How to Choose an Extractor Fan: Extract Rate, Noise and Control Types Explained

Get the extractor fan wrong and you'll either have a damp bathroom or a fitting that's too loud for the space. There are three variables that matter: extract rate, noise level, and trigger method. Here's how to spec each one correctly.

Extract Rate: How Much Air Do You Actually Need?

Extract rate is measured in litres per second (l/s) or cubic metres per hour (m³/h). Building Regulations Approved Document F sets minimum ventilation rates for domestic bathrooms.

Minimum extract rates (Approved Document F):

• Bathroom (with bath or shower): 15 l/s (54 m³/h)
• Shower room only: 15 l/s minimum — go higher if the shower is a power shower or steam unit
• Toilet (WC only, no bath/shower): 6 l/s (22 m³/h)
• Utility room: 30 l/s (108 m³/h) if there's a washer/dryer venting into the room
• Kitchen: 30 l/s (108 m³/h) continuous, or 60 l/s (216 m³/h) intermittent extract

For most domestic bathrooms a 100mm (4") fan running at 15–20 l/s is adequate. For large bathrooms, wet rooms or en-suites with steam showers, go to 150mm (6") or specify a fan at the higher end of the 100mm range.

Oversizing slightly doesn't hurt. Undersizing leaves moisture in the room.

Fan Size: 100mm vs 150mm

100mm (4") fans are the standard for domestic bathrooms. They're compact, relatively quiet, and the duct work is easier to run through joists and walls. Most standard GU10 downlight-sized ceiling fans are 100mm.

150mm (6") fans are used where you need higher extract rates — large bathrooms, wet rooms, utility rooms and commercial toilet areas. They're louder and require a larger duct run, but move significantly more air. If you're fitting into a space that previously had a 100mm fan and condensation was always a problem, a 150mm is often the right upgrade.

Trigger Method: Which Control Type to Specify

How the fan is triggered affects both the wiring and the user experience.

Pull cord / manual switch: Simplest installation. Fan runs only when the occupant switches it on. Fine for second bathrooms or cloakrooms where it'll be used correctly. Problem: people forget to leave it running long enough after a shower.

Timer: Fan runs for a set period (typically 5–20 minutes) after the light switch is turned off. The correct specification for most domestic showers — it clears moisture after the occupant leaves. Standard wiring: common, neutral, and a switched live from the light circuit.

Humidity sensor: The fan monitors relative humidity and activates when moisture rises above the threshold, then runs until the air is clear. The professional choice for busy bathrooms and en-suites — no user error, no forgetting to turn the light off. Slightly more expensive but eliminates callbacks.

PIR (motion sensor): Activates on occupancy. Less common for bathrooms but used in commercial WCs and accessible bathrooms where a timer or pull cord is less appropriate.

Noise: What the dB Rating Means

Fan noise is rated in decibels (dB(A)). For context:

• <25 dB(A): Very quiet — suitable for en-suites and master bathrooms
• 25–35 dB(A): Quiet — acceptable for most domestic bathrooms
• 35–45 dB(A): Audible — fine for utility rooms, commercial WCs, external locations
• >45 dB(A): Noisy — specify only where extract performance is the priority

Cheap fans are often quoted with a noise rating measured under ideal conditions — in a real installation with duct bends and grilles, the actual noise will be higher. For bedrooms and en-suites, specify a fan with an independently tested quiet rating rather than just the manufacturer's figure.

Wiring Zones: Don't Get Caught Out

Under BS 7671 and Part P, bathroom fans must be installed in accordance with the zone requirements. In summary:

• Zone 1 (above the bath/shower): only SELV (12V) equipment — mains fans not permitted
• Zone 2 (1.2m outward from the bath/shower): IPX4 minimum rating required for mains equipment
• Outside zones: standard IP20 equipment acceptable, but IPX4 is good practice for any bathroom fan

In practice: always fit an IP44 or IPX4-rated fan in a bathroom or shower room. It's not worth the call-back or the EICR observation to fit an IP20 unit.

In Stock at APM — Acton Trade Counter

We stock extractor fans across the common domestic sizes — 100mm and 150mm, timer and humidity sensor variants. Walk in and we can help you spec the right unit for the job.

APM Plumbing & Electrical Supplies
24 Western Avenue, Acton, London W3 7TZ
📞 020 8702 8080 | 🌐 apmi.uk
Mon–Sat, trade counter open