Fan running, tent still humid? Give the wet air a way out first.
My tent is still humid with a fan running. How much air do I need moving and leaving?
GROWER ANSWER
A fan inside the tent moves the air around, but it takes no water out. Humidity only drops when wet air leaves the tent and drier air comes in. So a tent has two airflow jobs: exchange, an exhaust that replaces the tent's air every 1 to 3 minutes, and circulation, a gentle breeze through the canopy. Get the exchange working first.
Give the wet air a way out before you add another fan.
Two jobs, not one
Exchange: air leaving. The exhaust pulls air out high in the tent, and fresh air comes in low through the intake. Of the two jobs, this is the one that lowers the humidity. Every leaf gives off water all day, so with no exchange it stacks up.
Circulation: air moving. A clip fan keeps air moving above and under the canopy, so no damp pocket settles in a corner or inside the plant. It keeps water off the leaves, but it never removes any. Dancing leaves prove circulation, not exchange.
How much air should leave
Aim to replace the whole tent's air every 1 to 3 minutes. The fan that takes depends on the tent's size and on the filter and duct, which slow the air; the shelf guide below does the sizing.
The air going out has to come back in. As a rule of thumb, leave an intake opening 3 to 4 times the exhaust opening, low and on the opposite side from the exhaust, so fresh air sweeps across the plants on its way out.
Check the tent is breathing
The wall test. Zip the tent and run the exhaust. The walls should bow in slightly. Flat or puffed out means not enough air is leaving. Pulled in hard means the intake is too small.
The zipper test. Crack the zipper an inch and feel for air pulling inward.
The path. A crushed duct, a closed intake flap or a clogged pre-filter on the carbon filter each cut the air leaving.
How much air should move inside
Gentle is enough. The upper leaves should move gently, not whip. Aim clip fans at a wall, just over the canopy and under it, and keep every fan off the sensor and off wet pots.
A fan aimed straight at the plants can wind burn the leaves it hits: they curl and claw while leaves out of its path look fine. It can look like a watering or feeding problem. The tell is location.
Where the exhaust gets its air
The exhaust can only pull in what the room around the tent holds. In winter the windows stay shut, and an exhaust venting into the same closed room lets the tent breathe its own wet air back in. If you can, vent it out of that room. If the room runs wet, put the dehumidifier in the room the tent draws from, not inside a small tent, where it adds heat.
Read the whole tent, not one corner
Check the corners, the center, the canopy top and inside the canopy at the same moment. In a well-mixed tent, aim for them to read within about 3°F and about 5 to 8 points of humidity. Wider than that, look for a dead corner, a fan blast, a wet pot or a bad sensor before you touch the controller. A packed canopy can hold a wetter pocket than the room reading shows.
Change one thing at a time. Turn the exhaust speed up or down about 10 to 20%, or re-aim one fan. Give it at least an hour to settle, then judge it across a full day and night before the next change. On a controller, move the humidity setting 3 to 5 points at a time.
What not to do
Don't add fans to a tent that isn't breathing. More stirring leaves the same wet air in place.
Don't leave a stopped exhaust with the lights on. Act in minutes: dim or switch off the light first, open the tent, and find out why the fan stopped before the light goes back up.
When it's something else
What humidity to aim for, and the jump at lights-off: see What humidity should I run, and what if I can't hit it?
Too dry, not too humid: see My tent is too dry this winter. What do I do?
The room is cold too: see How cold is too cold for my grow room at night?
Gray fuzz inside a flower: see our Bud Rot Survival Guide
On our shelf in Worcester: Inline and clip fans, carbon filters, ducting and fan controllers are in Climate, air & controls. Call (508) 459-9609 or get directions.
Keep going
Sizing the exhaust, filter and intake for your tent: see our tent airflow shelf guide.
Watch the episode: Why Your Grow Tent Is Still Humid: Airflow vs. Exhaust on MyRemy Live.
Air out first, then air around, one change at a time. The whole system for a steady grow is written down in the free Stability Playbook. Get the free Stability Playbook
More answers: all grow guides. Every shelf guide and maker feed chart: Grower Resources.