Ventilation and Airflow
Poor ventilation causes more indoor grow failures than almost any other factor. Heat stress, high humidity, mould, and weak stems all trace back to inadequate airflow.
The Three Airflow Roles
You need three types of airflow working together: (1) Extraction — an inline fan pulling stale hot air out through a carbon filter. (2) Circulation — an oscillating clip fan inside the tent moving air around the canopy, strengthening stems and preventing hot spots. (3) Fresh air intake — passive holes or a small intake fan pulling in fresh CO2-rich air.
Sizing Your Inline Fan
Calculate your tent volume in cubic metres (L x W x H). Your fan should exchange this volume every 1–2 minutes at minimum. A 60x60x140cm tent = 0.5m³ — a 4-inch (100mm) inline fan moving 100m³/h is more than adequate. An 120x120x200cm tent = 2.88m³ — a 5-inch (125mm) fan moving 250m³/h is appropriate.
Carbon Filter Setup
Connect your carbon filter to the intake side of your inline fan using ducting. The filter should hang inside the tent near the top where hot air accumulates. The fan then pushes filtered air out through ducting to outside the tent. Replace carbon filter media every 12–18 months when you notice smell getting through.
Negative Pressure
Your tent walls should bow slightly inward during operation — this is negative pressure and means your extraction is working correctly. It ensures all air leaving the tent passes through your carbon filter. If your walls bow outward, your intake is overwhelming your extraction and unfiltered air is escaping.
Quick Tips
- Run your fan on a timer — off at night can cause temperature swings. Keep it running 24/7 during flower.
- The oscillating clip fan should not blow directly onto your plants — aim it above the canopy for gentle movement.
- Insulate your ducting in hot climates to prevent condensation.