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Home > Help > What Extractor Fan Do I Need For My Bathroom?

What Extractor Fan Do I Need For My Bathroom?

Every bathroom in every house suffers with an excess of moisture. By their very nature, bathrooms create steam and if not removed will cause condensation on the walls and other surfaces, leading eventually to possible issues with mould, mildew and even breakdown of walls and surfaces.. The best way to address this is to remove the steam, while it is still steam. A well placed, well matched for your room extractor fan is what you need, and this section is designed to help you make the most informed choice.

Do you need a bathroom fan?

It is wise to have some form of ventilation in your bathroom, even if it is just a window. However, mechanical extraction is far less random than a window, with a fan installed you can be far more certain of the unwanted air making its way outside. Building Regulations part F requires that any fan in your bathroom must extract at least 15 litres per second (l/s) of air from the room. This extraction rate can also be written in metres cubed per hour (m3/h), in this case, 54m3/h. There is no problem with your fan being more powerful than this - in fact if you have a mould problem, it would be wise to go higher – but it can not be lower than that.

What different types of bathroom fan are there?

This is where the minefield begins, but don't worry, we are here to guide you through. Extractor fans that are suitable for bathroom installation can be broken down into three basic types:

• Axial – Designed for installation directly through an external wall, or in the ceiling on a very short duct run, these fans are what most people will think of when picturing a bathroom fan.

• Centrifugal – Similar wall or ceiling mounting options but with one big difference, a centrifugal fan generates far greater pressure, making it capable of extracting down much longer duct runs. Ideal for a bathroom with no external walls. Often including more switching and operating options, Centrifugal fans can be set to run at a low rate so there is always some flow through the room, even when there is no-one in there.

• Inline – As the name suggests, an inline fan sits within the run of ducting, completely out of sight. Installed in the loft space or ceiling void above the room, an inline fan produces a much greater extraction rate which makes it ideal for installations that either have long runs or serious condensation/ mould problems.

Are there different sizes?

When referring to a fan's “size”, we're talking about the diameter of the fan blade, and in turn the diameter of the surrounding “spigot” that points into the wall or ceiling. This will determine also what diameter duct pipe you use, and all related grilles and accessories. Most fans come in a standard 4 inch size, which will be perfectly suited to 95% of domestic installations, with some offering a 6 inch variant for larger rooms which can be used in kitchens too. Axial fans can also come in 5 inch versions, as can inline fans which can go all the way up to 8 inch versions. The main difference in all these sizes is extraction rate, the larger the fan blade, the more power. The larger room you have, the bigger fan you will need. This being said, there are less than 1% of domestic bathrooms worldwide that could not be more than adequately extracted by a 4 inch inline fan.

How do I know what size bathroom fan I need?

Understanding what fan will suit your bathroom comes down to knowing every aspect of your installation:

• Room size – Knowing the size of your room starts to give you a starting point of what extraction rate you need. To know what volume of air you needs to be extracted, you have to start by knowing what the volume of your room is. This is calculated by multiplying length by width by height. Let's take an average measurement: 3m long x 2m wide x 2.4m high = 14.4 cubic metres. This means that in order to change the air in that room once an hour, a fan would have an extraction rate of 14m3/h. Building Regulations require rate 4 air changes per hour, so we multiply that by 4, giving us a rate of 56m3/h or 15l/s. The Silent Tornado's extraction rate of 97m3/h would give you 6.73 air changes an hour.

• Length of ducting – As soon as you put a fan on any ducting, the extraction rate is affected. The air pressure within the duct will cause the air to slow as it is pushed farther away from the fan. This is where a centrifugal fan can be useful as they generate more pressure to counter the greater length of ducting.

• Installation position – Bathrooms are divided into zones depending on how close they are to the source of water. Which fan you fit will depend on what zone your are installing it in. More on that in a bit.

Which bathroom fan is best for my home?

Which fan is best comes again down to varying factors, every home and every installation is different. Is your bathroom next to a child's bedroom or is it an en-suite? Is your bathroom internal with no external walls? How do you want your fan to be switched on and off?


How loud your fan is can be a huge determining factor for what fan you decide to buy, no-one wants to be trying to have a relaxing bath with what sounds like a jet engine mounted in the wall. The noise level of extractor fans is displayed in dB(A) or decibels. It is worth bearing in mind that the decibel scale is not linear, and also doesn't correlate with general perceived ideas of sound. Decibels are a measurement of sound pressure, not actual volume. The difference between 20 and 30 decibels is less in volume than the difference between 30 and 40 decibels. Have a look at this decibel comparison chart to give you an idea of what decibel readings compare to.

