Confused about switching for a electric shower extractor fan.

It seems to me that ideally the shower extractor fan should come on automat ically when the shower is switched on using the power control on the shower unit itself. But I don't see being able to do that because it would involv e getting into the shower unit and connecting to the switched side of the s hower unit switch. Now I could probably do this but it seems it's not what is normal practice.
So what's the normal way of doing things? If I connect to operate from the light switch then what about in the daytime when you use the shower and wan t the extractor running but don't need the light on? Is it just expected th at any time you use a shower you must have the light on no matter the time of day?
And as the regs require this ventilation when the shower is in use surely t here's no point operating the extractor from a light switch because then it doesn't have to be used.
All seems odd to me and as I said...I'm confused.
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on 13/05/2018, me supposed :

Fit a PIR operated fan. That would cover the bath, or shower, or toilet being used in a room. Mine includes both a built in PIR and a humidity sensor. It comes on if you walk in the door and it continues to run after the bath or shower has been used, until the excess moisture has been dispelled to the outdoors.
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Seems alittle bit ad hoc though. I have no fan, I open a window! Brian
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I have no fan either and don’t bother to open the window, just leave the door open and that works fine even when I have the door closed when using it in winter to keep it warmer with a fan heater on in there.
We don’t have the silly rule about not having stuff plugged in in there and in fact the one I use most of the time is a combination shower room and laundry with the washing machine in it.

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On Sunday, 13 May 2018 11:56:49 UTC+1, Harry Bloomfield wrote:

But then you've got the situation where you go in to get the towels to wash , open a window, kill a fly etc., and the damn thing turns on unnecessarily for half an hour!
It just seems to me this hasn't been thought through properly. There should be a connection in the shower unit provided by the manufaturer which provi des power when the electric shower is turned on. It's only an extra ten or twenty watts after all so you wouldn't need to uprate the shower cable.
Anyway, reading around a bit it appears that most fans are switched from th e light switch and I'm not sure how that gets past building control when it can obviously be left off when showering?
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me brought next idea :

As you say, 20watts running for 20 minutes - so what!

If the shower in use were the only time the fan might be needed, I agree, but for most people it is needed when the bathroom is occupied, so an occupation switch (PIR) works better.

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On Sunday, 13 May 2018 14:13:25 UTC+1, Harry Bloomfield wrote:

Ah right. In my case though it's just a shower room, nothing else.
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I did a shower room for someone else. For that, the fan is on a run-on timer with the light as usual, but to get a longer run after a shower, it's also triggered by a pipe stat on the shower hot feed, so it will stay on until the pipe cools. That part of pipe has an 8mm parallel pipe which snakes up behind the bathroom mirror, so that when you use the shower, the mirror is heated. When the shower is finished, this loop up behind the mirror convects via the piece of pipe with the pipe stat on it, to cool that pipe down.
When I get around to it, I will write this up with diagrams.
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Andrew Gabriel formulated the question :

An interesting way of doing it, thanks..
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On Sunday, 13 May 2018 11:42:31 UTC+1, me wrote:

You can use a current sensing relay on the mains supply to the shower.
http://wiki.diyfaq.org.uk/index.php?title=Current_activated_switch
Owain
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On Sunday, 13 May 2018 14:04:40 UTC+1, snipped-for-privacy@gowanhill.com wrote:

I was an electronics engineer but my days of being bothered to cobble toget her electronic circuits are well gone. The principal is what I was after th ough. I just wish the shower manufacturer would build something into the sh ower itself. All it would need is an extra output on the shower power switc h and a bit of 1mm T&E could be run from it to the fan...job done.
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On Sun, 13 May 2018 06:14:26 -0700 (PDT)

You could join into the output side of the switch with some suitably sized cable, take that to a fused spur and then power the fan off that.
I've always thought that an auxiliary contact on the shower switch would be the easiest solution.
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On 13/05/2018 11:42, me wrote:

Put the fan on it's own switch.
Turn on fan when you want the fan on.
Turn off after use.
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Too primitive if you want the fan to run for different times after having a shower, multiple showers in a row with different people making the bathroom rather wetter and when you just want to pump the smell out after taking a dump after a decent curry the night before etc.
Makes a lot more sense to have a PIR, shower use sensor if it’s a primitive electric shower, and humidity sensor so that it stops when the bathroom is dry enough again etc and automatically handles very humid weather and very dry weather in summer, if you lot ever get anything like that.
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That is the remarkably primitive but transparent solution that I use. Seeing the fan has an overrun timer there is a neon on the ceiling switch for disambiguation.
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On 13/05/2018 23:45, Roger Hayter wrote:

I can also turn my lights on separately when I need to with the "other switch".
I have decided after much thought, to call the "other switch" the light switch.
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I have two! I call them the switch for the main lights and the switch for the recess lights. Why, I leave as an exercise for the reader. They are quite near each other, but it is far from the end of the world if one operates the wrong one(s).
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I thought the regs only needed fans in bathrooms with no window? In which case you'd always have the light on unless you were having kinky sex in the shower with an ugly mistress.
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On 13/05/2018 15:49, Jimmy Wilkinson Knife wrote:

Why would anyone have an ugly mistress?
Come on you have not thought this one through have you?
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Maybe she has other qualities?
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