Why is my extractor fan's outside vent the problem?
A fan can be working perfectly and still extract nothing if the outside end is blocked, painted over, or fitted with a grille that restricts it.
- Difficulty
- beginner
- Time
- 30 min
- Read
- 2 min
- Safety
- caution
Short answer
Go outside and look at the terminal while the fan runs. Nothing moving means the air is not getting out. Common causes are a shutter painted shut, an insect or bird nest, a fine mesh clogged with dust, or a grille with far less free area than the duct.
Diagnosis nearly always starts and ends at the fan, which is why this gets missed. The fan is only one component of a system that runs from the room, through a duct, to a terminal in a wall or roof — and the terminal is outside, out of sight, and exposed to weather, birds and paint.
Safety
Step by step
- Watch the outside terminal while the fan runs.Have someone switch the fan on while you watch from outside. A backdraught shutter should open, and you should feel air. Nothing at all means the restriction is between the fan and there — or right there.
- Check whether the shutter has been painted shut.External vents get painted along with the wall or the render, and the flaps get stuck closed. It is a very common cause and a very easy fix.
- Look for nests and insect blockages.Terminals are attractive nesting sites. A nest in the duct behind the grille blocks it completely, and it is a seasonal recurrence rather than a one-off.
- Check any mesh for clogging.Fine insect mesh clogs with dust and lint until it is effectively solid. Cleaning it restores the airflow immediately, and it is worth adding to a maintenance list.
- Consider whether the grille itself is too restrictive.A decorative or heavily louvred grille can have far less open area than the duct feeding it. That throttles the system regardless of how good the fan is.
- Check the terminal is the right way up and the right type.A shutter fitted upside down will not open properly, and a terminal intended for a different application can restrict flow or let water in.
- Look for water staining below the vent.Staining on the wall under the terminal suggests condensate running out of the duct, which points at an uninsulated duct as well as a terminal problem.
- Check the duct where it meets the terminal.A duct that has come adrift inside the wall, or was never connected to the terminal at all, discharges into the cavity or the loft rather than outside. That is a serious fault and it is common.
If it doesn't work
Fan sounds normal but no air outside.
Cause: Blocked or seized terminal, or a duct disconnected before it. — Fix: Clear and free the terminal, and confirm the duct actually reaches it.
Airflow good in summer, poor after decorating.
Cause: Shutter painted shut. — Fix: Free the flaps and clean off the paint; mask the shutter next time.
Recurs every spring.
Cause: Nesting. — Fix: Clear it, and fit a terminal with a proper cowl or guard designed to exclude birds without restricting flow.
Questions people ask
Can I fit a mesh to keep insects out?
A coarse guard designed for the purpose, yes. Fine mesh clogs quickly and becomes the restriction, so it defeats the fan it was meant to protect.
The duct vents into the loft — is that acceptable?
No. It dumps warm moist air into a cold space and is a common cause of loft condensation. It needs ducting through to an external terminal.