installation Notes:
The setup is pretty easy, it took me 30 minutes or less to set up my AC Infinity T4.
The most efficient setup would be routing the duct out a window so the hot air from your tent doesn’t just blow into the room where your tent is located, thus getting sucked back in and rendering the whole process useless.
Not everyone has the luxury of having a window near their tent. So if you’re forced to blow the tent air into the same room where it’s located, I highly suggest a portable air conditioner to re-cool that air so when it gets sucked back into your tent through the bottom tent windows, it cools your tent down.
Does It Matter What Carbon Filter I Choose For My Grow Tent Fan?
The one thing you do want to make sure of when picking a carbon filter for your grow tent fan is that the diameter of the connection point on the fan and the filter are the same. So if you have a four inch inline fan, make sure to get a four inch carbon filter.
Otherwise there isn’t much of a difference between carbon filters, they’re just big metal cannisters that filter your air through charcoal to remove contaminants and odor.
Do I Need an Oscillating Fan in Addition to my Inline Fan?
The inline fan removes and replaces the air in your grow tent, but doesn’t really circulate it well. So you should consider adding an oscillating fan to your grow tent to keep the air moving, which prevents mold and mildew spores from taking root. It’ll also help spread around fresh air for the plants to absorb.
If you look at the fans I recommended below, you’ll see there are small inexpensive oscillating fans you can clip onto the grow tent poles. You can also just get a standing fan.
Which Grow Tent Fans I Recommend:
AC Infinity makes incredible grow tent fans as long as you’re willing to buy the ducts and hangers separately, which is straightforward – simply match the duct diameter to your fan diameter.
Why are the AC Infinity fans so much better than others? This fan comes with a programmable controller to allowing the fan to turn on when a variety of conditions are met.
For example, you can set the fan to turn on at a certain humidity level. You can also set the fan to turn on when temperatures reach a certain level. You can also utilize timer and alarm warnings.
The technology used in AC Infinity fan motors allows them to run very quietly. I was pretty surprised how quiet my AC Infinity T4 sounded at full power.
Other fans are designed to deprive the motors of power in order to adjust fan speed, causing the fan motors to get noisy. AC Infinity engineered a different method altogether that works much more efficiently and quietly.
These fans are priced similarly to other fans without the technical features, so it makes sense to go with one of these. The fans are made in 205cfm, 351cfm, and 750cfm (4-inch, 6-inch, 8-inch) variants. Click here to check them out on Amazon.
If you don’t want to source all the parts on your own, this kit comes with a fan, 8 feet of ducting, clamps, hangers, and a carbon filter. It comes in two different sizes for different grow tent sizes.
The only downside with this kit is the fan isn’t programmable to turn on when certain criteria like temperature or humidity are met. The fan is simple on/off. Although people have reported being satisfied having the fan run all the time. It’s up to you.
Secret Jardin made this 6-inch clip-on oscillating fan with grow tent owners in mind. The clip fits securely on grow tent support poles. The fan will provide your grow tent with proper air circulation needed to prevent mold and powdery mildew from growing.
I’d recommend pairing one of these oscillating clip fans with an inline fan to maintain the perfect growing environment in your grow tent.
Keeping Your Grow Tent Cool
Sometimes you need more than just an inline fan to keep your grow tent temperatures under control. My best advice without making you read a whole article is to consider a portable A/C for the room your tent is located, or to consider adding CO2 which allows you to grow a few degrees hotter than normal.
Ideal Grow Tent Temperature and Humidity for Growing Cannabis:
An ideal temperature for cannabis plants is 70-80 degrees Fahrenheit. So if you’ve got a small grow light in a small tent, or other circumstances allowing you to maintain those temperatures without an inline fan, consider yourself lucky.
Cannabis plants in the seedling and vegetative phase grow best with 60% to 70% humidity. Cannabis plants in the flowering phase grow best with 40% to 60% humidity.
Using an inline fan is also a great way to replace CO2 inside your tent. As your plants grow, they’ll start to consume a considerable portion of the available CO2 in your grow space. One method to avoid that is by pumping out the old air and replacing it with new air.
Other Resources for Growing Cannabis Indoors:
In addition to picking the right size fan, I wrote several helpful lists and guides to help you complete your home grow the right way on the first try. Some other articles I highly suggest would be Picking the Right Size Grow Tent, This Year’s Best LED Grow Light List, and Understanding Grow Light PAR PPFD and Wattage.
Lastly, don’t forget to visit my home page for a complete list of helpful resources.
Grow Tent Fan & Carbon Filter FAQ:
How long will it take to hang my grow tent fan and duct?
30-45 minutes is all you’ll need.
What should I do if there isn’t a CFM output for my exact grow tent size?
The AC Infinity fans have multiple speed settings. So if you only require 100cfm, you can purchase the 205cfm (smallest model) and use the middle of the 10 speed options.
If I’m buying parts separately, how do I know if they’ll all fit together?
These parts are all measured by diameter. If you purchase a 6″ fan, make sure to purchase 6″ ducting and a 6″ filter. That’s all you need to know.
Do I need to hang the carbon filter?
Since the carbon filter is where air will be entering your fan, you should hang it near the top of your tent to capture the hottest air. Hanging the filter low or placing it on the floor would be inefficient.
Which end of the grow tent fan do I connect the carbon filter?
Connect the carbon filter to the “front” of the fan, meaning the fan is sucking air through the filter, not exhausting air through it.
What size fan will fit in my grow tent?
Most grow tents fit 8″ ducting, some may only fit 6″.
What if the size of my duct is too big for the duct hole on my grow tent?
If you have 8 inch ducting for a 6 inch grow tent outlet, you’ll need to buy a ducting reducer.
There’s too much negative pressure in my grow tent
Either lower the speed on your fan, open up the passive air intake windows on the bottom of your tent more if possible, or consider installing an active air intake fan, which is the same as your other fan but on the bottom of your grow tent pushing air inwards.
Can you wash or clean carbon filters after extended use?
You can clean a carbon filter with pressurized air. The filter is good for 1 year of use before it needs cleaning.
Can you place a carbon filter on each side of your fan?
Yes, but it may decrease your fan’s power efficiency a little bit. Make sure your fan is powerful enough to handle an extra carbon filter.
Do all carbon filters come with pre-filter wrap for the exterior of the canister?
Pre-filter wrap is not included with all carbon filters, make sure to check the product page details if you want a pre-filter included.
What’s the largest outer dimension of my carbon filter?
Add ~2 inches to the outlet size. The canister diameter on a 4 inch carbon filter would be more or less 6 inches.
How long do carbon filters last?
Carbon filters last at least one year of use.
Do I need inline and oscillating fans in my grow tent?
Inline and oscillating fans serve different purposes in the grow tent, so you should consider using both if possible.