Short answer
Yes. In a vented tent, run the exhaust fan 24/7 and let a temperature and humidity controller slow it, not switch it off. Circulation fans always run 24 hours. The one exception is a sealed CO₂ room, where the exhaust stays off during lights-on and air conditioning plus a dehumidifier do the work. Grower practice.
Why
At lights-off the air cools, so the same water vapour gives a higher relative humidity and surfaces approach the dew point [3]. An exhaust that stops at that moment traps the moisture.
Humid, still air in dense colas is how Botrytis bud rot and powdery mildew start [1][2]. Night is when it happens.
In flower, a stopped fan also lets the tent go neutral or positive, and smell escapes through every zip.
In veg there is more leeway: smell is low and the exhaust can follow the light, as long as night-time RH stays in range.
The numbers
| Situation | Exhaust | Circulation fans |
|---|---|---|
| Veg, vented tent | 24 h on a controller, or follow the light if RH stays in range | 24 h |
| Flower from about week 2, vented tent | 24 h, slowed at lights-off | 24 h |
| Drying in the tent | 24 h, low speed, filter on | indirect only, never on buds |
| Sealed CO₂ room, lights on | off except emergencies | 24 h |
| Lights-off RH in flower | ≤ 55 % | |
| Leaf-level air speed | 0.3–1.0 m/s (60–200 ft/min) |
What to do
- Fit a speed controller tied to temperature and RH.
- Set a lower night speed rather than off.
- Log RH through the lights-off transition for 48 hours.
- If nights run cold, slow the fan further and add a small thermostat-controlled heater.
- Keep circulation fans on day and night.
Common mistakes
- Fans off at night "to let the plants rest".
- Cutting the exhaust to keep a cold tent warm.
- A timer that switches the exhaust fully off.
- Running exhaust flat out in a sealed CO₂ room.
Related
- Full article: Airflow
- Humidity by stage
- How do I lower the humidity in my grow tent?
- How do I stop my grow tent from smelling?
- Terms: exhaust fan · oscillating fan
