Wiring a extractor fan (2023)

Is it possible to wire an timed extractor fan (with light) so that it comes on when the shower isolation switch is on, e.g. switch on power to shower and extractor comes on, switch off power and extractor overruns until preset time is reached?

I can only see us using the extractor when using the shower and would rather not have it come on when switching on the main lights to the bathroom ;)

Obviously the shower switch is a DP 50A pullcord type.

Thanks

Dave

In message , Dave writes

Tricky. Theoretically you could just wire it purely from the shower switch since it has the neutral, feed and switched feed, but the fans and their wiring aren't really suitably protected for a high current circuit and getting 6 or 10mm into the fans terminals is going to be a tough job. :)

The only idea that comes to mind is a little junction box adjacent to the isolator switch with a couple of suitably rated fuses inside for the feed and switched feed. This comes under the heading of dodgy dabble though. Anything like two standard size unswitched spur fuse plates is going to look pretty ugly.

In theory, yes you could. The practical implications of needing to fuse down to (probably) 3A for the fan will make it very difficult to wire.

How about fitting a humidistat fan instead? These will only spring into life when it gets moist in there. Saves it running just because someone forgot to pull the shower switch.

Wot Clive said.

You can get fans with humidistats, which would save a lot of "dodgy dabbling" around.

Owain

Unless I'm missing something here - if you're talking about THE shower isolation switch then AFAICS that wouldn't be a good idea because the whole point of them is that you switch them off, and the shower is then isolated and safe to work on (some even have padlock fitments on them). If the fan remains live after you've turned off the isolation switch, then by definition it's no longer an isolation switch.

Why can't you have say, a 1-gang light switch outside the bathroom to operate the fan on overrun), with the fan cable wired via the isolator switch?

David

How about taking the permanent live and neutral from the lighting circuit and the switched live from the permanent live via the contacts of a relay whose coil is powered by the shower switch?

It would work, but still leaves the relay rather unprotected on a 45A circuit.

You might find that 40A would generate enough magnetic field to toggle a reed switch in close proximity to one of the shower wires though.

Dave brought next idea :

That can be arranged, but it would be a custom job...

I would be inclined to suggest a current transformer on the shower supply cable then have the output of the CT cause a relay to to make to power the fan.

Easy way is to use a PIR plus humidity switched fan made for the job. They start working as soon as you walk in, run for a set time and keep will also continue to run based on the humidity level.

In message , John Rumm writes

Or at least with a few loops wrapped round the reed switch.

In message , Roger Mills writes

Because theoretically the relay itself would require a fuse to protect it's wiring. It also raises the issue of having two feeds to a single enclosure so that when Mr DIY decides to work on it and turns the fan circuit off he can get plated from the shower feed.

I'm not wanting to do any dodgy wiring, I have enough of that already!

Regarding the humidistats, are they adjustable?

My understanding is that I've got to have a 3 pole isolation switch for the extractor anyway, please correct me if I'm wrong. Is there any regulations as to where this should be when it is outside of the room?

Many thanks

Dave

Yup...

You can get fans with them built in, or TLC do a standalone Humidistat you can use to control whatever you want.

They normally have a knob that lets you set the humidity threshold they kick in at, and that will range from "fan runs continuously" to "fan needs to be submerged" before it starts!

You need an isolator. A three pole one is only require if you have a fan that uses switched and unswitched live connections (i.e. ones activated from the light that also have a timer run-on). Humidistat fans don't require both.

The positioning of the isolator follows normal placement rules for accessories in bathrooms- i.e. it needs to be appropriate for the location. In Zone three this can mean an ordinary face plate isolator switch or a pull cord version if you prefer. Some place the isolator outside the bathroom - high up on a wall, although technically speaking it ought to be of a type you can lock off if you do this, I don't think I have every seen anything other than a normal fan isolator switch used.

As another possible option, could I wire a timed extractor to the light switch and place the 3 pole isolator in place so that the fan can be switched on/off regardless of the light? This way I can switch on the extractor when I want a shower.

I'm not overly fond of the 3 pole pull cords as they are really bulky, unfortunately, and If I have a normal isolator switch it would have to be located on the wall of our hall landing, which I don't like the idea of.

Cheers

Dave

That is the way the isolator is supposed to be wired. Take a live, neutral, and switched live from the ceiling rose (or equivalent) of the light fitting, via an isolator to the fan.

(it gets a bit more complex if the fan wants fusing at 3A - then its simpler to use a FCU to break into the lighting radial and derrive a 3A protected feed that can then supply the light and thence fan)

Are they? Just a normal face plate sized thing. Stick it on a dry lining backbox sunk into the ceiling and they look neat enough.

Why "have to"? There is no regulatory requirement to place it outside. You can mount a conventional switch type isolator on the ceiling if you prefer.

Yes - as I suggested yesterday....! David

On Fri, 09 Mar 2007 03:19:57 GMT, Clive Mitchell mused:

Not a problem, 3A fuse in an HRC enclosure from a consumer unit fitted on the DIN rail next to the contactor.

That's why you're meant to label enclosures containing supplies from multiple sources.

Except that if you're going to switch it on and off manually, there's no point in having a timer. The timer relies on having a permanent live - and if you switch *that* off with the 3-pole isolator, the fan will stop instantly.

Could you not have a cord-operated momentary switch on the fan itself, and leave the 3-pole isolator on all the time (except when you really need to isolate it)? Then you simply have to give the cord a nudge, and the fan will run for the time set on the timer.

I'm thinking that a timed unit may not be required after all. Having thought about it, having a timed extractor come on with light pull means I would need to have the main lights on in order for it to work. With it getting lighter in the mornings I'm more put off by this option.

If I purchase a decent non timed unit (possibly vent-axia luminAir) I can use a DP isolator to switch the unit on/off when required, which would be whenever we use the shower.

Dave

On 12 Mar 2007 04:15:40 -0700 someone who may be "Dave" wrote this:-

Provided that the switch is located in a suitable place.

A humidistat fan will do that for you, and keep it running after the shower until the air is dry enough again.