Grow Space Air Exchange

A sealed growing space runs out of carbon dioxide within an hour or two of the lights coming on, accumulates heat continuously and builds humidity as the crop transpires. Air exchange addresses all three, and the sizing rule is simply the volume divided by how often you want it replaced.

The usual target for a vented tent is one complete exchange every one to three minutes. A 1.2 × 1.2 × 2.0 m tent holds 2.88 m³, so a one-minute exchange needs about 173 m³/h, which is 102 cfm. Rounding up to the next standard fan is normal practice and rarely wrong.

Rated fan airflow is measured with nothing attached. A carbon filter alone can cost 25% of it, and ducting, bends and a silencer take more — a fan rated at 200 m³/h may deliver 120 in a real installation. Size on the free-air requirement and then buy above it, or fit a speed controller to a larger fan, which is quieter at part load than a small fan at full speed.

One important exception: a CO₂-enriched room is not ventilated on this logic at all. Every air change throws away the injected gas, so enriched rooms are sealed and instead handle heat and humidity with air conditioning and dehumidification. Choosing between the two approaches is the central design decision in a grow room, and mixing them — enriching a vented space — wastes the gas entirely.