2. Calculate the correct required extraction rate: Extraction rate is calculated using the following formula:
Performance in m3/hr = Room Volume x Number of required air changes /hr
The following table shows number of typical air changes:
Domestic Environments:
Bathrooms & shower rooms 3-8
Toilets-domestic 3-10
Kitchens-domestic 10-15
Utility rooms 15-20
Bedrooms 2-4
Halls & Landings 3-5
Garages 6-8
Commercial Environments:
Meeting rooms 4-8
Offices 6-8
Restaurants & Bars 6-10
School rooms 2-3
Canteens 8-12
Shops 8-10
Changing Rooms with showers 15-20
Toilets-public 10-15
Workshops 6-10
Hospital rooms 4-6
So, to take an example of domestic bathroom: 2m x 2m x 2.5m = 10m x 8 (room changes) = 80m3/hr
3. Examine the fan's technical specifications: You have just established that you need a fan capable of extracting 80m3/hr. But this information has no worth if you don't use it to choose the correct fan. If a manufacturer or supplier does not state a fan's extraction rate then it's probably because they have something to hide! As a rough guide, 4 inch axial fans range from between 75m3/hr at the lower end to 95m3/hr at the very highest.
4. Understand the bathroom zoning system: UK bathrooms are divided into four separate zones. If your fan is to be situated in zone one or two then it must be either an SELV 12v or IP*5 rated. If it is to be installed in zone 3 then it can be a standard mains (240v) fan.
5. Take noise level into consideration. Far too often domestic purchasers look only at a fan's extraction rate. This is just one metric by which a fan should be judged and a noisy fan can become a real annoyance. These days there are so many quiet fans and silent fans available that this doesn't need to be an issue. A good quiet or silent fan will operate around 24dB(A). At this level the fan is virtually silent and will not disturb anyone while others are answering a call of nature in the middle of the night!
6. Don't forget aesthetics! For some it's the first consideration but others completely forget about how the fan will look once in situ. In the last five years designs have changed: the old slatted front grille or louvre, whilst far from obsolete is not as common as in the past. Many fans nowadays utilise a solid front panel, resembling a tile extracting through the side vents. Others use an 'opening eye' front louvre for a really cool look.
7. Think about how you want the fan to work: Extractor fans can be switched on and turned off in a variety of different ways, including: via the light switch, integral pull cord, timer, humidistat and PIR. These methods have been designed to accommodate different lifestyles and needs. For example: If your bathroom has lots of natural light, or if you are a landlord letting a property you should consider installing a fan with humidistat. This will turn the fan on when relative humidity reaches a pre-set level meaning steam is extracted even when the user forgets to turn the fan or lights on.
8. Fans with timers, humidistats and PIRs require three core and earth cable which includes both a switch live and permanent live. Such that the fan always has power going to it, but will only turn on/off when the switching mechanism is triggered. If you are replacing an old fan, it is important to remember that unless your existing fan has a timer you will not be able to use a timer on the new fan without changing the wiring because your existing standard fan will have been wired up with two core and earth cable.
9. Ducting! It seems so simple, how could you get ducting wrong!? Firstly, avoid, if at all possible, putting bends in your duct run. Bends and kinks will increase the resistance in the duct impairing the fan's performance. Secondly, use duct clamps. These secure the duct to the spigot of the fan and external grille preventing any extracted air escaping into the duct void.
10. Don't forget the back draught. By virtue of the fact that you are connecting the outside world to the inside of your home via a four inch pipe you are potentially going to suffer from chilly back draughts. Whilst easily prevented these are often overlooked and once the fan is installed much more hassle to remedy. Some fans these days come with integral back draught shutters, but many don't so either fit an inline back draught shutter which sits in the length of ducting or fit a gravity grille on the external wall.
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