Intake and outtake exhaust fan

I have a 25 ft x 25 ft empty room with a vivason 4X4 grow tent. I have a homemade commercial grade humidifying system that will dial into the exact humidity so that’s no problem. I can dial in to the exact number I want for heating and or air conditioning inside the tent so that’s no problem. I have a KS5000 4x4 light that’s no problem

My only issue is I control the heat by the heat I control the air by the air condition however, I have a Riverside exhaust fan hanging from the roof the charcoal filter and I have an inlet that is on exhaust fan at the bottom of the tent for intake fresh air.

I need to know how long should I run the exhaust fan inside the tent when and how long I have no problem putting it on a timer I just don’t know when to run it and when to turn it off. Same scenario with the intake fan for fresh air coming into the tent when should I turn it on when should I turn it off.

Any help will be appreciated


I run my exhaust and circulation fan 24/7


How about your intake fan bringing in fresh air from outside the tent?


Yes fresh air is important to plants you want good air exchange in your tent also the circulation fans keep the plants moving which helps straighten them and helps prevent powdery mildew


Good evening @Lostgirl welcome to the community!

You’ll want your exhaust fan on constantly… Plants need fresh air.

I use inkbird temp and humidity controllers.

On the temp controller there’s a heat plug and cooling plug… The heat has a oil filled radiator heater… The cool has an inline fan blowing in cooler lung room air.

On the humidity controller the two plugs are humidify and dehumidify… The humidify I have a humidifier… On the dehumidify I have another inline fan blowing in drier air when specifically needed… The lung room has a dehumidifier in it.

With this my temps and humidity have been perfect and stable.

Also the constant exhaust helps after lights out when the temps drop and dew point & humidity can spike… Keeping things healthy and stable.

Are you utilizing VPD to set your environment and DLI to set your lights? Plants have a optimal environmental conditions VPD…vapor pressure deficit.
Also, only need so much light per day for optimal growth… DLI… Daily Light Integral.

As @dirtydave mentioned fans inside help circulation and help strengthen the plants.

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I have the ink bird humidifier dehumidifier thermostat I basically use the same as you do.

I was concerned on how to run the exhaust fan or should I say how long. As both of you suggested I’ll simply keep it on all the time.

I have a heater inside the tent that has a thermostat built into it. I will have it set 77° so when temperatures drop to 76 the heater will come back on. As far as temperatures getting too hot outside the tent I have the air conditioned cooling the room.

I can put the intake in line fan that I have four fresh air coming in and put it on a timer it’s a 4-in fan. How long do you think I should have it pulling fresh air into the tent from outside and how long should it be off any ideas?


I would run both 24/7. Good air exchange is important. You could get by with a passive air exchange system with exhaust still 24/7 if you wanted. This is how i do it, but inline fan is good to bc you can control the amount of negative pressure by just having slightly less air in then out for odor if thats a concern.



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Welcome to the community , Exhaust fan will need to run 24/7 also If your fan did not come with a variable speed I would add one. Happy growing


Provided that the exhaust fan is ultimately strong enough to cool the tent down, I’d go passive intake & provide enough intake area to let the exhaust fan reach its max flow at full power. That’s the most efficient way to do it. Then you can dial in some negative pressure by slightly reducing the intake area from there.

If you need two fans due to having to run two controllers- one for air temp & one for humidity control, set the fans up in parallel/ side by side as exhaust fans.

You want the whole intake/exhaust setup to be as efficient as possible - not stealing airflow from the fan(s). Long duct runs, bends in the duct, fans in series, will always steal air flow. Intake dust screens, light traps on the intakes, carbon filters all can steal airflow if they are small enough to be restrictive. Best to avoid fans in series, & excess ducting & bends, & if you make the intake areas & carbon filter surface areas big enough, eventually they won’t steal flow.


As far as setting a timer for the input fan…

Sorry… The controller usually takes car of it… But it only goes on for a few minutes when needed.

I second that @PhotoFinisH Keep it simple
6 inch Exhaust fan on 24/ 7 (4x4 space) pulling hot stale air out an passive holes open at the bottom pulling in fresh air . I have never had a problem.


Thank you all for the advice. Shipping disaster has prolonged my receipt of seeds. Just getting the tent together. Can’t wait to get started! Thank you all. :kissing_heart: