How Often To Run Exhaust Fan In Grow Tent?

Trying to keep your grow tent cool? It can be a challenge both in the summer or in naturally hotter places like extra attic space. In this article we’ll tell you how often to run your exhaust fan in your grow tent for optimal temperatures.

The quick answer is to run it all day from sunup to sundown and adjust it throughout the summer as sunrise changes. In addition you should vent more during hot spells and you can run it less during cooler times.

The answer highly depends on where your grow tent is setup. The situation around it plays a huge role too.

Is it in the attic? Is it in a south facing room with lots of open windows? Have you tried any options to optimize the temperature in the room? For more tips on all of those see our full article on grow tent and grow room temperature control tips.

Note: If you click some of the links in this article we may earn a small commission at no additional cost to you.

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Understanding The Basics – What Does An Exhaust Fan Do?

Exhaust fans do one thing – they mix and distribute air. If you’re trying to cool your tent it’ll do no good to have an exhaust fan all day inside a hot room where you keep your grow tent in the attic.

We can further break down this distribution into a few basic functions.

Exhaust fans in a grow tent provide three basic functions:

  1. Exhaust fans distribute temperature – No more hot tops and cool bottoms. They match the temperature inside the tent with the room around it. Fans won’t cool a grow tent in a hot room. This is key to understand.
  2. Exhaust fans distribute humidity – Again they will mix the humidity within the grow tent and over time make it match the humidity in the room where you keep your grow tent. Watering your plants inside the tent will raise humidity. Adding exhaust will mix that humidity outside the room. Do you live somewhere dry like Colorado? Your exhaust fan may be lowering the relative humidity inside the tent from optimal levels.
  3. Exhaust fans distribute air (CO2 and O2) – Your fans will pull in fresh CO2 for your plants and exhale spent O2. Fast growing plants need this circulation to grow optimally.

Your plants needs for temperature, humidity, and CO2 distribution will vary throughout the day and night.

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Does Exhaust Fan Need To Run 24/7?

No. Your exhaust fan needs to run so that they following optimal conditions are met. These keep your plants happy. When super optimized (i.e. kept in a tight range) they can supercharge the growth of your plants.

  1. Daytime temperature of 65 – 85 Degrees F
  2. Nightime temperature of 60 – 70 Degrees F
  3. Daytime humidity of 75 – 90%
  4. Nightime humidity can drop to 65 – 75%
  5. CO2 should be maintained at the following
    • 800-1000 ppm for seedlings and most growth stages
    • 350-400 ppm minimum
    • WARNING – CO2 PPM over 2,000 can be harmful to both people and plants. Target ppm at 800 – 1,000. No more than 1,300 ppm. Some say 1,500 ppm is ideal but if you are adding CO2 be sure you know what you’re doing and how to maintain safe levels.
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How Long To Run Fan On Seedlings Stage

See the guidelines above. Seedlings will do best when temperatures are 70 Degrees F during the day, no less than 60 Degrees F at night, with humidity at least 65%, and CO2 between 350 ppm and 1000 ppm.

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How Long To Keep Fans On Why Lights Are On?

Exhaust fans should always be on when the lights are on. This may be less true for LED lights IF they you don’t have too many in your grow tent.

Either way, it’s still good practice to keep your exhaust fans on while the lights are on.

Set them on the same timers.

Keep in mind again that exhaust fans only distribute temperature with the rest of the room.

They match it.

If you’re house isn’t air conditioned, if the grow tent is in an attic or warm room, if it’s hot in your house in general then no amount of ventilation through the grow tent will help.

Working on cooling the rest of the room with these tips first then go to exhaust to cool your grow tent.

How Long To Run Fan On Flowering And Fruiting Stage

Run the fan only as long as is needed to keep the inside of the grow tent in optimal temperature ranges of 65-85 Degrees F. At 95 Degrees F most plants, including heat loving tomatoes, stop functioning well.

If you’re fruiting later in the winter you’ll need to monitor how long the fans are on accordingly.

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How Long To Keep Fans On Why Light’s Are Off?

Generally there’s no need to keep the fans on when the lights are off. You will need to test this for your situation though.

It’s quite possible it gets nice and hot in your grow room that houses your grow tent. Plus if your lights and your fan are on different timers this could case a problem.

First just test having the fans on the same timer as the lets. Monitor the temperatures carefully for 2 hours after both the fans and lights cutoff.

Do the temperatures drop or at least stay below 85 Degrees F? Then you’re fine.

If not, consider having the exhaust fans on a separate timer. Run them at least one hour longer than the lights.

Check the temps though. You may need to extend that to 2 hours or more if you’re working in a hot room.