Bathroom Exhaust Fans: How to Choose One That Works

Why the fan matters more than people think

A shower puts a surprising amount of water into the air. If that moisture is not taken out of the room, it settles on the coolest surfaces — the ceiling, the top of the walls, the window frame and the silicone joints — and stays there. Do that every day and you get peeling paint, swollen doors and cabinetry, and mould in the shower and on the ceiling.

Opening a window helps, but only when it is open and the weather allows. A correctly sized exhaust fan, ducted to the outside and left running long enough, is what keeps a bathroom dry all year. In a room with no window at all, it is the only thing doing the job.

The main types of bathroom exhaust fan

Ceiling-mounted fans. The most common type. The fan and motor sit in the ceiling with a grille below, and a duct runs through the roof space or ceiling cavity to the outside. They suit most houses and many apartments.

Wall-mounted fans. These go through an external wall and discharge straight outside with little or no duct. They are simple and effective where the bathroom has an outside wall, and there is no long duct to lose airflow. In an apartment, a new hole in the facade usually needs owners corporation approval.

Inline (remote) fans. The motor sits in the ceiling space or a cupboard, part-way along the duct, with only a grille in the bathroom. Because the motor is away from the room, they are usually much quieter, and one unit can sometimes serve two grilles, such as a bathroom and a separate toilet. They cost more and need access for servicing.

3-in-1 combination units. A fan, light and heat lamp or heater in one ceiling unit. They save ceiling space and suit smaller rooms, but the fan component is often modest. Check its airflow rating on its own, not just the heating output.

Sizing: airflow versus room volume

Fans are rated by how much air they move, in litres per second (L/s) or cubic metres per hour (m³/h). One litre per second equals 3.6 cubic metres per hour, so a 25 L/s fan moves 90 m³/h.

To size a fan, start with the room:

1. Measure the room volume. Length × width × ceiling height, in metres. A bathroom 2.4 m by 2.0 m with a 2.4 m ceiling is about 11.5 m³.
2. Choose an air change rate. For a bathroom, somewhere between eight and fifteen air changes an hour is a sensible target — at the higher end for a busy family bathroom or a windowless ensuite.
3. Multiply. 11.5 m³ × 10 air changes = 115 m³/h, or about 32 L/s.

The figure on the box is measured in ideal conditions with no duct attached. Once you add a duct, bends, a grille and an outlet, real-world airflow drops, sometimes by a lot. So pick a fan with some margin above your calculated figure rather than one that just scrapes past it.

There are also minimum exhaust rates for bathrooms set out in the building rules. Your builder, electrician or certifier can confirm the current figure for your job — treat it as the floor, not the goal.

Ducting: where the air goes

This is where most installations fall down.

Discharge to the outside. Older bathrooms often have a fan that simply blows into the roof space. Warm, wet air released there condenses on the underside of the roof, the framing and the insulation. For new work, the National Construction Code's condensation-management provisions generally require exhaust from bathrooms to discharge to the outdoors, directly or via a duct. The detail varies with the building and the edition that applies, so confirm the current requirements with your builder, electrician or certifier before you buy.

Keep the duct short and straight. Every metre of duct and every bend adds resistance. Rigid, smooth-walled duct flows better than flexible duct, and flexible duct should be pulled taut rather than left sagging or crushed. Use the diameter the fan is designed for; reducing it strangles the airflow.

Fit a backdraft damper. A simple flap in the fan or at the outlet stops cold air, dust and insects coming back down the duct when the fan is off.

Apartments are different. Many apartment bathrooms exhaust into a shared vertical riser, and the building's rules may limit what can connect to it. We cover that in more detail on our ensuite and powder room page.

Let air in. A fan cannot pull out more air than can get into the room. A gap under the door of around 10–20 mm, or a door grille, gives it replacement air.

Noise: sones and decibels

A fan that is too loud gets switched off, which defeats the purpose. Manufacturers quote noise in sones or in decibels (dB or dB(A)). Lower is quieter in both. Roughly, a fan around one sone is a quiet hum; three or four sones is clearly noticeable.

The trade-off is that more airflow usually means more noise. Ways around it:

• An inline fan, with the motor away from the room.
• A slightly larger fan running on a lower speed setting.
• A well-supported, properly fixed duct, so it does not vibrate against the framing.

Controls: run-on timers and humidity sensors

The steam does not leave when you do. A fan wired only to the light switch stops just as the room most needs it.

• Run-on timer. The fan keeps going for a set time after the light is switched off — often ten to twenty minutes.
• Humidity sensor. The fan starts or keeps running while the humidity is high, and stops when the room has dried out.
• Separate switch. Fine as a backup, but relies on people remembering.

For rentals and family bathrooms, a timer or humidity sensor is the most reliable choice.

Positioning the fan

• Opposite the door, so replacement air sweeps across the room on its way to the fan.
• Over or near the shower, where the steam is made. Your electrician will confirm the fan is rated for its position relative to the shower.
• Away from the supply of fresh air, so it does not just pull air from the doorway straight back out.
• Clear of joists and obstructions, so the duct can run as short and straight as possible.

Installation and maintenance

Fixed wiring must be done by a licensed electrician. They will check the fan's rating for its location, connect it, and fit any timer or sensor. The ducting and outlet should be installed so they are sealed and weatherproof.

Once it is in, maintenance is simple:

• Clean the grille every few months — lint and dust build up quickly.
• Test it occasionally with a sheet of toilet paper held near the grille. If it is not pulled firmly towards the fan, something is wrong.
• Listen for rattling or grinding, which usually means a worn motor.
• Check the outlet outside is not blocked by leaves, nests or paint.

Getting it right in a renovation

A renovation is the best time to fix a fan that has never worked, because the ceiling is open and the duct route can be planned properly. The fan and its duct are sized and specified as part of our bathroom renovations, alongside the waterproofing and tiling. For what a renovation typically involves financially, see our bathroom renovation cost guide. If you are in the Parramatta area, we can look at your existing extraction when we measure the room.