How to fit a bathroom extractor fan

Inspired by a helpful YouTube guide. We drew a lot from the popular UK video “Bathroom Extractor Fans – a DIY Guide” by Georgina Bisby DIY, which does a clear job of walking through fan types, bathroom zones, extraction rates, and what to look for in a decent UK model. Worth watching before you head to the trade counter.

1. Understand bathroom zones before anything else

UK bathrooms are divided into zones that dictate where electrical fittings can go. Zone 0 is inside the bath or shower basin itself. Zone 1 is above the bath or shower to a height of 2.25 m. Zone 2 extends 600 mm outward from the edge of zone 1.

Most ceiling-mounted extractor fans sit outside all zones, which makes life simpler. But if your ceiling is low or the bath is in an unusual position, check before you choose a fan location. The fan needs an IP rating appropriate for its zone.

2. Choose the right fan

Axial fans mount directly on the wall or ceiling and push air short distances. They suit most bathrooms where you can duct straight outside. Centrifugal fans handle longer, more tortuous duct runs without losing too much performance. Inline fans sit in the roof space and are quieter because the motor is further away.

Match the extraction rate to the room. A bathroom needs a minimum of 15 litres per second (54 m³/hr). A larger bathroom or one with a separate WC should go higher. That said, a very powerful fan in a small room is just noisy and pointless.

3. Plan the duct route before you start drilling

The fan has to vent outside — not into the roof space, not into a wall cavity. Work out the shortest, straightest route. Every bend reduces efficiency, so keep it as direct as you can.

You will need a core drill through the external wall or a ceiling penetration into the loft and out through the soffit. Mark the route out in full before you commit to drilling anything.

4. Isolate the circuit properly

Turn off the relevant circuit at the consumer unit. Do not rely on switching off a light — the wiring in the ceiling may still be live. Test the circuit with a voltage tester before touching any cables.

In a bathroom the fan is typically wired to the lighting circuit via a fused spur, or it runs independently from the consumer unit. Some fans wire directly to the light switch so they come on when the light does. Others have their own timer or humidity sensor and need a permanent live supply. Decide which suits your setup before you start.

5. Cut the aperture and prepare the duct

Mark the fan location carefully — a ceiling rose usually covers a 100 mm or 150 mm hole. Core-drill or use a jigsaw to cut the opening cleanly. Push the duct through the ceiling void and core-drill through the external wall if going directly outside.

Fit an exterior grille with a backdraught shutter to stop cold air coming back in when the fan is off. Gravity-close shutters are better than spring ones, which can seize up with paint or dirt over time.

6. Wire the fan

Following the fan manufacturer’s wiring diagram, connect live, neutral, and earth. In the UK, current cable colours are brown (live), blue (neutral), and green-yellow (earth). Older grey or red-and-black cables follow a different convention — if in doubt, check before connecting anything.

If the fan runs from the light switch, you typically connect the live feed from the switch line so the fan activates when the light goes on. Timer fans keep running for a set period after the light goes off. Humidity-sensing fans require a permanent live and a switched live.

7. Fix the fan body and test

Clip or screw the fan housing into the aperture following the manufacturer’s instructions. Most fans clip into place from below. Restore power at the consumer unit and test. Check airflow at the exterior grille and confirm the fan goes off (or times out) as expected.

When to call a handyman

If the duct route is complicated, the ceiling void is awkward, or you are unsure about the wiring arrangement, it is worth getting someone in. Wiring errors in bathrooms are not just inconvenient — they are a safety issue. Richard can help with straightforward bathroom fan installs and will be honest if the job needs a Part P registered electrician for the wiring sign-off.