Newbie: bathroom fan wiring puzzle

[BrokenSword]

Hi all. I'm trying to sort out the extractor fan situation in my bathroom, and I'm trying to figure out what is happening with the wiring. The fan comes on when the light switch is on. It goes off when the light switch is turned off. So no timer, then.

But there's a second pull switch next to the fan which turns it on/off when the light switch isn't on. Fair enough. But when the light switch is on, and therefore the fan is on, and I pull the 'fan' switch it stops for a fraction of a second then carries on going. Every time I pull the fan switch. If I then turn off the light switch the fan will either stop or carry on going, dependent upon whether I pulled the fan switch an odd or even number of times. Switching off the lighting circuit MCB in the fusebox means neither pull switch works. There is no isolator switch anywhere obvious, unless its hidden in the loft.

So, with my limited knowledge of electrics, I'm seeing the fan supplied with a switched live from the light switch which 'over-rules' a switched live from the pull cord (which itself is supplied by a straight live from the lighting circuit, probably the bathroom ceiling rose).

I understand two-way wiring as far as hall/landing lights go but I don't get why one switch would over-rule another. Bad/clever wiring? Relays, contactors, some other magic box I don't understand?

I've had a cursory look around the loft (where the wiring should be) but to get at it I'll have to move a ton of crap the last owners left. So any suggestions at what I should be looking for would be very welcome!

 

[Adam_151]

You'll kick yourself....

(Though I must admit it took me a minute or so)

What you'll probably find is that the pullcord is a two way switch, with common wired to the fan, L1 to the switchwire from the lights and L2 to permanent Live

So the switch is acting as a changeover, in one position the fan is on all the time, in the other its on only when the lights are on, you see the fan slow a little when switching it as the contacts are 'break before make'

 

[ColJack]

exactly the same as I put in mothers kitchen for the 3 spotlights in the kitchen extension over the sink..

View media item 13403neutrals and earths omitted..

 

[BrokenSword]

Ahhh! Thanks for taking the time to reply chaps, that makes sense now. To be honest I hate the fact that it comes on when the light does, no fun in the winter when you're on the other side of a partition wall trying to sleep after a night shift. To be honest, I've never understood why it's the standard thing to wire it up that way. After all, you still have to switch it on manually when using it during the day, so it only works 50% of the time. Surely, if you had to switch the fan on manually all the time it would be more likely become a good habit, and you'd be less likely on a humid summer's day to think 'oh bugger, the fan's off' immediately after getting in the shower. Makes more sense to have it switched on automatically by the shower, if anything. But that would mean possibly crossing two circuits and a whole new world of pain.

So I'm going to lose the feed off the light switch to make the fan switch independent and put an isolator in. Although, with another dangling cord, the bathroom ceiling is starting to resemble an Australians hat.

EDIT: Actually, scrub that. I'll just have a straight live in from the lighting circuit, and replace the two-way fan switch with an isolator, which I'll use as a normal switch. Unless that's illegal or something. I'll check it out.

 

[ban-all-sheds]

Makes more sense to have it switched on automatically by the shower, if anything. But that would mean possibly crossing two circuits and a whole new world of pain.

It can be done:

//www.diynot.com/forums/viewtopic.php?t=20386


Or you can put a flowswitch in the water supply to the shower, which is what I'd do next time....

 

[BrokenSword]

Makes more sense to have it switched on automatically by the shower, if anything. But that would mean possibly crossing two circuits and a whole new world of pain.

It can be done:

//www.diynot.com/forums/viewtopic.php?t=20386


Or you can put a flowswitch in the water supply to the shower, which is what I'd do next time....

Ah, I see. I take it you had problems with your solution then?

I think I'm going to leave things alone, for fear of having to notify the 'whoever'. To be honest, the new building regs are a joke as far as I'm concerned. I mean, I can leave the old wiring the way it is, it's usable, seems to have been installed properly at the time, and I'm not breaking the law by doing nothing. I try to bring it up to current regs by putting a fan isolator in and the authorities want money off me so they can check it. So I'm paying £100 (or whatever) to avoid having to pull a cord once a day, they can bugger off. /Rant Over

 

[ban-all-sheds]

Ah, I see. I take it you had problems with your solution then?

No - still working fine after several thousand operations.

But it does contravene KISS.

 

[rebuke]

The even simpler solution than a flow switch, is to get a fan with a humidity sensor - then it turns itself on when you use the shower...

My one is wired up so the humidity sensor will turn it on whenever, and also hooked up to the lights - the noise isn't audible in the bedrooms, and it's useful to have the fan come on to clear odours as it were. I think I need to get a different fan though, I went for the cheapest one I could see with a humidistat, and the amount of air it actually pushes through is miniscule

 

[plugwash]

But it does contravene KISS.

Why? a relay/contactor seems the obvious soloution to such a switching problem to me.

 

[ban-all-sheds]

It's more complex than a flowswitch.

 

[SUNRAY]

I have had to operate bathroom fans by the shower a number of times. I KISS by picking up the heater supply after the control system [pressure switch etc] then a suitable fuse / isolator switch.
How hard is that?

 

[ban-all-sheds]

Sounds pointlessly difficult to me - modifying the internals of the shower, invalidating the warranty, finding a way to safely connect another large diameter cable to the heater can.....

 

[SUNRAY]

Sounds pointlessly difficult to me - modifying the internals of the shower, invalidating the warranty, finding a way to safely connect another large diameter cable to the heater can.....

So far it has always been blatently simple to do so, certainly easier than fitting additional water operated switch or botching relays into an undersized enclosure which doesn't give the required automatic control. Why do you reckon it'll invalidate the warrenty? and why such a large cable when it only goes a couple of inches before it reaches a 1A fuse?

 

[ban-all-sheds]

Modifying a consumer product always invalidates the warranty.

Take your point about the cable size, as long as it complies with 434.2.2.

 

[SUNRAY]

Modifying a consumer product always invalidates the warranty.

Take your point about the cable size, as long as it complies with 434.2.2.

First of all I find the rating of components and wiring within showers leaves something to be desired and wonder how many actually comply [ie 2 15A switches wired with 1.5mm tri rated on the pressure sensor to control a 8.5KW heater] but how many shower units fail within the warranty period? Admittedly the flimsy plastic attachments get broken regularly but not the main unit.
Pulling a 1/4 spade off a tag, installing a piggyback connector and pushing original onto that or adding another ring crimp onto the threaded stud of a heating element in no way invalidates any warranty, certainly the additional 10W or so of load is irrelevant Apart from anything else it can so easily be reinstated before the word warranty rears its head.

I have done maybe 50 of these sofar [not always shower units] and not had to go back on any of them. Some of them have been in multiples and need to control a common fan or damper and lighting in wich case the wiring goes back to a control panel of some sort. I have also fitted isolating relays within the unit and used 24V control circuits.

My experience of showers is that the majority have their isolators left permanently switched on [some people don't even know there is an isolator] so any realistic control of fans etc needs to be done from within the unit.

Thinking about it now a nice way to do this would be to fit a current transformer switch on the main feed, which could be anywhere including the CU, theres food for thought.