Wiring new Extractor Fan into Existing Light Switch in Bathroom


Postby gcharlt3 » Sun Apr 16, 2017 10:41 pm

Hi all
Want to fit an extractor fan with timer above bath I have a pull switch that operates bathroom light at present can i utilize that to wire it up? if so what is best way for wiring up would I need an isolator ?
Thanx appreciate any guidance
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Postby kbrownie » Mon Apr 17, 2017 7:35 pm

I few things to consider first:
1) At what height is the fan to be mounted above the bath? As if you have a low ceiling, you may need to apply for building control consent.
2) Does RCD protection exist on the circuit being used? As this is a requirement for locations with baths and showers, it is also required for newly buried cable in walls, at less than 50mm depth that are not mechanically protected.

Then the secondary things to consider are:
For fan overruns, you will require a perm live and switch line, these can be found at the switch. You will also need a neutral, which are not often found at a pull cord switch and often not at a wall switch, so the neutral will be found at the light fitting, you will also find the switch line at the light fitting and depending on the set-up, you may find the perm live there also.
I would also recommend the use of a three pole fan isolation switch, but it is not a requirement, but nevertheless wise to. Also I would always run a CPC/earth between all accessories, even though the fan will not have terminal for one.
So the method you use would depend on the configurations of your circuits, it maybe that some alterations are made, so that it can be done.
But the most important points are that of building controls and RCD protection and that the work is done and completed safely..
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Postby ericmark » Fri Apr 21, 2017 6:58 am

What kbrownie says is correct, no need for me to repeat it.

Building regulations are not retrospective, however it would make sense to follow the current regulations.

So to switch with light switch is dependent on the window, with no window in the bathroom there is nothing wrong with using the light switch, also with an opening window there is no requirement for a fan so again nothing wrong with using a light switch.

However if there is a window, but it does not open, then there has to be another way to switch on the fan as well as the light switch, in fact there is no requirement to have it connected to light switch, so three main methods are:-
1) A simple push switch with built in timer.
2) A PIR.
3) Push switch plus lights.
For the latter you need a double pole switch, so although it is physically the same switch, electrically they are independent, other wise when you push the button the lights would also come on.

In the main fans ask for a 3A fuse, this will not protect the fan, it's far too big, if the fan was to stall and overload the fans windings would burn out before the fuse, British plug fuses have two preferred sizes 3 and 13 amp so they are telling you which of the two to use. Where a lighting supply is used the standard 6 amp protective device will likely trip before a 3A fuse will blow so no point fitting a fuse.

However where the fan becomes faulty, you will often need to disconnect the fan before you can reset a trip, so you do need an isolator so if it goes faulty you can switch off the fan and still have lights, since lights in bathrooms now need RCD protection you want a double or three pole isolator.

Bathrooms are a special location so if you want to be legal then the cost of going through the local authority building control will be higher than getting a scheme member electrician to do the work for you and he can self certify. Rules do alter around the country, Wales, England and Scotland are all different. So check for your area.

Main reason for fitting a fan is the double glassing no longer allows the window to open, however if the fan blows out, air needs to replace what it blows out, under I think Part F building regulations you can't fit a fan with an open flue, seems odd but open flue means the device draws air from inside the house, it does not mean you can stick your hand up the flue. So be it bathroom fan, cooker hob, or tumble drier first look at fires fitted in the house.

There is a way around the problem, there are heat recovery units made where the heat exchanger is built into the pipe going outside, so they look like a simple extractor fan, but instead of needing vents in the bathroom door to replace the air, the replacement air is drawn from outside, since outside air contains less moisture than inside air they work better than simple fan, although more expensive around £400.

The problem with a modern house is we have gone around stopping all the drafts, and you can't pump air out unless it can get in somewhere. I remember a bus with air vents which would keep passengers cool and the door opened in and could not be opened until fans were turned off.

Before you even think about wiring, have a look at house design and work out how that air gets back in.
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