What size extractor fan do I need for my bathroom?
A bathroom fan needs to extract at least 15 litres per second (l/s) while it is running, under Building Regulations Approved Document F. Other rooms have their own minimum rates.
The figure on the box is not the whole story. Long or bendy duct runs reduce how much air a fan actually moves, so we size the fan for the duct route as well as the room.
These are the minimum intermittent extract rates, meaning the rate when the fan is switched on:
- Bathroom or shower room
- 15 l/s
- Kitchen, extracting through a cooker hood
- 30 l/s
- Kitchen, fan elsewhere in the room
- 60 l/s
- Utility room
- 30 l/s
- Toilet (WC)
- 6 l/s
Which type of extractor fan is best?
It depends mainly on how far the air has to travel to get outside. Short runs suit a standard axial fan; longer runs need a stronger centrifugal or inline fan.
- Axial fan
- The common slim wall or ceiling fan. Best for short runs straight through a wall.
- Centrifugal fan
- Pushes air harder, so it copes with longer ducts and bends.
- Inline fan
- Mounted in the loft or ceiling void, with a grille in the room. Usually quieter.
- Continuous-running fan (dMEV)
- Runs quietly all the time and boosts when the room is in use.
- Window fan
- Fitted into a glass pane. Double glazing usually needs a new unit made with a hole.
Does an extractor fan need an electrician to fit it?
In most cases, yes. Bathrooms have extra wiring rules, and some fan work must be notified to building control under Part P.
Bathrooms are split into zones based on how close you are to the bath or shower. Zone 1 is the space directly above the bath or shower tray, up to a set height. A fan in a zone needs a suitable IP rating, which shows how well it keeps water out. In zone 1, many manufacturers call for a SELV fan, which runs at a safe extra-low voltage from a transformer placed outside the zones.
A new circuit is notifiable under Part P. Adding to an existing circuit in a bathroom may need notifying too, depending on where the work is. Norman works as both an electrician and a plumber, so the fan, the wiring and the ducting are handled by one person on one visit.
Should my extractor fan have a timer or a humidity sensor?
For most bathrooms, yes. A timer keeps the fan running for a few minutes after you leave, and a humidity sensor (humidistat) turns it on when the air gets damp.
Most fans are wired to come on with the light switch, with a separate fused isolator so the fan can be switched off safely for cleaning or repair. The control options are:
- Timer overrun
- Runs on for a set time after the light goes off, clearing leftover steam.
- Humidistat
- Senses moisture and runs until the room dries. Useful when tenants forget.
- Light switch only
- Simple, but steam often lingers once the fan stops.
- Pull cord or separate switch
- Handy in kitchens and utility rooms where lights stay on for long periods.
- Continuous running
- A low, steady extract all day, with a boost when needed.
Where should an extractor fan duct go?
Always to the outside air, through an external wall, soffit or roof outlet. Ducting a fan into the loft space just moves damp air into your roof, where it can cause condensation, mould and rot.
Older fans that blow into the loft are a common fault, and they are worth checking if your bathroom never seems to clear. For a new fan on a brick wall, we usually core drill a neat round hole through the wall. It is noisy and dusty for a short time, but we protect the room and clean up. Where a duct crosses a cold loft, insulated ducting helps stop condensation forming inside it and dripping back through the fan. We keep runs as short and straight as we can, because every bend reduces airflow.
How do I stop condensation and mould in my house?
Get moist air out where it is made, and keep the house evenly warm. A working extractor fan in each bathroom and kitchen does most of the job.
Many Leicester homes, especially Victorian terraces in areas like Clarendon Park, Highfields and the West End, have small bathrooms that were never built with a fan. Steam from showers and cooking has nowhere to go and settles on the coldest surfaces.
When condensation is a problem through the whole house, two options are worth knowing about. Continuous-running dMEV fans extract steadily from wet rooms all day. Positive input ventilation (PIV) is a unit, usually in the loft, that gently pushes filtered air into the house to dilute moist air. A fan will not cure damp from a leak, a failed gutter or rising damp, so those need ruling out first.
Signs of a ventilation problem include:
- Black mould on ceilings
- Often in bathroom corners and above the shower.
- Mould on window reveals
- The plastered sides of the window opening, where surfaces are coldest.
- Steamed-up windows
- Heavy condensation each morning, running onto the sills.
- A musty smell
- Especially in bedrooms, cupboards and behind furniture.
What do landlords need to know about damp and mould?
Damp and mould is treated as a serious hazard under the Housing Health and Safety Rating System (HHSRS), which councils use to assess rented homes. Working ventilation in kitchens and bathrooms is one of the simplest ways to reduce the risk.
Awaab's Law now applies to social landlords, setting time limits for investigating and fixing damp and mould. The private rented sector is expected to follow. The rules and timings are changing, so check current guidance for your property.
For landlords and letting agents, a humidistat fan runs without tenants needing to remember it. We can also replace failed fans between tenancies, and several rooms or properties can go on one request.