Why does my extractor fan drip water?

A duct in a cold loft is effectively a long cold surface with humid air passing through it. It is exactly the situation condensation needs, which is why this problem is common and why it is almost never the fan itself.

Safety

What you'll need

• Insulated flexible duct or loft insulation and tape
• A condensate trap if the duct rises
• Duct tape and clips
• A torch and boards for the loft

Step by step

1. Switch the fan off at its isolator.Usually a pull cord or a fused switch outside the room. Water and electricity in the same fitting is not something to work around.
2. Rule out an actual leak from above.Check the loft over the fan for a roof leak, a wet tank or a leaking pipe. A steady drip that continues when the bathroom has been unused for days is not condensation.
3. Confirm the timing.Condensate appears during and shortly after showers, worse in cold weather, and stops in summer. That pattern is diagnostic.
4. Inspect the whole duct run.Look for sags, uninsulated sections crossing the loft, and any part where the duct rises from the fan before falling to the terminal.
5. Insulate every metre in cold space.Either replace with insulated flexible duct or wrap the existing duct in loft insulation and tape it. This is the single most effective change.
6. Remove the sags.Support the duct on the joists or with clips so it falls continuously to the outside. Every low point collects water and eventually spills it back.
7. Fit a condensate trap if the duct rises.Where a ceiling fan ducts upwards, a purpose-made condensate trap sits below the fan and holds the water so it evaporates rather than dripping through the unit.
8. Check the external terminal.A blocked grille or a stuck flap means the moist air lingers in the duct rather than leaving, which increases condensation dramatically.
9. Consider the run-on timer.A fan that stops the moment the light goes off leaves a duct full of warm wet air to condense. A run-on of 15 to 20 minutes clears it.
10. Check the loft for damage.If it has been dripping for a while, look at the timbers and insulation above the bathroom for damp and mould.

Tips

• Insulating the duct fixes most cases on its own. It is a cheap afternoon in the loft.
• A longer run-on time reduces condensation as well as clearing the room — the duct gets purged with drier air.
• If the drip continues in warm dry weather, it is not condensation. Look for a genuine leak above.

Common mistakes

• Replacing the fan — The fan is rarely at fault. The water is forming in the duct, and a new fan in the same duct drips exactly the same way.
• Sealing the fan grille to stop the drip — It forces the water to find another way out, usually into the ceiling void where it does far more damage before anyone notices.

If it doesn't work

Questions people ask