Axial, Centrifugal or Inline Extractor Fan: Which Type Do You Need?

Axial, Centrifugal or Inline Extractor Fan: Which Type Do You Need?

Choosing the right extractor fan is not simply a case of finding one that fits the duct size. Axial, centrifugal and inline fans are suited to different installation conditions, particularly when it comes to duct length, resistance, airflow and where the fan can be installed.

For a simple bathroom extraction point through an external wall, an axial fan may be suitable. If the duct run is longer or more restrictive, a centrifugal or inline fan may be a better choice.

Quiet-Vent offers a growing range of extractor fans, including S&P axial bathroom fans, centrifugal extract fans and TD inline fans.

What Is an Axial Extractor Fan?

An axial fan moves air in a straight line through the fan, parallel to the motor shaft.

These are the familiar wall or ceiling-mounted extractor fans commonly found in bathrooms, toilets and en-suites.

Axial fans are generally best suited to installations with relatively low resistance, such as a short duct run directly through an external wall.

This makes them a practical option when there is a straightforward route from the room to outside.

Axial Fans Are Commonly Used For:

• Bathrooms
• Toilets
• En-suites
• Small domestic rooms
• Short, simple duct runs
• Direct wall extraction

The S&P SILENT-100 CDZ, for example, is a 100mm axial extract fan suitable for wall or ceiling mounting and connects to standard 100mm circular ducting.

What Is a Centrifugal Extractor Fan?

A centrifugal fan moves air differently. Air enters the fan before being redirected outwards by the impeller.

One of the main advantages is the fan's ability to operate against greater system resistance than a typical domestic axial fan.

Resistance increases when air has to travel through features such as:

• Longer duct runs
• Multiple bends
• Smaller ductwork
• Grilles and terminals
• Other restrictions within the ventilation route

This makes centrifugal fans useful where a standard axial fan may struggle to provide the required airflow.

Centrifugal Fans Are Commonly Used For:

• Bathrooms with longer duct runs
• Toilets
• Utility rooms
• Apartments or internal rooms further from an external wall
• More restrictive duct layouts
• Domestic and light-commercial applications

The S&P EBB-100NS centrifugal fan is specifically designed for installations where the ventilation system has greater resistance than a standard axial fan can comfortably overcome.

What Is an Inline Extractor Fan?

An inline fan is installed within the ductwork itself rather than having the motor directly behind the visible room grille.

This means the fan can often be positioned within a ceiling void, loft or another suitable accessible area along the duct run.

It is worth noting that "inline" describes the way the fan is installed rather than one specific impeller design. Many modern inline fans, including S&P's TD range, use a mixed-flow design that combines characteristics associated with axial and centrifugal airflow.

S&P describes its TD-SILENT range as low-profile mixed-flow fans designed for ducted installations.

Inline Fans Are Commonly Used For:

• Bathrooms and shower rooms
• Longer duct runs
• Ceiling-void installations
• Residential ducted ventilation
• Commercial and light-commercial applications
• Projects where the fan needs to be positioned away from the room

Quiet-Vent stocks a range of S&P TD inline extractor fans, including standard and SILENT versions across several duct sizes and airflow duties.

Axial vs Centrifugal vs Inline Fans: Quick Comparison

Fan Type	Typical Installation	Best Suited To
Axial	Wall or ceiling	Short, low-resistance duct runs
Centrifugal	Wall, ceiling or recessed depending on model	Longer or more restrictive duct runs
Inline / Mixed-Flow	Installed within ductwork	Ducted systems, ceiling voids and applications requiring greater airflow capability

The correct choice still depends on the individual fan's performance curve and the resistance of the complete ventilation system.

Does Duct Length Affect Which Extractor Fan You Need?

Yes. Duct resistance is one of the most important factors when choosing an extractor fan.

Every metre of ductwork creates resistance to airflow, and fittings such as bends, grilles and reducers can add further pressure losses.

A fan may provide a high airflow figure under unrestricted conditions but deliver less air once connected to a real duct system.

For a short route directly through an external wall, an axial fan may provide an effective solution.

For a longer route containing several bends, a fan capable of overcoming greater static pressure may be required. This is where centrifugal and mixed-flow inline fans can become particularly useful.

The fan should therefore be selected based on the complete installation, not simply the diameter of the duct connection.

Are Inline Fans Better for Long Duct Runs?

Inline fans can be particularly well suited to ducted installations because many models are designed to provide strong airflow while operating against system resistance.

For example, the S&P TD-350/125 SILENT Inline Fan is a low-profile mixed-flow fan designed for installation within circular ductwork.

