Why isn't my extractor fan clearing the steam?

What this usually means

An extractor fan's job is to remove moist air from the room faster than it is produced, so humidity falls and steam clears. Whether it does that depends not on whether the fan spins but on the actual volume of air it moves — and that is frequently far below the fan's rating. Steam left hanging after a shower, or a kitchen that stays humid after cooking, is the visible sign that the real extract rate is too low for the moisture being generated.

The usual reasons are about installation as much as the fan itself. A fan undersized for the room cannot keep up; long, kinked, crushed or uninsulated ducting adds resistance and condensation that throttle the airflow; a poorly positioned fan does not capture the moist air at source; and a blocked grille or external terminal chokes it further. Even a good fan performs poorly through bad ducting, which is why the installed system, not the fan's badge rating, determines what actually leaves the room.

Behaviour matters too. Fans need to run during the moisture-producing activity and for a period afterwards to clear the pulse of steam; switched on briefly or not at all, even a capable fan leaves moisture behind. Because the consequence of failing extract is humidity that condenses on cold surfaces and grows mould, the right response is to measure what the fan actually moves and find the limiting factor — sizing, ducting, position or use — rather than assume a spinning fan is enough.

Common causes

Signs and symptoms

A fan's effective performance is its operating point on its pressure-flow curve once installed, not its free-air rating. Every metre of duct, every bend, every reduction in diameter and any blockage adds resistance, pushing the fan to a higher pressure and a lower flow; long, kinked or crushed ducting can cut the delivered airflow to a fraction of the rating. Uninsulated ducting through cold spaces also lets the moist extract air condense inside, adding water and resistance. So the installed extract rate — what actually leaves the room — is governed by the whole system.

Clearing moisture is a question of that extract rate against the moisture generation and the room volume. Bathing and cooking release a sharp pulse of vapour; an adequate extract rate, capturing the moist air near its source and running through and beyond the activity, removes it before it can settle. An inadequate rate — from undersizing, ducting losses, poor position or short run time — leaves the humidity high, so the dew point rises to meet cold surfaces and condensation and mould follow, exactly the outcome the fan was meant to prevent.

This is why diagnosis measures airflow rather than trusting the fan to spin. Measuring the actual extract rate at the grille, inspecting the ducting and terminal, checking the fan's position and run-on, and relating it to the room's moisture and the home's airtightness, identifies the limiting factor. The fix follows the finding — resize the fan, improve the ducting, reposition it, clear the terminal, or set continuous or run-on operation — restoring genuine extract so the steam clears and humidity stays healthy.

How to make an extractor fan actually clear the steam

Measure what the fan really moves and find the limiting factor — sizing, ducting, position or use — then correct it so moisture leaves the room.

1. 01

   Measure the actual extract rate

   Check the real airflow at the grille rather than assuming a spinning fan is extracting.

2. 02

   Improve the ducting

   Shorten and straighten runs, replace crushed or kinked duct, and insulate duct through cold spaces to restore airflow.

3. 03

   Right-size and reposition the fan

   Fit a fan rated for the room's moisture and site it to capture steam at source.

4. 04

   Clear blockages

   Unblock the internal grille and the external terminal so air can flow freely.

5. 05

   Run it long enough

   Run the fan during and after bathing or cooking, or use continuous or run-on operation, to clear the moisture pulse.

6. 06

   Verify the steam clears

   Confirm humidity falls quickly and steam clears after use, and that condensation and mould reduce.

How to prevent it coming back

• Specify extract rated for the room and ducted properly.
• Keep ducting short, straight, sound and insulated through cold spaces.
• Run extract during and after moisture-producing activities.
• Keep grilles and external terminals clear.