Where should a bathroom extractor fan vent to?

A fan can only be as good as its duct. Most weak, dripping or useless bathroom fans are perfectly serviceable units connected to a duct that undoes everything they do.

Safety

What you'll need

• Rigid duct in the correct diameter, or insulated flexible duct
• Duct tape and clips
• An external grille with a backdraught flap
• A core drill or a long masonry bit for a wall exit

Step by step

1. Decide the shortest sensible route.Straight out through the nearest external wall is the best option available. Every metre of duct and every bend costs extraction, and bends cost far more than straight runs.
2. Match the duct size to the fan.A 100mm fan needs 100mm duct. Reducing it, or fitting flexible duct that has been squashed, throttles the fan and the noise goes up as the airflow goes down.
3. Prefer rigid duct.Rigid plastic duct has far less resistance than corrugated flexible. If flexible is unavoidable, pull it taut so the corrugations are stretched out rather than concertinaed.
4. Set a fall towards the outside.Any warm air in a duct will condense. A slight continuous fall to the terminal means that water runs outside; a sag anywhere means it collects and eventually runs back into the fan.
5. Insulate any part crossing a cold space.Duct in a cold loft is where most condensation forms. Insulated duct, or normal duct wrapped in loft insulation, keeps the air above dew point for the run.
6. Terminate outside, properly.A purpose-made wall or roof terminal with a backdraught flap and a bird guard. Not a hole in a soffit, not a loose end in the loft, not a tile vent with no duct attached to it.
7. Seal the duct joints.Tape every joint. Air escaping into a wall cavity or a ceiling void is the same problem as terminating in the loft, just harder to see.
8. Keep the terminal clear of openings.Away from windows that will be opened, away from an air brick, and not straight into a passage where people walk through the steam.
9. Check the loft afterwards.If the duct previously ended in the loft, expect damp timber and possibly mould. Inspect and let it dry with the new route working.
10. Get the wiring done by an electrician.Any change to the fan's supply, its isolator or its position is notifiable work in a bathroom. The duct is yours; the cable is not.

Tips

• Every 90-degree bend costs roughly the same as several metres of straight duct. Design the route to avoid them.
• A duct that runs uphill from the fan needs a condensate trap at the fan end, or the water comes back through the unit.
• Check the external flap opens when the fan runs — a stuck flap makes a perfectly good fan useless.

Common mistakes

• Terminating the duct in the loft — It moves all the moisture from the bathroom into the roof space, where it condenses on cold timber and the underside of the felt. It causes loft condensation, mould and eventually rot.
• Using long runs of concertinaed flexible duct — The corrugations create enormous resistance. A fan rated at 90 cubic metres an hour can deliver a fraction of that through a sagging flexible duct with three bends.

If it doesn't work

Questions people ask