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Grow Tents and Fans: 11 Things to Know

Indoor growing offers a plethora of benefits. You have complete control of the growing conditions and there’s little limitation to what you can grow. Many people focus on all the other factors required for uninterrupted growth of their plants and often forget about ventilation.

While it may seem like a corner fan will handle all your ventilation needs, it won’t. Grow tents require adequate ventilation to ensure all your plants thrive. If you don’t know the fundamentals of using fans in a grow tent, we’ve got you. Here are eleven things to keep in mind about grow tents and fans.

Photo of a vintage fan with streamers tied to the front. The fan is on and the streamers are flowing.
Photo by Marilyn Gould

Why Do You Need Fans in Your Grow Tent?

You need a fan and an air extraction system in your grow tent to keep your grow tent fresh. Here’s why:

  • Remove excess heat from grow lights: too much heat can impair a plant’s growth.
  • Control humidity: Plants release water vapor throughout the day. Without proper ventilation, uncontrolled humidity may cause pests and diseases.
  • Control CO2: Plants need CO2 for their nutrient cycle, and they get this from the outside air. Ventilation ensures the circulation of fresh air, thereby boosting plant growth and yields.
  • To strengthen the plant: Continuous airflow makes the plant stems strong, thereby ensuring they can handle the weight of their yield when they start bearing fruit.

How Many Fans Do You Need in a Grow Tent?

As a general rule of thumb, you should have at least two medium-sized fans to ensure sufficient airflow in a grow tent. You can place one fan above the plants and the other on the ground. Oscillating fans are the best because they don’t concentrate air on one spot of your plants, thereby preventing windburn.

To determine if you need more fans, have a feel of how even the breeze is inside your grow tent.  If one area doesn’t seem to get any breeze, try adjusting the fans for an even distribution of air or add an extra fan. Below are guidelines for how many fans you may need for a few different sized grow tents.

2X2 Grow Tent

A 2×2 grow tent is quite small, so you don’t need too many fans in this type of tent. However, ventilation is still crucial in such a small space to ensure your plants have optimal growth. For such a space, you should have at least one exhaust fan to expel the air from the inside to the outside and bring in fresh air. Additionally, you’ll need a second fan inside the grow tent to circulate air within the grow tent.

2X3 Grow Tent

Similar to a 2×2, a 2×3 is also quite small. For this tent, you will also need at least one exhaust fan to ensure a continuous exchange of air in and out of the tent and at least one oscillating fan to ensure proper airflow within the tent.

2X4 Grow Tent

For this grow tent, you need at least two oscillating tents inside to keep the air flowing and one exhaust fan to ensure there’s a continuous flow of fresh air within the tent.

3X3 Grow Tent

For this grow tent, you need at least two oscillating tents one exhaust fan to ensure there’s a continuous flow of fresh air within the tent.

4X4 Grow Tent

For this grow tent, you need at least two oscillating tents inside to keep the air flowing and one exhaust fan to ensure there’s a continuous flow of fresh air within the tent.

5 X 5 Grow Tent

For a 5×5 grow tent, you need to ensure sufficient air movement because this size of grow tent holds more plants. As a general rule of thumb, have one oscillating fan strategically located above the plants and one on the ground, ensuring they’re releasing a gentle breeze. It’s also advisable to have an exhaust fan to ensure you’re expelling hot air and bringing in fresh air continuously.

Where to Place Fans in a Grow Tent?

You have two fan placement options in a grow tent. You can place the fan outside or inside the grow room. What’s the difference? If you place the fan outside, it will pull air from inside the tent to the outside through a duct on the top corner of the tent.

If you decide to place the fan indoors, you’ll also need a filter. You should place the filter at the top of the tent where the hottest air flows. Placing the fan inside is the easiest approach because you can use hangers, and the filter goes a long way in boosting ventilation. In outdoor setups, you may have a challenge getting the fan in the perfect position.

Should You Use Oscillating Fans in A Grow Tent?

Oscillating fans are perfect for grow tents because they offer many benefits, including:

  • Affordability
  • Effectiveness
  • Ease of set up
  • They blow air above, under, and around your plants.
  • They’re easy to move around, thereby allowing you to experiment with placement.

If you decide to use oscillating fans, it’s best to invest in several fans because you need adequate ventilation throughout the tent. Also, make sure you’re not facing the fans directly towards the plants to prevent windburn.

Should Fans Be Left On When Grow Tent Lights Are Off?

Yes. Although your plants are not producing as much moisture or heat when the lights are off, you still need to ensure adequate airflow for optimum growth. When the lights are off, reduce the speed of the fan or set it to run for only 15 minutes per hour. The air circulation will ensure adequate airflow when it’s dark.

Do Grow Tents Need Fans for the Exhaust Vents?

Yes. Setting up oscillating fans in your grow tent will go a long way in moderating the temperature and humidity in the tent. However, fans alone are not enough. You need an air extractor system in place to keep the grow tent fresh.

An air extractor fan system cycles out the old air and brings in fresh air, which your plants need to thrive. The extractor fan system should replace the air in your grow tent every minute or at least every three to five minutes. It is therefore crucial to invest in the right-sized fan for your exhaust vents.

Photo of grape vines growing in a hooped outdoor grow tent
Photo by Zdasiuk

How Big Should Grow Tent Fans Be?

When you’re replacing air in a grow tent, the general rule of thumb is to replace all the air once per minute. This is known as the cubic feet per minute (CFM) rating. To figure out CFM, you’ll need to know:

  • Your grow tent dimensions
  • The exhaust efficiency

To get the total volume of your grow tent, multiply the length x width x height.  The result is the cubic measurement of your room. Hold on to the number you got. Next, you need to multiply it by the efficiency drop.

