# Should my exhaust fan run 24/7?

> Yes, in most vented tents the exhaust fan should run 24/7, slowed by a temperature and humidity controller rather than switched off. Lights-off is when humidity spikes and, in flower, when smell builds, so an off fan invites bud rot and odour. Circulation fans should always run 24 hours. The exception is a sealed CO₂ room, where exhaust stays off during lights-on.

- Canonical: https://genetixbay.com/knowledge/answers/run-exhaust-fan-24-7.html
- Topic: Environment · Type: How-to
- Evidence: Grower practice · Status: Draft: awaiting expert review
- Full article: [Airflow: air exchange, canopy air speed and fan placement](https://genetixbay.com/knowledge/environment/airflow.md)
- Also asked as: Should the exhaust fan run all the time? | Should I turn my fans off when the lights are off? | Can I put my inline fan on a timer? | Exhaust fan at night in flower
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Short answer

Yes. In a vented tent, run the [exhaust fan](https://genetixbay.com/knowledge/glossary/exhaust-fan.md) **24/7** and let a temperature and humidity controller **slow it**, not switch it off. [Circulation fans](https://genetixbay.com/knowledge/glossary/oscillating-fan.md) 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](https://genetixbay.com/knowledge/glossary/dehumidifier.md) do the work. *Grower practice*.

## Why

At lights-off the air cools, so the same water vapour gives a higher [relative humidity](https://genetixbay.com/knowledge/glossary/relative-humidity.md) and surfaces approach the [dew point](https://genetixbay.com/knowledge/glossary/dew-point.md) [3]. An exhaust that stops at that moment traps the moisture.

Humid, still air in dense [colas](https://genetixbay.com/knowledge/glossary/cola.md) is how *[Botrytis](https://genetixbay.com/knowledge/glossary/botrytis.md)* bud rot and [powdery mildew](https://genetixbay.com/knowledge/glossary/powdery-mildew.md) 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

1. Fit a speed controller tied to temperature and RH.
2. Set a lower night speed rather than off.
3. Log RH through the lights-off transition for 48 hours.
4. If nights run cold, slow the fan further and add a small thermostat-controlled heater.
5. Keep circulation fans on day and night.

## Common mistakes

- [Fans](https://genetixbay.com/knowledge/glossary/fan-leaf.md) 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](https://genetixbay.com/knowledge/environment/airflow.md)
- [Humidity by stage](https://genetixbay.com/knowledge/environment/humidity-by-stage.md)
- [How do I lower the humidity in my grow tent?](https://genetixbay.com/knowledge/answers/how-to-lower-humidity-in-a-grow-tent.md)
- [How do I stop my grow tent from smelling?](https://genetixbay.com/knowledge/answers/how-to-control-grow-smell.md)
- Terms: [exhaust fan](https://genetixbay.com/knowledge/glossary/exhaust-fan.md) · [oscillating fan](https://genetixbay.com/knowledge/glossary/oscillating-fan.md)

## Follow-up questions

**Can I turn the exhaust fan off at night to keep the tent warm?** Turning the exhaust fan fully off at night to keep a grow tent warm is a common mistake. Humidity spikes at lights-off, and still, damp air in dense colas is where bud rot starts; in flower, smell also builds and escapes. Instead, slow the fan with a thermostat or speed controller, and add a small thermostat-controlled heater if the tent still runs cold.

**Should circulation fans run at night?** Yes, circulation fans in a cannabis grow should run 24 hours a day, including lights-off. Humidity rises when the lights go off, and moving air dries the microclimates inside dense colas where Botrytis and powdery mildew start. Keep leaf-level air speed around 0.3–1.0 m/s (60–200 ft/min): leaves should flutter gently, never whip.

**Is a fan speed controller better than a timer?** Yes, a speed controller tied to temperature and humidity is better than an on/off timer for a grow-tent exhaust fan. It keeps the tent under steady negative pressure and holds climate within range, ramping up when lights are on and slowing at lights-off. A timer that cuts the fan entirely lets humidity, heat and smell swing, which is harder on plants and neighbours.

## Sources

1. Punja ZK, 2021. Emerging diseases of Cannabis sativa and sustainable management. Pest Management Science 77:3857–3870. https://pmc.ncbi.nlm.nih.gov/articles/PMC8451794/
2. Utah State University Extension. Hemp bud rot (IPM fact sheet). https://extension.usu.edu/planthealth/ipm/notes_ag/hemp-bud-rot.php
3. Allen RG et al., 1998. FAO-56 Chapter 3 (vapour pressure). https://www.fao.org/4/x0490e/x0490e07.htm