Best Inline Fan for a Grow Tent

The inline fan does three jobs in a grow tent: it removes the heat your light puts out, it pulls humid air out before it causes mold, and it drags air through the carbon filter that controls odor. Get the size wrong and you either cook the canopy or run a loud fan flat out all day.

Size the fan before you shop

Use the formulas LONGWELL publishes with its fan calculator. Our fan CFM calculator runs the same math.

1. Tent volume. Length × width × height in feet. A 4x4x6.5 ft tent is about 104 cubic feet; one air exchange per minute means a base of about 105 CFM.
2. Add for resistance. Carbon filter +25%. Flexible duct +5% per 5 feet. Each 90-degree bend +10%. A hot room adds about 30%.
3. Check the heat load. Required CFM ≥ light watts × 3.412 ÷ (1.08 × allowed temperature rise in °F). LONGWELL notes that a 480 W LED in a normal room is covered by the airflow from step 2 in a 4x4.
4. Pick the duct size. Up to about 190 CFM: 4-inch. Up to about 400 CFM: 6-inch. Up to about 750: 8-inch.

Worked example (4x4x6.5 with filter, 10 ft of duct, two bends): 105 × 1.25 × 1.10 × 1.20 ≈ 173 CFM, which LONGWELL rounds to 190–200 CFM with margin. That is a 6-inch fan running well below full speed, or a strong 4-inch EC fan near its limit.

Why oversize? Fans get much quieter and cheaper to run at partial speed. LONGWELL’s figure is about 60% of the airflow for about 22% of the power at 60% speed, and roughly 11 dB less noise. A fan with headroom also has capacity left for a hot July afternoon.

What to compare on a spec sheet

• Rated CFM is free-air flow with nothing attached. You will not get it through a filter.
• Static pressure (Pa or inches of water) tells you how well the fan pushes through a filter and long duct. Higher holds airflow better under load. Few brands publish it.
• Motor type. EC (electronically commutated) motors with PWM speed control are quieter and more efficient at low speed than AC motors on a variable-voltage dial.
• Controller. A dial sets one speed. A temperature/humidity controller ramps the fan when the tent warms up or gets humid, which is what you want in practice.
• Rated noise (dBA) is measured by the manufacturer at a distance it chooses. Use it to compare fans from the same brand, not across brands.

AC Infinity CLOUDLINE T6: best overall 6-inch

Manufacturer specs: AC Infinity’s current page lists 425 CFM at 30 dBA. Older AC Infinity listings and most retailers show 402 CFM and 32 dBA for the same model, so check which figure your listing uses. Static pressure 503 Pa, EC motor with PWM control, dual ball bearings rated for 67,000 hours, IP44, and a controller with temperature, humidity and VPD triggers on a 12 ft sensor probe. 2-year warranty.

Owner feedback: SimpleHydroponic’s roundup of owner reports says owners commonly run it below half speed in a 4x4, and that its plastic housing feels less premium than the price suggests.

Full details in our CLOUDLINE T6 review.

AC Infinity CLOUDLINE S6: the same fan, simpler control

The S6 uses the same 6-inch fan as the T6 (425 CFM / 30 dBA on AC Infinity’s current page) with a 10-speed manual controller instead of the automated one. It is compatible with AC Infinity’s UIS controllers, so you can add automation later. If your room temperature is steady and you will set one speed, this is the sensible saving.

Vivosun AeroZesh T6

Vivosun’s authorized dealer Trimleaf lists the AeroZesh T6 at 408 CFM, 3,100 RPM and 35 W, with a PWM-controlled EC motor, steel housing, an E12 controller with 10 programmable speeds and a temperature/humidity probe that adjusts speed automatically. Noise figures differ between listings, so we do not quote one. We could not find a published static-pressure figure.

For comparison, the R6 fan bundled in Vivosun’s 4x4 starter kit is listed by Vivosun at 390 CFM, 45 dBA and 70 W. Check which fan a kit actually includes.

Small tents: AC Infinity CLOUDLINE S4

For 2x2 to 2x4 tents a 4-inch fan is usually enough. HWGrow compared a 2021 AC Infinity CLOUDLINE S4 with a Vivosun 4-inch fan and measured their wall-power draw at 28 W and 59 W. At those draws, running continuously for 90 days uses 60.48 kWh versus 127.44 kWh. The AC Infinity also looked stronger in a duct-sleeve test, which the reviewer is careful to say is not a CFM measurement. HWGrow quotes AC Infinity’s current S4 listing at 226 CFM and 28 W, and the Vivosun model at 190 CFM and 60 W.

Getting the install right

• Mount the carbon filter at the top of the tent, where the warmest and smelliest air collects, and pull air through it into the fan.
• Keep duct runs short and straight; every bend costs about 10% of your airflow.
• Leave a passive intake of at least 1 square foot per 400 CFM of exhaust (LONGWELL) so the tent stays under slight negative pressure.
• Replace the carbon every 12–18 months, sooner in humid conditions, per LONGWELL.

Our grow tent setup guide walks through the whole install in order. If humidity stays high even with good exhaust, see dehumidifiers for grow tents.

How these picks were made

We sized the fan first using published formulas, then compared fans that meet a 4x4 tent’s need on published static pressure, motor type, controller features and warranty, plus the one independent power measurement we could find. We did not test these fans ourselves; see how we research.