Video – “What Happens When a Centrifugal Fan Runs in Reverse”?

One of the most common faults discovered during LEV commissioning and Thorough Examination and Test (TExT) is a centrifugal fan that has been wired incorrectly and is rotating in the wrong direction. Surprisingly, when a centrifugal fan runs in reverse, the airflow does not reverse direction. Instead, the fan continues to draw air through the system but typically at a dramatically reduced performance level, often achieving only a fraction of its intended airflow.

This OXYL8 training video demonstrates exactly what happens when a centrifugal fan rotates backwards and explains why checking fan rotation should form part of every commissioning exercise and every Thorough Examination and Test. The resource also highlights the value of modern contactless fan rotation testers, which allow engineers to verify motor rotation safely and quickly without making physical contact with the fan, motor shaft or electrical wiring.

Why This Guidance Matters

Many engineers incorrectly assume that if a centrifugal fan rotates backwards the airflow direction will also reverse.

In reality, centrifugal fans continue to draw air into the inlet and discharge it through the outlet even when rotating in reverse. However, the airflow volume and pressure generated by the fan are significantly reduced. In many cases a reversed fan may achieve only around 40–60% of its intended performance, resulting in inadequate contaminant control despite the system apparently operating normally.

This is why fan rotation should never be assumed and should always be verified.

Why Do Fans End Up Running Backwards?

Most LEV systems use three-phase motors.

Following installation, electrical maintenance work, motor replacement or changes to power supplies, the phase sequence can inadvertently be altered. When this occurs, the motor rotates in the opposite direction and the fan impeller also rotates backwards.

The fault is surprisingly common and can remain undetected for years if fan rotation is not physically checked.

The Key Lesson from the Video

The video clearly demonstrates that:

• Airflow does not reverse.
• Air is still drawn into the hood.
• The system may appear to be functioning normally.
• Performance can be substantially reduced.
• Exposure control may no longer be adequate.

This is why airflow measurements alone may not immediately reveal the underlying fault unless the engineer understands the expected system performance.

The Importance of Fan Rotation Checks

A fan rotation check should form part of:

• LEV commissioning
• Fan replacement works
• Motor replacement works
• Electrical maintenance activities
• Thorough Examination and Test (TExT)

Simply observing that air is moving through the system is not sufficient evidence that the fan is operating correctly.

Verification of rotational direction provides confidence that the fan is operating in accordance with its design specification.

Using Contactless Fan Rotation Testers

One of the safest and quickest methods of checking fan rotation is by using a contactless fan rotation indicator.

These instruments:

• Require no contact with live wiring
• Require no access to rotating components
• Can be used with many enclosed fan motors
• Provide immediate indication of rotational direction
• Reduce the need for intrusive testing

The OXYL8 team routinely recommends the use of contactless rotation testers during commissioning and TExT activities because they provide a rapid and reliable method of confirming correct motor rotation.

LEVShop supplies a range of fan rotation indicators and phase rotation testers suitable for LEV testing applications.