A closed tent builds its own climate within hours: the light heats things up, the plants transpire, the CO₂ runs out, and everything quietly drifts off track. By the end of this step, you'll know how to size your extraction without trusting the number on the box, organise the air inlet and outlet, move air around without stressing your plants, and manage temperature, humidity and VPD with real decision-making logic, phase by phase.
Why the air has to be renewed continuously
Your plants consume CO₂ and transpire water; your LED gives off heat. In a closed tent, the CO₂ runs out and growth slows, the temperature climbs, humidity saturates until mould appears. The extractor fan expels this stale air and draws in fresh air: it's the lungs of your tent.
Remember the mechanism that justifies this whole step: transpiration. A plant drinks through its leaves. It evaporates water through microscopic valves, the stomata, and that evaporation pulls sap up from the roots, nutrients included. If the air is already saturated with moisture, transpiration stops, and with it the plant's entire internal transport system. Air that is renewed and well circulated is literally what keeps the sap moving.
The basic benchmark: renew the tent's air volume once per minute. That's the real airflow to maintain in operation, with filter and ducting installed.
Field tip: a LED runs cooler than an old HPS, but it doesn't do away with ventilation, whatever its wattage (sizing the LED itself is covered in the Lighting step). Air renewal remains essential for CO₂, humidity and residual heat.
Keep this figure in mind: out of 100 litres poured at the base of your plants, around 90 come back out as vapour, drawn up by the roots then released by the leaves. Every watering is therefore a full-scale test of your extraction.
Renew the air before you try to condition it: properly sized extraction on its own solves most temperature and humidity problems. The air conditioner and the dehumidifier are only back-up, never substitutes.
