An extractor must move air at an adequate rate, through a duct that does not defeat it, to outside. The common failures are an undersized fan, a long flexible duct with sags, discharge into a loft, and no overrun timer.
Why it matters
A bathroom generates a large volume of water vapour in a short time. A shower puts a substantial amount of moisture into the air in a few minutes, and that moisture has to go somewhere.
If it is not extracted, it condenses on the coldest surfaces in the room – the window, the external wall, the ceiling in a corner – and stays there. Persistent surface moisture is what mould grows on. People then treat the mould, which returns, because the cause was never addressed.
It also migrates. Moist air leaving an under-ventilated bathroom condenses in the next cold place it finds – a loft, a wardrobe against an external wall, a corner of a bedroom. A bathroom ventilation failure frequently shows up as a damp problem somewhere else entirely.
Sizing
Building Regulations set minimum extract rates for bathrooms, expressed as a volume per second, with a higher rate where there is no openable window. Those are minimums.
What actually determines whether a fan achieves its rated performance:
- The rated figure is measured in free air, with no duct attached. Connect it to a duct and the actual performance falls – sometimes very substantially.
- Duct length reduces it further with every metre.
- Every bend reduces it, and a tight bend reduces it more than a swept one.
- Duct diameter matters enormously. Reducing from a larger to a smaller diameter increases resistance sharply.
- The terminal – a grille with a poor free area, or a cowl with a flap that sticks, adds resistance.
The practical consequence: a fan rated comfortably above the requirement, on the end of a long flexible duct with three bends and a reduction, may be moving considerably less air than the room needs. This is why so many bathrooms with a working fan still suffer condensation.
A noisy fan is a fan people turn off, disconnect, or avoid using. Fan noise is rated, and a quieter fan that is actually used extracts more over a year than a powerful one that nobody switches on. Centrifugal and inline fans are generally quieter at the grille than axial ones, because the motor is further away or better isolated.
Ducting, which is where installations fail
| Fault | Effect |
|---|---|
| Flexible duct with sags | Water condenses in the low points, sits there, and progressively blocks the duct. The classic failure |
| Flexible duct compressed or kinked | Massive increase in resistance |
| Long duct run | Progressive loss of performance with length |
| Tight bends | Each one costs performance; two 45-degree bends beat one 90 |
| Reduced diameter | Sharp increase in resistance |
| Discharging into a loft | Not permitted, and it puts all that moisture into the roof structure. Creates a far worse problem than it solves |
| Uninsulated duct in a cold loft | Condensation forms inside the duct and runs back into the fan |
| Terminal flap stuck shut | Fan runs, nothing leaves. Check it from outside with the fan on |
| No back-draught protection | Cold air and smells come back in when the fan is off |
Rigid duct, kept short, with swept bends, insulated where it passes through cold space, falling slightly towards the outside terminal, is the answer. Flexible duct is used because it is quick and it accommodates a sloppy route, and it is the single biggest reason bathroom extraction underperforms.
Types of fan
Axial
The common wall or ceiling fan with a propeller. Suited to short duct runs – straight through an external wall is ideal. Performance falls off sharply with duct length, so an axial fan on a long run is a poor combination.
Centrifugal
Uses an impeller to generate more pressure, so it tolerates longer duct runs and more resistance. Generally the right choice where the duct run is more than very short.
Inline
The fan unit sits in the duct, typically in a loft, with only a grille visible in the ceiling. Quiet at the grille because the motor is remote, powerful, and able to serve more than one grille. Needs accessible mounting for maintenance, and needs the duct insulated on both sides of the fan.
Mechanical ventilation with heat recovery
A whole-house system that extracts from wet rooms and supplies fresh air to habitable rooms, recovering heat from the outgoing air. Appropriate in a very airtight house or a full renovation, and substantial over-provision for a single bathroom.
Controls
- Overrun timer. The fan continues for a period after the light is switched off. This is essential – most of the moisture is still in the room when somebody leaves it. A fan without overrun extracts during the shower and stops at the moment it is most needed.
- Humidity sensing. The fan runs when humidity rises and stops when it falls, regardless of the light. Better than a timer because it responds to actual conditions, and it works when somebody showers with the light off in daylight.
- Pull cord or separate switch, which relies on people remembering.
- Trickle or continuous running. Some fans run continuously at a low rate with boost on demand. Effective and quiet, and it addresses background moisture as well as shower peaks.
- Electrical zones. Bathroom electrical work is governed by zone requirements and it is notifiable. The fan’s position determines its required rating.
The rest of the picture
Extraction is necessary and not sufficient. Condensation is a balance between moisture production, ventilation and surface temperature.
- Replacement air. Extracting air requires air to come in. A bathroom with a well-sealed door and no undercut is a bathroom where the fan is fighting a partial vacuum. A gap under the door is not a draught problem; it is how the fan works.
- Surface temperature. A cold external wall or a cold window will condense moisture however good the extraction. Insulation and heating both help – which is one of the arguments for underfloor heating in a bathroom.
- Heating the room rather than leaving it cold. A cold bathroom condenses more.
- Closing the door during and after a shower, so the moisture is contained where the fan can deal with it rather than migrating through the house.
- Not drying laundry in the bathroom, which adds a great deal of moisture over hours.
- Wiping down the screen and tiles, which removes moisture the fan then does not have to.
Mould keeps coming back? Go outside while the fan is running and check air is actually coming out of the terminal. It frequently is not.
Questions we get asked about this
Why does my bathroom still get mouldy with a fan fitted?
Usually the duct rather than the fan – a long flexible run with sags, a reduced diameter, or a terminal flap stuck shut. Go outside with the fan running and check air is actually leaving.
Can the extractor discharge into the loft?
No. It is not permitted and it puts all the moisture into the roof structure, creating a considerably worse problem. It must discharge to outside.
Does a bathroom fan need an overrun timer?
It should have one. Most of the moisture is still in the room when the light goes off, so a fan without overrun stops at the moment it is most needed. Humidity sensing is better still.
Do I need a gap under the bathroom door?
Yes. Air extracted has to be replaced, and if it cannot get in, the fan is working against a partial vacuum and moving far less air than it should.
Is a more powerful fan the answer?
Not usually. If the duct is the restriction, a more powerful fan on the same duct is louder and only marginally more effective. Fix the duct first.
What about a bathroom with no window?
The regulations set a higher extract rate where there is no openable window, and a humidity-sensing fan with overrun is the sensible specification. The ducting matters even more.
Need this doing?
Call and describe the problem — you will speak to an engineer, not a call centre. Written quotes, no estimates over the phone.