An inline fan (also called an inline duct fan or remote-mounted fan) is a ventilation device installed directly within ductwork, typically in an attic, crawlspace, or mechanical room. Unlike traditional exhaust fans that mount flush to a wall or ceiling, inline fans sit “in line” with the air ducts—hidden from view but powerful where it counts.
Inline fans create negative or positive pressure within the duct system to move air efficiently from one space to another. When used for exhaust, they pull stale, humid, or contaminated air from rooms through ducts and expel it outside.
| Feature | Inline Fan | Wall-Mounted Fan |
|---|---|---|
| Installation Location | Inside ductwork (attic, crawlspace) | Directly in wall or ceiling |
| Noise Level | Low (1-2 sone) | Moderate to high (3-5 sone) |
| Airflow Capacity | High (100-1000+ CFM) | Limited (50-150 CFM) |
| Duct Length Capability | Excellent (50+ ft) | Poor (struggles beyond 15 ft) |
| Cost | Higher ($100-$400) | Lower ($30-$150) |
The three main types—axial, centrifugal, and mixed-flow—use different internal designs to move air.
Axial fans use propeller-style blades that push air parallel to the motor shaft. They excel in high-volume, low-pressure scenarios over short ducts.
Centrifugal fans use a squirrel-cage impeller that builds much higher pressure, ideal for long ducts or complex duct layouts.
The key metric is CFM (Cubic Feet per Minute). Use this formula:
CFM = (Room Volume × Air Changes per Hour) / 60
| Room Size | Volume | CFM Required | Recommended Fan |
|---|---|---|---|
| 5′ × 8′ × 8′ | 320 cu ft | 43 CFM | 50-80 CFM |
| 8′ × 10′ × 8′ | 640 cu ft | 85 CFM | 110 CFM |
| 10′ × 12′ × 9′ | 1,080 cu ft | 144 CFM | 150-200 CFM |
Calculate the fan size you need:
Room Volume: 384 cu ft
Required CFM: 51.2 CFM
👉 Recommended: 80 CFM
Step 1: Cut the Duct Section
Measure and cut the duct cleanly where the fan will sit.
Step 2: Mount the Fan
Orient the fan correctly (arrow points toward exhaust). Secure to joist/rafter with mounting bracket.
Step 3: Connect Ductwork
Use duct clamps and aluminum foil tape for airtight seals.
Step 4: Electrical Wiring
Connect black-to-black, white-to-white, ground-to-ground. Use wire nuts.
Step 5: Test
Verify airflow direction with tissue test.
For more HVAC installation guides, see our HVAC Guides category.
| Model | Type | CFM | Noise | Price | Best For |
|---|---|---|---|---|---|
| AC Infinity CLOUDLINE S6 | Mixed-Flow | 402 | 2 sone | $$$ | Master bathroom |
| Fantech FR 140 | Axial | 140 | 3.2 sone | $ | Budget bathroom |
| S&P TD-SILENT 200 | Mixed-Flow | 375 | <1 sone | $$$$ | Ultra-quiet premium |
Browse our inline duct fan products for specifications and pricing.
For industry standards, visit Home Ventilating Institute (HVI) and ASHRAE.
| Problem | Likely Cause | Fix |
|---|---|---|
| No airflow | Fan backwards, duct blockage | Check arrow direction, inspect ducts |
| Loud noise | Loose mount, failing bearings | Tighten screws, lubricate or replace |
| Motor overheating | Blocked airflow, electrical fault | Clear obstruction, check wiring |
Lifespan: 10-15 years with proper maintenance.
