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[Sticky] What size fans are needed for proper air circulation in marijuana grow tents  


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April 16, 2019 2:31 am  

When planning for air circulation in marijuana grow tents or grow rooms, you’ll need to purchase several types of fans: circulation fans to provide air movement within the tent/room, and inline vent fans which exhaust air from the tent (and air cooled light hoods or cool tubes).

In this article we’ll discuss what size fans are needed for proper air circulation in marijuana grow tents.

But keep in mind that grow tents and grow rooms require both fans for circulation as well as separate fans for proper exhaust. Without exhaust fans your circulation fans will be blowing around stale, hot, humid air devoid of necessary carbon dioxide. Your crop will never reach its potential in that type of environment. So plan for proper air circulation and exhaust.

Why is air circulation needed for marijuana plants

Marijuana grows best when exposed to a constant supply of fresh air with lots of carbon dioxide. Plants consume CO2 similar to how humans require Oxygen. For outdoor grows this is provided by the wind. To replicate this indoors, its important to keep a constant stream of air movement to replenish the CO2 around your plants.

Your tent’s exhaust system will provide the intake of new air. You’ll need fans inside the tent for air circulation so this fresh air reaches all parts of your pants. Proper air movement will also prevent problems with plants, such as mold, mildew, and pests.

Plants respond to the wind by increasing the strength of their branches. Lots of air movement encourages strong, healthy branch structure which will be needed to keep top-heavy buds standing up close to the light.

Where to position fans in grow tents

Clip on fans are a great way to provide air circulation above the canopy in your weed grow tent
Clip on fans are a great way to provide air circulation above the canopy in your weed grow tent.

Your marijuana will grow best when you have multiple circulation fans in your tent. Having a fan circulate air across the top of the canopy dissipates hear from your grow lights, and provides CO2 to the plants’ grow tips and buds.

You want to position your fans so that they’re covering as much of the canopy as possible.  Leaves should rustle gently; avoid blasting air right at them or you may notice signs of wind burn.

Having an additional fan pointed below your canopy will help keep air from growing stagnant underneath. It will help reduce moisture buildup as well which can lead to problems.

A fan under the canopy will and can help dry out the top layer of coco between feedings, which reduces the chance of fungus gnats or other insects. It can also help dry out the outside of cloth bags between waterings as well, which prevents mildew and mold.

How much air circulation do marijuana plants need?

You want the fans to be pointed right at your plants so that they’re constantly rustling. The CO2 around your plants needs to be replenished at least every 5 minutes for optimal growth of marijuana plants. A gentle rustling is best. But based on your setup it can be hard to ensure uniform circulation.

Make sure not to overdo the air movement. Blasting a powerful fan may help the plants furthest away get nice air movement, but the plants right in front of the plant may show signs of wind burn.

Different lights may also impact how much air circulation your marijuana plants need. If you’re running 1000 watt air-cooled lights in a tent, you’ll still have to worry about temps and keeping the light far enough from your plants.

Try the hand test. If you place your hand under the light and it start to get uncomfortable, that’s exactly how your plants will feel.

Increasing circulation above the canopy can reduce heat to the tops of your plants. But it’s no cure for plants that are too close to hot lights, so always make sure to factor in your plant height when inducing flowering so that the additional growth in the flowering stretch doesn’t raise your plant tips into the burn zone.

What sizes and types of fans work best in grow tents

When choosing the best fan for your grow tent there are several consideration. In a large grow room, powerful oscillating fans can be used to maximize air coverage across a wide area.  This isn’t possible in a small grow tent, where clip on fans attached to a pole are a better option.

Clip on fans

Popular sizes for clip fans used in grow tents are 6 inches and 8 inches. Go with the larger size, and keep in mind that’s a measure of overall grill size and not of the fan blade length.

Clips fans mounted on tent poles can only rotate certain ways, your angles are limited because the material itself will get in the way of the shape of the clip and prevent your from fully rotating it. Having multiple clip fans is a great above-the-canopy circulation solution in a grow tent. Clip fans are very inexpensive. They don’t move a lot of air, but in a small to medium grow tent they do the trick well and are also a great way to save space.

Oscillating stand fans

Large grow tents need a larger solution.  Oscillating fans come in lots of sizes, including 8″, 10″, 12″, 16″, 20″, 24″, and 30″. Most of the larger sizes are available as stand fans. Since they deliver air in multiple directions, oscillating fans a great choice for ensuring lots of air movement around your happily-growing marijuana. If you have the room, go for it.

Large oscillating fans on stands work well in larger tent environments. Multiple fans are needed in bigger grow rooms, but the changes are if you’re utilizing a very large grow tent you’re probably not reading this tutorial on choosing the right tent fans.

Tower fans

Tower fans can provide circulation both above and below your plants at the same time. Many models oscillate so the can cover and entire tent. They’re a bit pricier than some of the cheaper tent fan options listed above, but if they can cover twice the area you’ll require less fans which can balance out the cost.

Under the canopy circulation

For underneath the canopy air movement, clip fans can be used there as well. High velocity floor fan models are even better since they come in larger sizes and can move more air. They come in sizes both small and large.

Look for fans with multiple fan speeds so you can fine tune the circulation based on your setup. You could even try to rig up a medium sized desk fan underneath, depending on how much room you have on the edges of your tent.

How many fans does your grow tent need?

I run a single clip fan atop the canopy, and two medium floor fans below.  This provides a gentle rustle to all four plants in my 4’x4′ foot grow tent.

With a 5’x5′ foot tent or 3’x5′ foot tent I’d add an additional clip fan on the other side of the canopy.  With a 3’x3′ tent, I would likely just use one above and one below.

For a very small space such as a tent or closet for mother plants, a single small fan would do the trick nicely.

Safely installing fans in grow tents

It makes sense to lift floor, desk, or vertical fans off of the floor to avoid risk of contact with water.

Always make sure your fan cord loops lower than the bottom of your fan before your run it up to exit the opening on the side of your tent. Since water is somewhat sticky it will follow the cord to the lowest point upon contact, and adding a dip in the cord will run the water off before it reaches the fan which is quite preferable than into the fan unit itself.

Use a GFCI outlet to run electricity into your tent, similar to what is used in bathrooms. That way you can avoid getting fried in the wrong way from your marijuana grow.


There are lots of considerations when planning for proper air circulation in marijuana grow tents and grow rooms. By following these steps you’ll help create optimum growing conditions for your plants, and you’ll be rewarded with a bountiful harvest of dank, high quality buds with maximum potency.

Whispererofweed, Bob Durban, Macky and 1 people liked
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April 16, 2019 9:31 am  
Posted by: GreenBud


Use a GFCI outlet to run electricity into your tent, similar to what is used in bathrooms. That way you can avoid getting fried in the wrong way from your marijuana grow.


Nice article mate Smile

...just a small translation for our european members...GFCI (Ground fault circuit interupter) is the same as an RCD (Residual current device) 😉

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April 16, 2019 9:51 am  

Very nice green bud, great Article mate! 

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February 22, 2020