What size extractor fan does a bathroom need?

Fan performance is quoted at free air, meaning with nothing attached. Add a duct, some bends and a grille and the real figure falls sharply. That gap is why so many correctly specified fans fail to clear a mirror.

What you'll need

• Room dimensions
• The proposed duct length and number of bends

Step by step

1. Start with the regulatory minimum.15 litres per second for a bathroom or shower room, 6 litres per second for a separate WC. Those are minimums, not targets.
2. Convert the units so you can compare.Fans are sold in both litres per second and cubic metres per hour. Multiply litres per second by 3.6 to get cubic metres per hour. 15 l/s is 54 m³/h.
3. Do the air-change check as a sanity test.Multiply the room's length, width and height for its volume in cubic metres, then aim for six to eight complete air changes an hour in a bathroom. Compare that with the regulatory figure and take the larger.
4. Discount heavily for the ducting.A fan quoted at 90 m³/h free air can deliver half that through three metres of flexible duct with two bends. Buy generously rather than exactly.
5. Check the fan's own duct guidance.Good manufacturers publish a performance curve or a maximum duct length. Beyond that length the fan is simply not moving the air it claims.
6. Consider an in-line fan for long runs.Where the duct has to be long or the bathroom is large, an in-line fan mounted in the loft with a plain grille in the ceiling moves far more air and is much quieter in the room.
7. Match it to the room's use, not just its size.A family shower room used four times each morning needs more capacity than a rarely used guest bathroom of the same dimensions.
8. Look at noise as well as flow.Fans are rated in decibels. A noisy fan gets switched off or its run-on gets disabled, which makes its rated flow irrelevant.
9. Choose the control to suit the room.A humidity-sensing fan runs when it is needed rather than when the light is on, which suits bathrooms used with the light off in daylight.
10. Have it wired by an electrician.Selecting and buying the fan is yours. Installing it in a bathroom is notifiable electrical work.

Tips

• Multiply litres per second by 3.6 to get cubic metres per hour — nearly all confusion in fan specs comes from mixed units.
• For anything but the shortest duct run, buy a fan rated at least half as much again as the minimum.
• An in-line fan in the loft is dramatically quieter in the bathroom, which means it actually gets used.

Common mistakes

• Buying to the minimum rating on the box — That figure is free air with no ducting. In a real installation the delivered flow can be half of it, which puts the room below the regulatory minimum.
• Fitting a bigger fan on the same poor duct — The duct is the restriction. A more powerful fan on a squashed flexible duct is noisier and only marginally more effective.

If it doesn't work

Questions people ask