How powerful it needs to be

Approved Document F, the Building Regulations guidance on ventilation in England, gives these minimum rates for fans that switch on when the room is used:

Room Minimum extract rate
Bathroom15 litres a second
Kitchen with a cooker hood that vents outside30 litres a second
Kitchen without one60 litres a second
Utility room30 litres a second
Separate toilet6 litres a second

Fans that run all the time on a low setting have different rates. A new edition of Approved Document F takes effect on 24 March 2027 for most building work, and it keeps the same minimum rates for these fans. Wales, Scotland and Northern Ireland have their own building rules, so check locally if you live there.

The airflow in a product listing is usually what the fan moves with nothing attached. Ducting, bends and the grille on the outside wall all reduce it, so choose a fan with some room above the minimum, and keep the duct short and straight.

What else to look for

  • A humidity sensor (humidistat). The fan switches itself on when the room gets steamy and off once it has cleared, so it runs long enough even if nobody remembers to leave it on.
  • A run-on timer. The fan keeps going after you switch the light off. In a bathroom with no window you can open, the guidance says the fan should carry on for at least 15 minutes after you leave.
  • Low noise. A quiet fan is one people leave running. Compare the dB figure: lower is quieter.
  • The right type for the route. The fans below are axial fans, which suit a short duct straight through an outside wall or window. A long duct through a loft or ceiling needs a more powerful centrifugal or in-line fan, which an electrician can advise on.
  • The right size hole. Most bathroom fans are 100mm. If you are replacing one, measure the existing hole and duct.

Our picks

These are chosen by their specs against the list above. We haven't tested them ourselves; the reasons say which spec puts each one here.

For most bathrooms

VENTS 100 Silenta-MTH

Type
Axial fan
Fan size
100mm
Airflow
21.7 litres a second
Power
7W
Noise
26dB
Cost per hour
0.2p
  • Timer
  • Humidistat

Rated at 21.7 litres a second against the 15 litres a second minimum, with a humidistat and a timer, so it runs whenever the room is steamy. Its 26dB rating is low enough to leave it running.

View on Amazon Manufacturer specs

For a fan that runs with the light

EnviroVent Silent 100 with timer (SIL100T)

Type
Axial fan
Fan size
100mm
Airflow
26 litres a second
Power
8W
Noise
26.5dB
Cost per hour
0.2p
  • Timer

A 100mm fan rated at 26 litres a second with a run-on timer but no humidity sensor. It suits a bathroom where the fan comes on with the light.

View on Amazon Manufacturer specs

For a kitchen

Manrose XF150T Kitchen Fan with Timer

Type
Axial fan
Fan size
150mm
Airflow
64 litres a second
Power
20W
Noise
40dB
Cost per hour
0.5p
  • Timer

A 150mm fan rated at 64 litres a second, set against the 60 litres a second guidance for a kitchen without a cooker hood that vents outside. It needs a larger hole than a bathroom fan.

View on Amazon Manufacturer specs

What it costs to run

Very little. The humidistat model above uses 7W, which is 0.2p an hour, and even running it two hours a day costs about 11p a month.

Costs use 26.32p per kWh, the Ofgem price cap, Direct Debit average for England, Scotland and Wales for 1 October to 31 December 2026. Your own tariff may differ. How we calculate.

Fitting it: when you need an electrician

Bathrooms have extra electrical safety rules because of the water. In England, under Part P of the Building Regulations, these jobs are notifiable:

  • adding a new circuit
  • replacing the consumer unit (fuse box)
  • adding to or altering a circuit in the space around a bath or shower, which runs up to 2.25 metres high and out to 0.6 metres from the edge of the bath or shower tray (or 1.2 metres from the shower head if there is no tray)

Notifiable work has to be checked. The simplest way is to use an electrician registered with a competent person scheme, who can sign off the work and send you a certificate. Otherwise a registered third-party certifier or building control must be told before the work starts.

Swapping an old fan for a new one on the existing wiring is not notifiable, but it still has to be done safely. If you are not sure how to isolate the circuit and connect a fan with a timer or sensor, use an electrician.

Getting the most out of it

  1. Leave it on while you shower and for at least 15 minutes afterwards, or let the timer or humidistat do it.
  2. Keep the bathroom door shut while it runs, so it pulls fresh air in under the door and not steam out into the house.
  3. Check it works: hold a sheet of toilet paper against the grille. If the fan doesn't hold it there, the fan, duct or outside flap may be blocked.
  4. Clean the grille every few months. Dust and fluff cut the airflow.

If the ceiling or walls already have mould, see how to get rid of black mould on walls, and check the room with a hygrometer once the new fan is in. What humidity to aim for.

Sources

Checked 15 October 2026.