Condensation usually persists when the fan cannot move enough air through the actual duct run, or when moisture remains trapped in a cold, poorly ventilated bathroom.
Choose the right fan for your bathroom and duct run
A bathroom should normally have at least 15 litres per second (l/s) of intermittent extract ventilation, or 8 l/s of continuous extraction, under Building Regulations Approved Document F.
Is an axial fan enough for your duct?
An axial fan is usually enough when the outlet is on the same external wall, the duct is smooth, and the run is as straight as possible. It can be a sensible choice for a compact Swansea bathroom where the fan discharges directly through a wall grille.
When do inline and centrifugal fans help?
Centrifugal and inline fans are better where static pressure is high. Static pressure means the push-back caused by the duct, bends, grilles and shutters, much like resistance when water travels through a kinked hose.
For a short external-wall route, a quiet axial fan can be enough. For a ceiling fan with more than roughly 3 to 5 metres of duct, several bends, or flexible ducting, a centrifugal or inline unit is usually the safer choice.
Choose the fan type by starting with the duct route, not the advertised free-air airflow. An axial extractor fan suits a short, smooth 100 mm wall duct with one external grille. Choose a centrifugal extractor fan where a compact fan must overcome a longer or more restrictive route. An inline extractor fan is often the better option for a ceiling grille connected to a loft or roof outlet, because the motor can sit away from the bathroom and handle greater static pressure.
Choose continuous dMEV where everyday humidity control is more important than occasional high-speed extraction, particularly in a windowless or cold bathroom. In every case, confirm that the bathroom fan ducting discharges outdoors and that replacement air can enter the room.
Size airflow for your bathroom volume and moisture
Start with the 15 l/s Part F minimum for an intermittent bathroom fan, then allow more capacity where the bathroom is large, cold, windowless or heavily used.
Convert l/s and m³/h correctly
One l/s equals 3.6 m³/h. Therefore, 15 l/s equals 54 m³/h, while 8 l/s equals 28.8 m³/h.
| Bathroom volume | 8 air changes/hour | 12 air changes/hour | Part F minimum |
|---|
| 6 m³ | 48 m³/h, 13.3 l/s | 72 m³/h, 20 l/s | 15 l/s intermittent |
| 10 m³ | 80 m³/h, 22.2 l/s | 120 m³/h, 33.3 l/s | 15 l/s intermittent |
| 15 m³ | 120 m³/h, 33.3 l/s | 180 m³/h, 50 l/s | 15 l/s intermittent |
When is 15 l/s not enough?
Fifteen l/s is a compliance baseline, not a promise that every bathroom will stay dry. A large family bathroom, an internal bathroom without a window, or a cold room with wet towels may need a fan that can sustain more extraction through the installed duct.
Compare airflow at a stated pressure, duct size, sound level in dB(A), controls and installation cost, rather than comparing headline l/s figures alone.
| Type and example | Published airflow | Best duct route | Noise and control | Typical fan price |
|---|
| 100 mm axial, timer or humidistat | Around 21 to 25 l/s, 76 to 90 m³/h | Short, straight wall route | Often 26 to 35 dB(A); timer or humidistat options | About £30 to £90 |
| Manrose MF100S centrifugal | Check current data sheet; designed for higher resistance | Longer 100 mm duct runs | Usually louder than silent axial units; timer versions vary | About £70 to £130 |
| 100 mm inline mixed-flow fan | Often 40 to 55 l/s, 144 to 198 m³/h | Long loft or roof routes | Motor noise can be remote; separate controller may be needed | About £90 to £220 |
| dMEV continuous fan | 8 l/s continuous, boost varies by model | Homes needing steady moisture control | Low-watt running; boost and humidity controls vary | About £80 to £180 |
Why does airflow drop in real ducts?
Flexible ducting can reduce performance when it is sagging, dirty, too long, crushed behind a ceiling, or narrowed by a reducer. Each bend and restrictive external vent grille adds resistance.
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Our recommendation
An adjustable humidistat fan can be useful in a shower room where people forget to use the switch. Choose one only after confirming that the duct route can carry the extracted air outdoors.
- Starts extraction when shower humidity rises, even if the light switch is not used.
- Adjustable humidity settings can reduce needless running during normal damp weather.