The efficiency varies based on the make and age of the filter and the length of the duct between the filter and fan, among other things. As a rule of thumb, if you have a short duct, multiply the volume of your room by two. And if you have a long duct, multiply it by three. The formula is as shown below:

CFM = Volume of room x efficiency drop

Once you have your CFM value, look for a fan that is higher than this number. CFM determines the fan’s specific diameter and strength. Below are some of the popular grow tents and the size of fans you’ll need.

Grow Tent SizeCFM for no DuctCFM for 1 Duct Bend
2×2 grow tent4055
2×3 grow tent5065
2×4 grow tent80110
3×3 grow tent90120
4x grow tent160215
5×5 grow tent240340

Note: Inline fans for grow tents are not manufactured to the exact CFM output you may be looking for. Therefore, it’s advisable to go over by 50 to 100 CFM when shopping for a fan. It’s also a good idea to go for a fan that has an adjustable setting so that you can tweak the CFM to your desired needs.

What is the Difference Between an Exhaust Fan and an Intake Fan for Grow Tents?

Exhaust fans, also known as extractor fans, are installed to remove the hot and humid air out of your grow tent. They are used along with ducting to channel the air away from the tent. On the other hand, intake fans are devices used to bring fresh air into a grow room from the outside or a different room.

An intake fan is crucial to the growth process because it keeps the air fresh and replenishes the amount of carbon dioxide and oxygen in the air. You can reverse an extractor fan to use it as an intake fan. However, to do this successfully, you need to know how to tweak the strength of the fan so that it functions correctly. An air intake fan should only have a quarter of the strength of an exhaust fan.

How Long Should Fans be Left on in Your Grow Tent?

Several factors determine how long you should keep your fans on in a grow tent. These include:

  • Size of the grow tent: Small grow tents require smaller intervals of turning the fans on compared to bigger ones.
  • Fan CFM: If your fan has a low CFM, you’ll need to turn it on for more hours than a fan with a higher CFM. Make sure you pick the right fan depending on the size of your grow tent.
  • Need of the fan: Fans are used for different reasons in grow tents, from controlling humidity to filtering the air. If you’re using it to lower humidity, for instance, you should turn it off when the humidity is back to the required levels. So, establish why you need the fan and adjust the duration based on your plants’ needs.
  • Light and dark hours: Plants produce a lot of heat and moisture during light hours, so it is recommended to keep the fan on throughout light hours. Lower the speed of the fan and keep it running for about 15 minutes per hour during dark hours because the plants won’t be producing as much heat at this time.
  • Stage of plant growth: Plants require different amounts of airflow depending on their stage of growth. Observe the stage of growth and adjust the airflow as needed.

Can You Have Too Many Fans in a Grow Tent?

Yes. There needs to be adequate planning before you invest in fans for your grow tent.  Before you purchase any fan, you should know your grow tent’s CFM to have an idea of how many fans you’ll need for optimum results. Also, you should ensure the room has a gentle breeze as opposed to a wind storm.

Too much airflow can have negative effects on your plants by causing too much transpiration or damage to your plants’ leaves.  The challenge with windburn is that it tends to mimic other plant problems. You may think your plants are suffering from nitrogen toxicity or watering issues, while the problem could be too much wind.

To know if you have a case of too much airflow, observe the leaves. The damage tends to be serious where the wind is highest than where it’s not blowing. Clawing, cupping, or curling of leaves and dead leaves are some of the signs of too much airflow.

How to Prevent Windburn

If you’ve calculated the CFM and you’re certain you have the right number and size of fans, and you’re still having windburn issues, try the following to fix the problem:

  • Ensure the placement of the fans is even above and below the canopy by moving the fans in a unified direction.
  • Set the fans to release a gentle breeze that is just enough to move the leaves and not the entire plant.
  • Avoid placing plants right in front of a fan even if the fan is small in size. This direct airflow may have a negative impact on the plant’s growth.
  • Stick to oscillating fans. They’re easily adjustable and pocket-friendly.

How to Make Grow Tent Fans Quieter

The noise caused by grow tent fans can be a nuisance. Luckily, you don’t have to live with it. There are several things you can do to make the fans quieter. These include:

Suspend the Fans in an Enclosure

Noise is created when vibrations from your fan go through the walls, ground, and ceilings. The best way to fix this problem is to isolate your fan from any surface by mounting it in an enclosure such as a wooden box. Use some bungee cords to suspend the fans from the ceilings. Do the same for the ducts. To achieve maximum noise reduction, do the same for all your fans and ducts.

Soundproof Your Grow Tent

Fans are not the only devices that emit noise from a grow tent. Air pumps and exhaust ducts can also be noisy. Therefore, even after quieting the fans, the noise may still be a nuisance. The best way to deal with this problem without doing major structural work is to soundproof the tent.

One fast and affordable option is to hang heavy vinyl curtains. However, if you want to block off more noise and vibrations, you’ll have to invest in heavy sound damping materials like rubber. If you’re in the set up stages of your grow tent, it’s advisable to lay soundproofing underlay on the floor. This addition will go a long way in keeping the noise at bay.

Closing Thoughts

Ventilation is a must-have if you want your plants to flourish in a grow tent. Fortunately, if you know your tent’s CFM, figuring out which fans and how many you need is a piece of cake. However, getting the right fan for your grow tent isn’t enough. You also need to know how to place the fans correctly and adjust their settings so that the airflow rate is favorable for your plants to thrive. With this guide, you can say goodbye to ventilation issues in your grow tent.