Quiet-Vent also stocks TD inline fans in several sizes and performance levels, making it possible to match the fan more closely to the requirements of the duct system.

However, being an inline fan does not automatically make a product suitable for every long duct run. Airflow requirements, pressure losses and the manufacturer's performance data should still be checked.

Which Type of Extractor Fan Is Quietest?

Fan type alone does not determine how quiet an installation will be.

Noise can also be affected by:

• Fan speed
• Airflow rate
• Duct diameter
• Number of bends
• Grilles and terminals
• Fan mounting
• Vibration
• Distance between the fan and occupied space

One advantage of some inline installations is that the fan motor can be positioned away from the room itself, although airborne noise can still travel through the ductwork.

S&P's TD-SILENT range uses sound-absorbing insulation and is specifically designed for applications where lower operating noise is important.

For more information on reducing ventilation noise throughout the whole system, read our guide to Quieter Ventilation: Units, Ducting and Air Valves Explained.

What About Timer, Humidity and PIR Controls?

After choosing the appropriate fan type, you should also consider how the extractor fan needs to operate.

Depending on the model, extractor fans may include:

Run-On Timer

Allows the fan to continue extracting for a set period after the trigger has been switched off.

Humidity Sensor

Responds to an increase in room humidity, helping provide additional extraction when moisture levels rise.

PIR Movement Sensor

Activates the fan when movement is detected within the room.

Continuous Running

Some extract fans are designed to operate continuously at a lower background rate before boosting when additional ventilation is required.

Quiet-Vent's S&P range includes several control options, so it is important to check the full product specification rather than choosing based on the fan type alone.

Which Extractor Fan Is Best for a Bathroom?

There is no single fan that is best for every bathroom.

For a bathroom with a short duct directly through an outside wall, a 100mm axial fan may be suitable.

If the bathroom is located further from the external wall and requires a longer or more restrictive duct run, a centrifugal fan may provide better pressure capability.

Where the ductwork runs through a ceiling void or loft, an inline fan can provide another option, particularly where the fan needs to be positioned away from the room.

The required airflow, duct route and installation conditions should all be considered before selecting a product.

Do Extractor Fans Have to Meet Building Regulations?

Ventilation requirements depend on the location and type of project.

For projects in England, the government's Approved Document F – Ventilation provides guidance on meeting the ventilation requirements of the Building Regulations, including extract ventilation from rooms where moisture and pollutants are produced.

Requirements differ across Northern Ireland, Scotland, Wales and the Republic of Ireland, so the regulations applicable to the specific project should always be checked.

The fan also needs to be suitable for its installation location, including any applicable electrical and IP-rating requirements.

Don't Forget the Ductwork

Even a well-selected extractor fan can perform poorly if the duct system creates unnecessary resistance.

Good practice includes:

• Keeping duct routes as direct as practical
• Avoiding unnecessary bends
• Using the correct duct diameter
• Preventing flexible duct from becoming crushed or excessively sagged
• Using suitable external terminals
• Making sure connections are properly installed
• Providing access where equipment requires future maintenance

The complete system needs to allow the fan to deliver the required airflow rather than unnecessarily restricting it.

Explore the S&P Extractor Fan Range at Quiet-Vent

Quiet-Vent now offers an expanded selection of S&P extractor fans, providing options for a wide range of domestic, trade and light-commercial applications.

The range includes:

• Axial bathroom fans
• Centrifugal extract fans
• dMEV fans
• Standard inline fans
• TD-SILENT inline fans
• Timer-controlled models
• Humidity-controlled models
• PIR-controlled models

Browse the full Quiet-Vent Extractor Fan range to compare available fan types, controls, duct sizes and airflow performance.

Need Help with a Ventilation Project?

Fan selection is only one part of creating an effective ventilation system.

The length and size of the ductwork, number of bends, required airflow and position of the extract point can all affect performance.

For larger ventilation projects, Quiet-Vent can also provide technical advice on system layout and compatibility through our Design Request service.

So, Which Type of Extractor Fan Do You Need?

As a general guide:

Choose an axial fan for straightforward, low-resistance applications such as short bathroom duct runs.

Consider a centrifugal fan when greater pressure capability is required because of longer or more restrictive ductwork.

Consider an inline fan when the fan needs to sit within the duct system, particularly for ceiling-void installations or more substantial ducted applications.

The best fan is ultimately the one that can deliver the required airflow against the resistance created by the actual installation.

Instead of choosing on duct diameter alone, consider the fan performance, duct route, controls, noise and application together before making your selection.