Axial vs. Centrifugal: Choosing the Right Extract Fans

by | Apr 27, 2024 | Last Articles | 0 comments

The Art of Adequate Ventilation: Extractors

There is such a diverse range of extractors to choose from the help comply with Part F of the Building Regulations. But which ones are any good? Read this blog to find out more.

The Big Fuss over Ventilation

Regulation 42 of the Building Regulations requires builders to provide evidence of air-flow testing of all fixed extractor fans to the Building Control Body (BCB) within 5 days of the test being carried out.

What is an Extract Fan or Extractor?

An extract fan, also known as an extractor, operates by creating a flow of air. Let’s delve into the mechanics of this essential ventilation fixture:

  1. Design and Operation:
    • The extractor fan consists of a rotating arrangement of vanes or blades, collectively known as an impeller.
    • When the fan operates, it forces air to move parallel to the shaft around which the blades rotate.
    • This movement of air serves to remove moisture, odours, and other contaminants from a specific area, such as bathrooms and kitchens.
  2. Importance:
    • Dampness can cause significant problems, including mould growth, unpleasant smells, and damage to windows and fixtures.
    • Properly installed extractor fans help displace moisture and maintain good indoor air quality.
  3. Ventilation:
    • Extractor fans need to vent outside.
    • Venting outside ensures that moisture, odours, and contaminants are effectively expelled from the room or building.
    • For domestic settings, such as bathrooms and kitchens, extractor fans should either be fitted to exterior walls or have ducting leading to an exterior wall vent.
  4. Electricity Consumption:
    • The electricity usage of an extractor fan depends on its size, power rating, and usage duration.
    • Domestic extractor fans typically have a relatively low power rating and do not consume significant electricity during typical usage.
    • Larger industrial fans or those used continuously for extended periods will consume more electricity.

Remember, these unassuming extractor fans play a vital role in maintaining a fresh and healthy environment in our homes!

Axial vs. Centrifugal: Choosing the Right Extract Fans

Axial Fans:

  1. History and Design:
    • Axial fans are the oldest design, dating back to Persian windmills in A.D. 500.
    • Their blades rotate around a fixed axis, pulling air in parallel to the axis and expelling it in the same direction.
    • Think of a ceiling fan: it draws air down from the ceiling and forces it straight down.
  2. Performance:
    • Axial fans move air at a high flow rate but operate at low pressure.
    • They’re ideal for general-purpose applications, such as cooling spaces (homes, offices) and equipment (computers, vending machines).
  3. Energy Efficiency:
    • Axial fans require less power input to run.
  4. Best Uses:
    • Cooling spaces and equipment.
    • Non-ducted applications.
    • Increasingly common due to technology integration.

Centrifugal Fans:

  1. History and Design:
    • Centrifugal fans are a more recent invention.
    • They operate by accelerating air radially outward from the centre.
    • Their design allows for higher pressures.
  2. Performance:
    • Centrifugal fans are powerful and can move air at higher speeds.
    • They’re suitable for creating a cooling effect in rooms or enclosed spaces.
  3. Noise and Weight:
    • Centrifugal fans are quieter than axial fans.
    • They tend to be heavier.
  4. Application Considerations:
    • Choose centrifugal fans for specific needs, such as when noise reduction is crucial or when housing units in weaker infrastructure.
    • They excel in harsh environments and have longer durability.

Regulatory References:

  • NHBC Fact Sheet Regulation 42:
    • Builders must provide evidence of testing all fixed fans to the Building Control Body (BCB) within 5 days of the test.
    • The Domestic Ventilation Compliance Guide (dVCg) outlines approved procedures for measuring airflow rates.
    • Challenges with axial fan testing led to collaborative solutions.
  • BSRIA 46/2022:
    • BSRIA’s guide provides detailed procedures for testing domestic ventilation systems.
    • It accounts for revised guidance in Part F of the Building Regulations (England and Wales).
    • Focuses on accurate airflow rate measurement, especially with the rise of Mechanical Ventilation with Heat Recovery (MVHR) systems


When it comes to ventilation, understanding the differences between axial and centrifugal fans is crucial. Let’s break it down:

  1. Axial Fans:
    • Design: Blades rotate around a fixed axis, moving air parallel to the axis.
    • Performance: High flow rate, low pressure—ideal for general use.
    • Energy Efficiency: Requires less power input.
    • Best Uses: Cooling spaces, non-ducted applications.
  2. Centrifugal Fans:
    • Design: Accelerate air radially outward from the center.
    • Performance: Powerful, higher speeds—suitable for creating cooling effects.
    • Noise and Weight: Quieter but heavier.
    • Application Considerations: Choose for specific needs, harsh environments, and durability.

Remember, proper ventilation ensures fresh air and a healthier environment!