- A timer overrun continues clearing steam after the shower has ended.
Check availability →
Set a humidistat without heat loss
A sensible starting point is often between 65% and 75% relative humidity, then adjust after observing how the bathroom behaves.
A useful comparison only works when the figures are like for like. Check whether the stated bathroom fan airflow is measured in free air or at a declared static pressure: a fan rated at 90 m³/h with no duct connected may deliver substantially less once bends, flexible duct and an external grille are fitted. Compare the sound rating in dB(A) at the same speed, the running watts for continuous operation, available timer or humidity control, warranty length and the cost of ducting, roof terminals and electrical work.
For shower moisture removal, a quieter fan with a proven duty point through the installed duct can be a better purchase than a louder high-output model whose published figure applies only to an unrestricted test setup.
Fix duct faults and safety issues before buying
Persistent condensation needs a simple diagnosis before another fan is bought.
Check these causes of recurring mould
- Check whether the external vent grille is blocked by paint, lint, insects or a stuck shutter.
- Check whether the duct is smooth, sealed, supported and discharged outdoors rather than into a loft.
- Check for wet towels, indoor clothes drying and frequent showers adding moisture faster than the fan can remove it.
- Check for adequate background heat, trickle vents and a gap under the bathroom door for replacement air.
- Check for plumbing leaks, roof leaks and water ingress before treating the issue as condensation.
When should you call an electrician?
Ask a properly registered electrician to assess a new fan location, a long duct route, a replacement that needs wiring changes, or a fan in Zone 1 or Zone 2.
Do not rely on a new extractor fan where there is a plumbing leak, penetrating rain, widespread mould, a blocked outlet, or condensation across the whole home. Those signs need the moisture source diagnosed first, because a stronger fan cannot correct failed insulation, inadequate heating or water entering the building fabric.
Noise and electrical safety should be planned alongside extraction performance. A bathroom fan noise level is affected not only by the motor but also by turbulent grilles, undersized ducting, loose flexible duct and sharp bends. Use the manufacturer’s specified duct diameter, keep the run taut and supported, and isolate an inline unit from timber where practical to limit vibration. The chosen fan and its controls must be suitable for the bathroom zone and installed in line with the manufacturer’s instructions; a qualified electrician can confirm the required IP rating, isolation arrangements and wiring method.
Positioning the motor remotely in a loft can reduce noise in the room, but the duct route still needs to be short, sealed and insulated where it passes through cold spaces to avoid condensation within the duct.
Common questions
The best fan delivers enough real airflow through your installed duct, not just a high free-air rating. Use axial fans for short straight routes and centrifugal or inline fans for longer, restrictive routes.
A windowless bathroom usually benefits from a timer, adjustable humidistat or continuous dMEV fan. It should meet at least 15 l/s intermittent extraction or 8 l/s continuous extraction and discharge outdoors.
Do I need a powerful fan for a large bathroom?
A large bathroom often needs more than the 15 l/s minimum after allowing for volume and duct resistance. Calculate room volume, then consider 8 to 12 air changes per hour as a practical screening range.
How long should a bathroom fan run after a shower?
A bathroom fan should usually run for between 15 and 30 minutes after showering. Longer run time may be needed in a cold, windowless or heavily used bathroom.
Should I leave the bathroom door open or closed?
Keep the door closed during the shower so steam does not spread through the home. Open it after the main steam has cleared if the room lacks replacement air and the fan is still running.
Why does my extractor fan run but condensation remains?
Condensation can remain when the duct is blocked, too long, crushed, leaking or discharged into a loft. It can also result from weak heating, missing replacement air, wet clothes drying or a plumbing leak.
Choose the fan that can work in your home
The right bathroom fan is the one that matches the room, the duct and the moisture source.
The essential points:- Meet at least 15 l/s intermittent extraction or 8 l/s continuous extraction for a bathroom.
- Match fan type to the duct route, because long or flexible ducting can cut real airflow sharply.
- Use an adjustable humidistat with a 15 to 30 minute overrun where shower moisture is regularly left behind.
- Investigate leaks, insulation, heating and blocked outlets when mould or condensation continues.
Related sources
These articles can help you explore the topic in more depth:
