An exhaust fan is a ducted fan, usually an inline type mounted in the duct, that pulls air out of the grow space and sends it outside the room or building. It is the main tool for removing heat from lights and water vapour from transpiration. When the space is sealed apart from passive intake vents, the exhaust also creates negative pressure, so odour leaves only through the carbon filter.
In practice
Size the fan from the volume of the space and the restrictions in the line. A 1.2 × 1.2 × 2 m (4 × 4 × 6.5 ft) tent holds about 2.9 m³ (about 102 ft³). A common rule is one full air exchange every 1–3 minutes, then add 25–50% for a carbon filter, bends and long ducts. That gives roughly 100–200 CFM (170–340 m³/h) for this tent, more under hot HID lights.
Use a speed controller or a climate controller rather than running at full speed all the time. Keep ducting as straight and short as possible; each sharp bend costs airflow. Mount the fan with vibration isolation to reduce noise.
Don't confuse with
- Oscillating fan: circulates air inside the space without removing any.
- Intake fan: pushes air in; running it stronger than the exhaust loses negative pressure.
