Why does cold air blow back through my extractor fan?

An axial fan has almost no resistance when it is off. Whatever the wind does outside is transmitted straight through the duct into the room, which is why an exposed elevation on a windy day can feel like an open window.

Safety

What you'll need

• A replacement external grille with a flap, or a wind cowl
• A screwdriver and a brush
• A ladder if the grille is high

Step by step

1. Establish where the air is coming in.Hold a tissue near the fan grille on a windy day. Movement confirms the fan; still tissue means the draught is coming from somewhere else in the room.
2. Check the shutter at the fan.Most fans have small plastic flaps behind the grille that swing open when it runs. They stick with dust and hair, or get painted shut during decorating.
3. Clean the fan grille and shutter.Take the cover off, wash it, and clean the shutter so it swings freely under its own weight. Do this with the fan isolated.
4. Check the external grille.Outside, the terminal should have a flap or a set of louvres that hang closed. Many cheap grilles have none, or the flap has broken off.
5. Replace the external grille with a flapped type.A grille with a properly weighted or hinged flap costs very little and is the fix in most cases. Fit one with a bird guard as well.
6. Consider a wind cowl on exposed walls.On a high or wind-facing elevation, a cowl or a wind-resistant terminal deflects the pressure so a simple flap can stay shut.
7. Check the duct is not acting as a chimney.A vertical duct to a roof terminal can pull air continuously by stack effect. A shutter at the fan end is essential in that arrangement.
8. Fit a shutter kit if the fan has none.Separate backdraught shutter units fit between the fan and the duct. They are inexpensive and are the tidiest answer for a fan without an integral flap.
9. Do not just block the fan.Taping over a fan solves the draught and creates a condensation problem. If the fan is genuinely unnecessary, remove it and make good the wall properly.

Tips

• Clean the fan shutter every time you clean the fan. Dust is what stops the flap closing.
• A flapped external grille with a bird guard is a few pounds and solves the majority of backdraught complaints.
• Never block a bathroom fan permanently — the moisture has to go somewhere and it will find the coldest surface instead.

Common mistakes

• Taping over the fan in winter — It stops the draught and stops the ventilation. The moisture that the fan was removing then condenses on windows and walls, which is a bigger problem than the draught.
• Fitting a fixed louvre grille outside — Fixed louvres keep rain out but do nothing about wind pressure. The terminal needs a moving flap or a cowl to stop backdraught.

If it doesn't work

Questions people ask