Why Extractor Fans Fail: Four Real Faults Found on Site

A fan can sound fine, look fine and still be moving almost no air. The only way to know what the complete installation is actually delivering is to measure it.

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A running fan is not necessarily a working fan

Airflow is affected by the complete installation: the fan and its setting, every bend and joint, the duct material, the external terminal and the route available for replacement air.

A motor running proves only that the motor has power. The figure printed on the box is not necessarily the airflow achieved after installation. A calibrated test confirms what the system actually delivers.

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Fault 1: The wrong fan

A low-cost fan with almost no spare capacity once installation resistance was added.

The bathroom target was 15 L/s, but the installed fan was only rated at around 14–15 L/s before ductwork, bends, shutters and the outside terminal were taken into account.

With virtually no spare capacity, even a modest amount of resistance could pull the installed airflow below the required rate. Paying a little more for the right fan can cost far less than dealing with future mould, damp and damaged finishes.

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Fault 2: Crushed ductwork

Flexible ducting folded in on itself, leaving a much smaller opening than the fan connection.

Flexible ducting can appear connected while folds, kinks and compression dramatically reduce the clear area available for air to move.

The motor may still run normally, but the restriction increases system resistance and the airflow reaching the room can fall sharply. Keep duct routes short, smooth and properly supported.

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Fault 3: Slightly unattached ductwork

This one looked minor, but the airflow figures showed otherwise.

Before: the slightly unattached duct delivered 14 L/s.

After: the correctly attached duct delivered 18 L/s.

14 L/s to 18 L/s

Correctly attaching the duct increased the measured extract rate by 4 L/s — approximately 29%. Every connection matters: attach, support, seal, then measure.

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Fault 4: Backdraught damper fitted backwards

The fan motor could be heard running, but a backdraught damper supplied with its flaps assembled backwards was blocking the airflow.

The closed face of the backwards damper.

The direction in which the damper flaps should open.

0 L/s to 29 L/s

Before correction, the measured result was literally 0 L/s. Once the damper was corrected, the same system delivered 29 L/s on boost mode.

Without a measured airflow test, this could easily have been mistaken for a working fan because the motor could still be heard.

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Maintenance matters too

Dust and debris around the impeller and casing can reduce airflow and increase noise.

Dust, debris, painted external terminals, clogged insect screens and stiff shutters can all add resistance over time. A visual switch-on check will not quantify the loss; a measured airflow test will.

What a proper airflow test can reveal

• Wrong fan or incorrect setting

• Crushed, restricted or disconnected ductwork

• Blocked external terminal

• Backwards or jammed backdraught shutter

• Poor replacement air

• Heavy contamination

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A failed reading is the start, not the end

At Airtightness UK, we do more than record pass or fail. We use calibrated measurements and practical investigation to identify the likely cause, then retest after correction to show the measured improvement.

Real results from site

• Backdraught damper corrected: 0 L/s to 29 L/s on boost

• Duct correctly attached: 14 L/s to 18 L/s

For Part F airflow testing, commissioning or diagnostic support, call 0330 043 8801 or email enquiries@airtightness.uk.

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