Tent circulation fans: When to start using?

I understand the purpose of a tent fan.

Temp and humidity control
Simulates natural environment
Helps plants “breath”
Strengthen plant
Prevention of bud rot, etc.

I am trying to understand when to start using one.

I am 3 weeks into my second grow and have been able to control my temp and humidity without the use of the tent fan. Currently have tent dialed in at 70F and 75% RH. 4 Gelato Autos in water only soil grow in a 4 by 4 tent. I intend to drop humidity by 5% each week until it gets to Veg range.

I have not used the tent fan yet because I have not needed it.

Specifically I am interested in:

When you start to use your fans?
Do you have more than 1 fan?
Specific Triggers for on or off?
Continuously on?
Scheduled timed on/off? At what interval?
Off at night?
Position in tent?
Bottom blowing up?
Top blowing down?
If you SOG or SCROG…one below and one above?

I appreciate any guidance you have!!!
Happy Growing!!!

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My goal is always getting horizontal air flow at the “forest floor”. Seedlings I protect from too much breeze, but once past a week or so, I want to see leaves wiggle here and there all the time. Moving air and light, kills things that live in the night…… in practice, in a small tent, a chest high fan in one corner blowing almost straight down will do it. Picture the air dumping down and billowing out. It’s taking exhaled air and decomp to the filter.


This pertains to circulation fans only… I use two, One below the canopy and one above in between canopy and light. They are on 24/7 as even a slight breeze when they’re small helps to strengthen the stock as well as what you listed. Oscillating fan above canopy. If they’re very small you can get away with using one fan.

Note on exhaust fan… Plants need CO2 and depending upon their size will use it up in a tent fairly quickly. Regardless of your temperature and humidity, replace the air in near tent / room / grow area once an hour to replenish the CO2…

VPD (vapor pressure deficit) find a chart online for cannabis. It will tell you the proper temperature and humidity for each plant stage. Look for one that uses leaf temperature, room temperature and humidity. If you don’t have a laser thermometer to measure leaf temperature then you can assume 3 to 4° less than room temperature for leaf temperature if using an LED light.

Good luck…

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@Jgrowman --Welcome to the forum!!!

Thank you for the recommendations…I have more tent fans ordered…one of the two I had on my first grow crapped out. They were cheap anyway. I have AC infinity fans on the way that I can at least set on a schedule.

I do have my exhaust fan cut on and off at intervals unless triggered by something else in order to turn over the air.

As far as VPD goes, the sensor calculates it, but I have to check on the Leaf Temp setting, I don’t think I have it adjusted correctly.

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I have my fans running constantly while growing. Only time I turn them off is after the grow room has been cleaned, dried & shut down for summer. Then I grow outside.


Just curious… what are you running for 'sensor’s ?? I’m using inkbird for co2 and rh.

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@regnartS I have a couple of AC Infinity controllers that monitor temp and humidity. It calculates VPD and operates exaust, circulation fan and lights. It has a leaf offset setting as well as data logging through the phone app. Although these are the bluetooth based ones. I would prefer the wifi ones so you don’t have to be in proximity to change settings. You can also download all data if needed. It has a few flaws and interface issues.

I don’t have a co2 monitor yet.

Hmm, pretty nifty! I have a couple spider farmer ones with similar controllers… but nothing that fancy. Ill have to look into what ac infinity has.

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