For a bathroom you want a 100mm fan moving at least 15 litres per second; for a kitchen it's 30 l/s if it sits over the hob and 60 l/s if it doesn't. Get those two numbers right and almost everything else is preference.
The trouble is that nobody sells fans in litres per second on the box. They sell them on "quiet", on backlights, on humidistats — and on a photograph of a bathroom that looks nothing like yours.
We've been supplying extractor fans over the counter at Villa Road since 1975, and the mistake we see every week isn't buying the wrong fan. It's buying the right fan and then hanging it on the wrong end of a three-metre concertina of flexible duct.
Here's what actually matters.
1. Size it in litres per second, not in "powerful"
Approved Document F sets minimum intermittent extract rates for UK homes, and they're the only spec that isn't marketing:
- Bathroom or shower room — 15 l/s
- Kitchen — 30 l/s if the fan is above the hob, 60 l/s if it's anywhere else in the room
- Utility room — 30 l/s
- Toilet / WC — 6 l/s
A standard 100mm domestic fan lands somewhere between 20 and 25 l/s. That comfortably clears a bathroom and comfortably fails a kitchen — which is why kitchen extraction is normally a cooker hood or a 150mm fan, not a 4-inch one.
Watch the units. Some fans are rated in cubic metres per hour instead. Divide by 3.6 to get l/s: a fan quoted at 74 m³/h is doing roughly 20.6 l/s, which is a bathroom fan, not a kitchen one.
The Knightsbridge EX1TWH is the one we hand over most often: 100mm, IP45, 24 l/s, 230V and 9.7W, with an overrun timer built in. It's the default bathroom fan — enough extraction for the regs with headroom, and low enough power draw that nobody notices it on the bill.
2. IP rating decides where it can physically go
Bathrooms are split into zones. Zone 0 is inside the bath or shower itself, Zone 1 is directly above it up to 2.25m, Zone 2 extends 0.6m beyond that.
Nothing mains-voltage goes in Zone 0 — ever. For Zones 1 and 2 you need a minimum of IPX4, and the circuit must be RCD protected.
Every fan in this guide is IP45, which clears that bar with room to spare. That's deliberate: buy IP45 and you stop having to measure whether the fan is 0.55m or 0.65m from the bath.
Trade Tip: IP45 is about splashing, not steam. Every domestic fan copes with humid air — that's the job. The rating is there for the day the shower head gets pointed the wrong way, or somebody rinses the tiles down with a hose in a wet room. It is cheap insurance against a fault you will never see coming.
3. Timer, humidistat, or neither
Three options, and they suit genuinely different situations.
Plain fan. On with the light, off with the light. Fine for a downstairs WC. Useless for a shower room, because the steam is still in there when you flick the switch off.
Overrun timer. Runs on for a set period after the light goes off. This is the right answer for the overwhelming majority of bathrooms, and it's why almost everything we stock has one.
Humidistat. Senses humidity and runs until the room is actually dry, regardless of the light. This is the one to fit where the fan has a real problem to solve — a bathroom with no window, a family of five, a wall that's already showing black spotting in the corners.
The Knightsbridge EX1THWH carries both — overrun timer and humidistat, 100mm, IP45. If you're fitting a fan because of a damp problem rather than because the regs say so, fit this one and stop thinking about it.
4. The bit everyone gets wrong: the ducting
You can fit a perfect fan and get terrible extraction. It's almost always the duct.
Four rules, in order of how often we see them broken:
- Shortest run you can manage. Every metre and every bend costs you airflow. A fan rated 24 l/s on the bench is not doing 24 l/s through four metres of squashed flexible.
- Fall away from the fan. Condensate forms inside the duct. It needs to run outwards to the grille, not back into the motor.
- Never terminate in a loft. Venting warm wet air into a cold roof space is how you get a rotten roof. It must go outside, through a wall or a tile vent.
- Pull the flexible taut. Concertina duct left compressed is a series of baffles. Stretch it out before you clip it.
The Knightsbridge EX4DUCT kit is 100mm aluminium ducting expandable to approximately 3m with two jubilee clips included. Pair it with an EX009W or EX009B 100mm grille — both have a fly screen, which matters more than people expect in a Birmingham summer.
5. The isolator nobody budgets for
Here's the one that sends people back to the counter.
Trade Tip: a timer or humidistat fan needs a triple-pole isolator, not a double-pole one. The fan has a permanent live, a switched live and a neutral — three conductors to break. A double-pole switch leaves one of them live at the fan, which means anyone working on it later gets a surprise. Every fan isolator we sell is 10A triple pole for exactly this reason.
It's a small part with a specific job, and it's the single most common thing left off a bathroom fan quote.
The Knightsbridge M1100 is the metal-clad 10A triple-pole version, IP20, 230V — the one for a loft or a cupboard. If it's going somewhere visible, the FP1100BC flat-plate in brushed chrome does the same job in a finish you won't mind looking at.
6. Where DIY stops
Swapping a like-for-like fan on an existing, correctly-isolated circuit is within reach of a confident DIYer.
Running a new spur into a bathroom is not the same job. A room containing a bath or shower is a special location under Part P in England and Wales, and the work needs certifying. If you can't self-certify it, that's an electrician's job — and it's worth saying plainly that "it works" and "it's compliant" are two different tests. Rules differ in Scotland; check your local requirement.
We'll happily talk it through over the counter before you commit either way.
Extractor fan quick-reference
| Room | Minimum rate | Typical fit | Control |
|---|---|---|---|
| Bathroom / shower room | 15 l/s | 100mm, IP45 | Overrun timer |
| Bathroom with damp issues | 15 l/s | 100mm, IP45 | Humidistat + timer |
| Toilet / WC | 6 l/s | 100mm | Timer or plain |
| Utility room | 30 l/s | 150mm | Timer |
| Kitchen (over hob) | 30 l/s | Cooker hood | Manual |
| Kitchen (elsewhere) | 60 l/s | 150mm | Timer |
Expert Help in the West Midlands
We've been trading from Villa Road in Handsworth since 1975, and the trade counter is open to everybody — installers and homeowners alike.
Bring the room dimensions and a photo of where the duct has to run. Two minutes at the counter will tell you whether you need a 100mm fan and a timer, or whether you've got a 150mm problem — and we'd far rather sort that out before you've drilled the hole.
Browse the full range in Extractor Fans & Ventilation, or come and see them.
Frequently asked questions
Can I fit an extractor fan directly above a shower?
Not in Zone 0 — that's inside the shower enclosure itself, and nothing mains-voltage belongs there. Directly above it is Zone 1, where you need at least IPX4 and RCD protection. Every fan in this guide is IP45, so it clears that; the SELV question only arises for fans fitted inside the enclosure.
Why does my extractor fan keep running after I turn the light off?
That's the overrun timer doing its job — clearing the steam that's still in the room after you've left it. Most are adjustable. If it's running for an unreasonable length of time, the timer setting is turned up, not broken.
Do I need an isolator switch for a bathroom extractor fan?
Yes, and for a timer or humidistat model it needs to be triple pole so all three conductors are broken. It's a small extra part that makes every future repair safe, and it's the item most commonly left off a fan quote.