Typical dB measurements for axial bathroom extractor fans range between 35dB(A) and 45dB(A) with the more powerful centrifugal fans between 40dB(A) and 55dB(A). In recent years however the trend has moved towards even lower decibel levels. As fans get larger and the flow rate gets higher, the decibel level also rises. The Silent Tornado, for example, is whisper quiet at 24dB(A). Please note that manufacturers' quoted decibel levels are taken 'at 3 metres' - This is the industry standard. Furthermore, these measurements are taken in test environments which may not necessarily replicate your installation. Also bear in mind that any fan activated during the night - by a call to nature for example - will sound louder than during the day.

Internal bathrooms

As mentioned above, if you have a bathroom that has no external walls, you have no choice but to extract down a long run of duct. Most of the time this is not more than 3-4 metres, but it can end up being more if the run has to go round structural elements or all the way up to a roof. Centrifugal and inline fans are a great solution for long duct runs thanks to the greater pressure they produce.

Switching operations

All fans maybe wired into the lighting circuit, so that they turn on and off with your lights, or a separate remote switch, allowing them to be operated independently. Further to that, extractor fans have a range of extra switching options which suit different applications and requirements. Many fans are available in three versions - Basic, Timer and Humidistat, but some ranges also have pull cord and PIR versions.

• Basic models are operated by the light or remote switch only.

• Timer models feature a timer which keeps the fan running for a set period after the light or remote switch is turned off. This ensures all steam has been extracted from the bathroom.

• Fans with Humidistats will turn on automatically when humidity in the bathroom reaches a pre-set level and off when the humidity falls back to its preset level. These are great in bathrooms where the lights (and fan) are not always turned on manually and are popular with landlords, or in those installations where a fan is only required to extract steam.

• Fans with integral Pullcord are rarer these days, but allow for local operation.

• PIR (Passive Infra Red) sensors operate the extractor fan automatically when somebody enters the room.

Installing your fan

Choosing where to install your fan is a combination of electrical safety and common sense. The first and most important consideration is electrical safety.

When fitting a fan in your bathroom, you must understand which Bathroom Zone you are installing in. There are three bathroom zones, numbered 1, 2 and 3. Zone 1 is basically the area of the bath or shower cubicle and up to 2.25 metres high, Zone 2 is 60cm outside of Zone 1 in all directions and Zone 3 is then anywhere outside of that. Where possible, it is best to try to fit your fan in Zone 1 where it is close to the source of the steam. This means the fan is extracting the steam directly out of the room as it is being produced, minimising time for it to condensate on the walls.

In order to install a fan in Zone 1, you must adhere to IEE Wiring Regulations BS7671, Requirements for Electrical Installations Section 601 – which basically says that if you are installing in Zone 1, the fan must either be Low Voltage or IP Rated to IP45.

Low Voltage fans are powered by a 12 volt feed from a step down transformer, usually supplied with the fan. Although they provide a safe operating voltage in case of water ingress, they are a little more challenging to fit as the transformer itself must be put in Zone 3, or buried in the wall. The easiest solution for this is place the transformer above the ceiling but that isn't always possible.

A far easier solution is to install a fan that is IP rated. The IP rating system basically tells you how penetrable a fitting is, perhaps a light fitting or a fan. The first number tell you how penetrable it is by solid items, from something the size of a hand at 1 all the way up to fully dust tight at 6 and the second number shows its resistance to liquids, with protection from water droplets at 1, all the way up to fully submersible at 8. A bathroom fan suitable for Zone 1 must be rated at IP45, although you may see it rated as IPX5 as the rating for solids is less important in a bathroom.

Do fans have to be vented outside?

When installing an extractor fan, you must extract the stale air and moisture to an external vent in a wall, roof tile or roof soffit. Extracting moisture and steam into a loft space will cause mould build-up and eventually structural problems. A surface mounted axial fan will not be able to adequately extract outside with any real power if the duct run is longer than 3 – 4 metres.

How much does it cost?

