A grow tent inline fan does three jobs at once: it removes heat, controls humidity and pulls air through a carbon filter for odour.
The carbon filter is what makes the sizing awkward, because it can absorb half the airflow the fan was rated for.
Size a grow tent inline fan on heat load rather than tent volume, then check the fan curve at the loaded filter pressure. A typical 1.2 by 1.2 metre tent with 400 W of lighting needs roughly 250 to 350 m³/h delivered, which usually means a fan rated well above that in free air.
Sizing on heat, not volume
Air change rules give a number that is usually too low once lighting heat is included.
- Add up lighting and equipment wattage inside the tent.
- Decide the temperature rise you can accept, commonly 4 to 6 K.
- Airflow in m³/h equals watts divided by 0.335 times that rise.
- A 400 W tent at 5 K therefore needs about 240 m³/h.
- Compare against 30 to 60 air changes per hour and take the larger.
- Finally, add margin for summer ambient temperature.
Heat almost always governs, which is why a grow tent inline fan sized on volume alone runs hot in July.
The carbon filter changes everything
| Component | Clean | Loaded |
|---|---|---|
| Carbon filter | 80–180 Pa | 200–300 Pa |
| Pre-filter sleeve | 10–30 Pa | 60–90 Pa |
| Flexible ducting, 5 m | 50–150 Pa | Worse if compressed |
| Silencer | 30–80 Pa | Unchanged |
| Each bend | 10–25 Pa | Unchanged |
| Outlet mesh | 20–60 Pa | Clogs with dust |
Consequently a fan quoted at 400 m³/h free air may deliver only 220 m³/h installed. Our inline duct fan range publishes full curves for exactly this reason.
Grow tent inline fan and filter position
Two arrangements work, and one is clearly better for most setups.
Pulling through the filter keeps the fan clean, quieter and easier to maintain. Pushing through it works too, though the fan then handles unfiltered air and collects dust on the impeller.
Heat and odour rise, so a filter at the top of the tent removes both more effectively. It also keeps the ducting short, which matters given how much pressure the filter already costs.
Negative pressure and odour control
Odour escapes through gaps, so the tent must sit slightly below the pressure of the room around it.
- Extract slightly more than you supply, so the walls pull inward gently.
- Seal cable and duct ports properly, since these leak most.
- Check the zips, which are a common escape path.
- Avoid excessive negative pressure, which stresses the frame.
- Use a passive intake rather than a powered supply where possible.
- Finally, confirm odour control by walking around the outside.
Slight inward bowing of the walls is the visual confirmation you want. Our grow room ventilation guide covers the larger-scale version.
Keeping it quiet
Grow tents usually sit inside homes, so noise is often the limiting factor rather than airflow.
- Use speed control and run slower during dark periods.
- Fit a silencer on the discharge, allowing for its pressure drop.
- Isolate the fan with rubber straps rather than rigid mounting.
- Use rigid duct where possible, since flexible duct rattles.
- Avoid mounting the fan directly against a stud wall.
- Finally, choose a mixed-flow or EC unit, which run quieter at part load.
A larger fan running slowly is nearly always quieter than a small one running flat out.
Common mistakes
Four errors account for most disappointing grow tent inline fan setups.
Sizing on free-air airflow. Using a cheap axial inline fan with a carbon filter. Running long compressed flexible duct. Finally, forgetting that filter resistance doubles as the carbon loads, so the setup drifts out of specification over months.
For general ventilation rate guidance, ASHRAE Standard 62 provides the underlying framework.
Grow tent fan FAQ
What size fan for a 1.2 by 1.2 metre tent?
Usually a 150 mm fan rated 400 to 600 m³/h in free air, which delivers roughly 250 to 350 m³/h once a carbon filter and ducting are fitted.
Should the fan pull or push through the filter?
Pulling is better in most setups, since it keeps the fan clean and quieter. Pushing works, though the impeller then collects dust and needs cleaning.
Do I need a passive intake?
Yes, otherwise the fan cannot draw air. A passive intake also keeps the tent slightly negative, which is what stops odour escaping.
Why is my tent still hot?
Usually undersized airflow once the filter is included, or an intake that is too small. Measure the actual airflow rather than trusting the rating.
Size a grow tent inline fan on heat load, read the curve at the loaded filter pressure, and keep the tent slightly negative. Everything else is ducting detail.
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