How Often To Run Exhaust Fan In Grow Tent?

Every grow tent needs a proper ventilation system to ensure healthy plant growth. Perhaps you already know that using an extractor fan is one of the most effective ways to vent the humid air out of your grow room. However, one common question every grower repeatedly asks is how often to run exhaust fan in grow tent.

Well, the simple answer is 24 hours, 7 days a week. You must leave the extractor fan active in your grow tent during the daytime and night. If not, your plants may develop bud rot issues or even suffer from heat stress.

But there is more! So, read on to find out everything there is to know about the ideal running time of your inline fan.

How Often To Run Exhaust Fan In Grow Tent?

You can run your exhaust fan all day or simply 24/7. This is because even the slightest changes in humidity and temperature can seriously affect your yields. Besides, the air inside your grow tent must be replaced with clean air every minute or 3 minutes, especially in the summer.

However, in some cases, you may not need to run your extractor fan continuously based on factors such as:

1. Types of grow lights


Fluorescent and HPS grow light lamps generate excess heat compared to their LED counterparts. So, if using HPS lights, your grow tent could get hot more quickly than when using LED bulbs.

In this case, you should have your exhaust fan constantly running to vent the excess heat. However, when using LED lights, you may not need to run the extractor system all day, especially when the lights are off.

2. Climatic conditions

If you stay in a naturally hot area, you should leave your fan running all day. The same applies during the summer season.

During winter, you do not have to run your air extractor fan all day. This is because your grow tent might get too cold because of pulling in so much cold air. Alternatively, you can run the fan throughout the day but at lower speeds.

3. Growth stage of your plants

During the seed germination stage, your plants need at least 18-24 hours of light and mild humidity. Therefore, at this stage, I suggest running exhaust fan 24/7.

At the vegetative stage, most plants require 18 hours of light and 6 hours of darkness. Therefore, you must leave your extractor fan on throughout the day and probably switch it off when the grow lights are off.

During the budding and flowering stage, plants need 12 or more hours of darkness to begin flowering. So, you can run the fan continuously during the day and switch it off an hour later after turning off the lights.

4. Daytime or nighttime

During the day, the lights in your grow tent should stay on to ensure your plants receive maximum sunlight. While the grow lights are on, your inline duct fan should run all day to keep the tent cool.

On the other hand, at night time, you can run your fan for a while and gradually reduce its speed after switching the lights off. And after several hours, you can turn it off.

5. Humidity levels

At the seedling stage, your plants in the grow room require humidity levels ranging from 70-75%. During the vegetative stage, they need 50-70% moisture levels and 40-50% in the flowering stage. So, when the humidity levels are above 70%, you will have to leave your exhaust system running all day.

Related topic How to Raise Humidity in Grow Tent without Humidifier?

Is A Grow Tent Exhaust Fan Necessary?

Yes, and for many reasons. First, an extractor fan system cycles out air, helping protect your plants from heat stress. And without an extractor fan, your plants are more susceptible to mold and bud rot caused by excessive moisture in the tent.

Besides that, using an inline duct fan can help keep pests away, including spider mites and aphids, which tend to thrive in hot and humid conditions. Another reason a fan is necessary is that it helps provide your plants with CO2 for photosynthesis.

Check out this grow tent ventilation diagram to better understand how an exhaust fan works.

How Much Airflow Does A Grow Tent Need?

This entirely depends on the size of your tent. Generally, the unit for measuring the rate of air is Cubic Feet Per Minute (CFM). Now, to determine the airflow requirements of your grow tent, you first calculate the volume of your grow area. The answer you get, divide it by the time it takes to replenish the air inside the grow space.

To find the volume of your tent, simply multiply its length, width, and height. For instance, if your grow area measures 4 by 4 by 6 feet, its volume will be 96 cubic feet in size.

Now, since your extractor fan needs to replenish the air in your grow space every minute, this means your tent needs 96 CFM of airflow. However, if your extractor system were to replace the air in the tent every 3 minutes, the required airflow should be 96 divided by 3 to get 32 CFM.

Below is a table comparing different sizes of grow tents and how much airflow your exhaust fan should produce to avoid too much ventilation.

Grow Tent SizeAirflow Requirements (CFM) Per MinuteAirflow Requirements (CFM) Every 3 Minutes
2 by 2 by 83211
4 by 4 by 69632
4 by 8 by 619254
5 by 5 by 717558
8 by 8 by 6384128

Should I Run Exhaust Fans On Or Off During Dark Period?

Your plants in the grow tent need fresh air throughout, even during the dark periods. But you are allowed to adjust the speed of the fan. Otherwise, you might have to deal with bud rot and mold problems because of too much moisture in the grow space. However, if you have a proper grow tent oscillating fan setup, you can turn them off.  


Now that you know the benefits of running an inline duct fan continuously, you might have some questions. Below are answers to some frequently asked questions.

Does My Carbon Filter Need To Run 24/7 Along With The Extractor Fan?

If you want zero odors in your grow tent throughout, you should let your carbon filter run during the day and night. However, you must replace the filters every 18-24 months of continuous usage.

How Often Should You Run An Exhaust Fan In Your 3x3x5 Grow Tent?

Unless you want your plants to get moldy or be susceptible to spider mites, you should run your extractor fan continuously. And it does not matter how many grow tent oscillating fan setups you have.

What Is The Difference Between An Intake Fan And An Extractor Fan?

An intake fan brings in cool and fresh air into the grow tent while its exhaust counterpart removes the stuffy air from inside. As a result, this helps keep the grow room cool 24/7.


Your plants in the grow tent need regular air circulation to produce maximum yields. For this reason, the answer to the question of how often to run exhaust fan in grow tent is continuous. Even during the dark period, you should leave your inline duct fan running to keep humidity levels in control.

However, depending on the type of lights you are using, the plant’s growth stage, and humidity levels, you can decide to turn off the fan. Or, you could just reduce the speed of the fan.

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