The cost of having a a bathroom fan fan installed is dependant on a couple of factors; 1, Which fan you have chosen and what accessories it requires... and 2, how much your electrician will charge to install it. A fan plus accessories should cost somewhere around £60 - £100 depending on the fan and what bits you need. An electrician will charge somewhere between £50 and £100 for a morning's work, which is all a fan installation should take. Some may be a little more involved than others, you may only be replacing an existing fan and don't need any accessories which is a half hour job; or you may be installing an inline fan with a long duct run and a roof tile outlet, which will be a little more involved.

Installation tips and Troubleshooting

Every so often, a fan can end up not performing how you expect. Quite often, it is because of an oversight in installing or setting up the fan. Here are a few tips and tricks we have gained over the years to help you.

Backdraught Shutter

A backdraught shutter can be an invaluable addition to your installation, there's nothing worse than a fan that allows cold air back into your bathroom, especially in the winter. It may sound like an easy fix, but the best place to fit one is not always obvious to some installers. A lot of wall mounted axial fans these days have a backdraught device of some form built into the back of the fan itself, but inline fans quite often need one as an additional accessory. As air travels down a duct run, it slows down. Still pushed by the air coming behind it, it will still exit the exterior grille, but it doesn't travel as fast as when it exited the fan itself. The trick, as we have discovered, is to fit the backdraught shutter as close to the exhaust end of the fan as possible. If placed closer to the exterior grille, as the air slows it will struggle to push the shutter open, creating a blockage in the duct. When placed at the exhaust end of the fan, the faster moving air pushes the shutter open more easily, making blockage much less of a problem.

Condensation Trap

Condensation traps are designed to.. trap condensation. Very simple. Understanding where is best to place them is however not so forthcoming. The first thing to understand is that condensation traps are designed to work in vertical sections or ducting, they don't work in a horizontal duct. Although they can be used with any vertical run of duct, they are best suited to be used in conjunction with a roof tile or through roof vent to the outside world. Particularly in cold weather, this is where steam will immediately turn to water and run straight back down the pipe. Fitting the condensation trap as close to the underside of the roof vent as possible will be the most effective position. While a condensation trap is designed for use with solid pipework - and it is worth bearing in mind that condensation traps are for use with 110mm soil pipe, not 100mm duct pipe – you can use PVC ducting pipe on the underside of the condensation trap as it is just about stretchy enough to go over the larger diameter. Ducting Best Practise Horizontal ducting runs will still suffer condensation within them, although not as much. There are a couple of ways to combat this that are easy to put in place, even in existing installations.

• Pulling the duct so that it is as taught as possible between connection points is a really good piece of advice to go by. If there are dips and troughs in the duct, this is a perfect place for condensation to build up and eventually create pools of water in the duct. These pools at best restrict the diameter of the ducting, at worst can cause ducting to rupture and spill lovely foul smelling water in your loft and through your ceiling.

• Any condensation occurring within an inline fan and horizontal duct installation is more likely to happen after the fan motor, in the exhaust side of the duct run. This can be combatted again by having a slight downward angle to the ducting after the fan. This can easily be achieved by mounting the fan on a raised platform or screwed to a joist.

• Bends in ducting are often unavoidable, having to get over and around joists and other obstacles in your attic or ceiling void is part of the joy of installing an extractor fan. Something to bear in mind when planning your duct run is that a 90° bend in your duct will add the equivalent of 1 metre of air pressure to the system. If you have a 3 metre run from fan to the outside wall but have two 90° bends, that's a duct run of essentially 5 metres. That's the difference between having an axial fan and an inline or centrifugal fan installed.

Minimising Vibrations

Having an improperly fitted extractor fan in your attic can be a real problem, especially if it rattles around and wakes you up in the night when someone uses the bathroom. A great trick to solve vibration with a ceiling or loft mounted fan is to mount it on a rubber base. These can be actually quite expensive for a not particularly high tech item, so a great installers trick is to screw the fan down onto four rubber grommets that any electrician should have in abundance in the back of his van, or can easily be obtained from any DIY store.

Air Starvation

One of the single biggest reasons for calls for advice here at Extractor Fan World is when a customer has bought a more powerful fan to combat condensation in their bathroom and it still hasn't solved the problem. This is nine times out of ten because the fan is being starved of air, or there is poor airflow through the room. Think of it like being suffocated, as the fan is trying to take air out of the room, more needs to come in to replace it, otherwise it's like not being able to breathe. To fix this, you need to get more air coming in when the door is shut. You can either fit a small passive vent in the door which can be opened and closed as you need or, simply shave 5mm or so from the bottom of the door so that more air can come in underneath it.

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