AC Infinity RAXIAL S4 Inline Booster Duct Fan 4 - Variable Speed, Low

In this review the AC Infinity RAXIAL S4 is presented as a compact inline booster fan intended for anyone needing a straightforward way to increase airflow in HVAC ducts or ventilate small home spaces. The bottom line: it is best for homeowners and hobbyists who want a durable, low-noise booster with a simple three-speed controller, because its unibody die-cast construction and dual ball bearings deliver steady operation while keeping vibration and maintenance to a minimum.

Key Features

• Inline booster design: boosts existing airflow in ducts to improve ventilation in attics, basements, bathrooms, workshops and similar spaces.
• Variable speed controller: three-speed control lets you balance airflow and noise to suit different rooms and times of day.
• Unibody die-cast construction: a seamless housing reduces vibration and improves durability for long-term, stable performance.
• AC shaded-pole motor: uses balanced steel blades and permanently lubricated dual ball bearings for quieter continuous use and less maintenance.
• Compact size and airflow: the 4 in duct fits tight spaces while delivering up to 106 CFM for targeted boost applications.

Who It's For

The RAXIAL S4 is a good fit for homeowners, workshop users, or DIY HVAC improvers who need a compact inline fan to increase ventilation without a big installation footprint. Its modest airflow and low 28 dBA noise level make it useful for intermittent boost in bathrooms, kitchens, or localized duct sections.

It is not aimed at large commercial HVAC projects or whole-house circulation where higher CFM or larger duct work is required; buyers who need very high static pressure performance or larger duct diameters should consider a larger industrial blower.

Pros & Cons

Pros

• Solid build thanks to the unibody die-cast housing that limits vibration and rattles.
• Useful three-speed controller for adjusting airflow and keeping noise down when needed.
• Quiet operation at typical settings, with a rated noise around 28 dBA for less intrusive running.
• Low maintenance motor with permanently lubricated dual ball bearings for continuous duty.

Cons

• Airflow is modest at 106 CFM, so some users report mixed results when expecting larger volume movement.
• Not suitable for large-scale HVAC needs or for ducts that require greater diameter or pressure capacity.

Specifications

Duct size	4 in
Dimensions	7.28 x 3.95 x 3.95 in
Airflow	106 CFM
Noise level	28 dBA
Motor type	AC shaded-pole induction
Bearings	Dual ball

Our Verdict

The RAXIAL S4 is a compact, well-built inline booster that represents good value for homeowners and small-scale users who need targeted airflow increases and low-noise operation. Its unibody housing and permanently lubricated bearings make it reliable for long runs, while the speed controller gives practical flexibility for everyday ventilation tasks.

Frequently Asked Questions

Will this fan fit a 4 in flexible duct?
Yes, the unit is specified for a 4 in duct and fits typical 4 in flexible and rigid ductwork used in homes.

How loud is the fan in normal use?
The manufacturer rating is about 28 dBA; most users find it quiet at lower speeds though noise perception varies by installation and mounting.

Is the motor maintenance free?
The AC shaded-pole motor uses permanently lubricated dual ball bearings, so routine lubrication is not required for normal operation.