A grow tent exhaust fan should replace all the air in the tent roughly once a minute: multiply the volume in m³ by 60 and add a 30–50% margin for the carbon filter and ducting. A 120x120x200 tent therefore needs about 225–260 m³/h, which means a 125 mm extractor fan rated at 300–400 m³/h, fitted after the filter and run through a speed controller.
In brief
- Formula: length × width × height (m) × 60 × 1.3–1.5 = airflow in m³/h.
- Installation order: filter at the top of the tent → fan → silencer → ducting → outlet.
- The tent walls should pull in slightly: that is the sign of correct negative pressure.
- A larger fan at 50–70% speed is quieter than a small one at full power.
How to calculate the airflow
One complete air change per minute is the most widely used benchmark: it removes the heat from the lamp and the moisture the plants transpire. With a low-power LED and a cool room, one change every 2 minutes can be enough; with HPS lamps or a warm room it is worth going up to 1.5–2 changes per minute. The margin is needed because the rated airflow is measured with no resistance: the carbon filter, metres of ducting and bends reduce it, sometimes by a third or more.
- Work out the volume: 1.2 × 1.2 × 2.0 m = 2.88 m³.
- Multiply by the changes per hour: 2.88 × 60 = 173 m³/h (one change per minute).
- Add the margin: 173 × 1.5 = 259 m³/h, rounded to about 260 m³/h.
- Choose a model with a slightly higher rating, here 300–400 m³/h, and a carbon filter rated for at least the same airflow.
| Tent (cm) | Volume | 1 change/min | With ×1.5 margin | Typical diameter |
|---|---|---|---|---|
| 60x60x160 | 0.58 m³ | 35 m³/h | 52 m³/h | 100 mm |
| 80x80x180 | 1.15 m³ | 69 m³/h | 104 m³/h | 100 mm |
| 100x100x200 | 2.00 m³ | 120 m³/h | 180 m³/h | 100–125 mm |
| 120x120x200 | 2.88 m³ | 173 m³/h | 259 m³/h | 125 mm |
| 240x120x200 | 5.76 m³ | 346 m³/h | 518 m³/h | 150 mm |
Extractor fans of 100 mm usually cover 100–250 m³/h, 125 mm models 250–400 m³/h and 150 mm models 400–800 m³/h: always check the model's data sheet.
Installation: filter, fan and ducting
- Hang the carbon filter at the top of the tent, where warm air collects, using adjustable straps.
- Connect the fan straight after the filter, with the airflow arrow pointing towards the outlet so it pulls air through the filter. It can sit inside the tent or just outside it.
- Use the same diameter for filter, fan and ducting. Reducing from 125 to 100 mm removes more than a third of the cross-section.
- Keep the outlet duct short, straight and pulled taut: every tight bend and every metre of flexible duct reduces airflow.
- Secure the joints with metal clamps and aluminium tape, and run the ducting through tent ports that match the diameter (see how to choose a grow tent).
Negative pressure and air intake
The fan has to remove more air than flows in on its own. That keeps the tent under slight negative pressure: the walls pull in a little and odour can only leave through the filter. For incoming air, a passive opening at the bottom, on the side opposite the fan, is usually enough.
- Opening area: at least equal to the cross-section of the exhaust duct, ideally 1.5–2 times larger, with fine mesh against insects and dust.
- Walls sucked in hard: open another vent or slow the fan. Walls straight: extraction too weak or intake too large; walls bulging: the intake fan is pushing harder than the exhaust.
- An active intake fan is only worth it in large tents or to draw air from a cooler room; its airflow must stay below the exhaust, at about 70–80%.
Where to vent the air
- Window: a panel or insert with a duct connector closes the gap; vent away from neighbours' windows and balconies.
- Another room: a ventilated space such as a cellar or a box room with a window; the heat and moisture will end up there too.
- Same room: this only works if it is large and ventilated, otherwise the tent sucks its own warm, humid air back in.
- Never into chimney flues or shared ventilation ducts; in a rented flat, no holes in the walls without the landlord's consent.
Speed controllers and climate controllers
- AC motors: controlled with a transformer controller (stepped, quiet) or an electronic one (stepless, may hum at low speed).
- EC motors: built-in or 0–10 V control, lower power use and quiet even at minimum speed; they cost more.
- A climate controller with a probe adjusts fan speed to temperature and humidity. Set a minimum speed, for example 30–40%, so you never lose air exchange or negative pressure.
To hold 20–28 °C with the lights on and the right humidity for each stage, see the guides to grow tent temperature and grow tent humidity.
Reducing noise and circulating air
- Oversize and slow down: a fan running at 50–70% is much quieter than a smaller one at full speed.
- Fit a silencer straight after the fan and use acoustic ducting instead of aluminium duct.
- Hang the fan from straps or anti-vibration mounts, with no contact with the frame, wall or floor.
- Whistling comes from narrow passages: remove kinks and reducers and clean the carbon filter's pre-filter.
- Inside the tent you need circulation fans: one 15 cm clip fan in a 60x60 or 80x80, two in a 120x120, above and below the canopy. The leaves should move slightly, with no direct blast on the buds.
Frequently asked questions
What exhaust fan does a 120x120 grow tent need?
With a height of 200 cm the volume is 2.88 m³ and you need about 225–260 m³/h including the 30–50% margin. Choose a 125 mm model rated at 300–400 m³/h and a carbon filter with the same airflow and diameter.
Should the exhaust fan run all the time?
Yes, around the clock: with the lights on at the speed that keeps temperature and humidity in range, with the lights off more slowly. Switching it off lets humidity rise and odour escape from the tent.
Where can I vent the air without a window?
Into another ventilated room, always after the carbon filter. Venting into the same room only works if it is large and ventilated; never into chimney flues or shared ducts.
How can I make the exhaust fan quieter?
Use a larger fan at reduced speed, add a silencer and acoustic ducting and hang it on anti-vibration mounts. EC motors are the quietest at low speed.
Fans, filters and ducting matched for diameter and airflow are available in the air extraction kits.
Updated on 13 October 2026.