Ventilation

Sizing the fan. Multiply the tent's length, width and height in metres to get its volume. Most growers aim to change all the air every one to three minutes, which is 20 to 60 changes an hour. A 1.2m by 1.2m by 2m tent holds 2.9m³, so 60 changes an hour needs about 175m³/h. Add around 25% for the carbon filter, and more again for long runs or tight bends, because both cut the airflow you actually get.

Matching the filter. Pick a carbon filter with the same neck size as the fan and an airflow rating at or slightly above it. An undersized filter chokes the fan, adds noise and saturates early. Mount the filter before the fan, so the fan pulls air through it and the tent stays under slight negative pressure.

Ducting and noise. Keep duct runs short, straight and the same diameter as the fan. Every bend and every step down in size adds resistance and noise. Acoustic ducting and a speed controller take most of the hum out of a room, and a larger fan turned down is usually quieter than a smaller one running flat out.

EC fans and running costs. EC fans draw noticeably less power than AC fans for the same airflow, and the gap widens when they are turned down. They also run cooler and quieter. For a fan that runs around the clock, that lower draw adds up over a year, so compare the wattage at the speed you will actually use.