Loft fan stuck on

[turip]

Hi there,

We live in an oldish house that has had a downstairs toilet added and an en suite in the master bed room. Both of theses rooms benefit from a fan which is turned on when you use the light switch in either. The fan is in the loft and it is connected with ducting. Normally the fan would switch off automatically, probably after 30 minutes or so. Lately however the fan has been stuck on and it won't switch off, I have to go to the switch board, if I flip the switch it resets the fans and turns off. I can't find a switch anywhere after looking around in the loft. The unit is a Vent Axia model (probably 5 years +) with inline duct fan.

I am not sure how to solve the problem. I am actually happy to turn the fan off altogether but I don't know how to do it. I could turn the power off, unscrew the plastic housing and disconnect the wires, but I am not sure what to do with the live wires. Any help much appreciated.

Thanks

 

[riveralt]

It sound as if the timer has gone on the fan - the fan will need replacing if that is the case.
You can get the fan to operate on/off from the light switch (no timer over run) - for it to do that you will need to put a link wire between the switch live and the permanent live at the fan.

 

[ericmark]

Looking at at PDF instructions it would seem you need to remove the L wire and place it in a connector of some other isolation method. It does not say how to then run it may need a link between Ls and L.

Once you find the connections at the fan likely you could remove the line feed in the centre of the ceiling rose and place it in the outer double connection so doing the same as above but at the ceiling rose.

I know the one I found was LED lamp with fan but likely even in old models wires still called L and Ls and removing feed from L should stop fan running on.

 

[JohnW2]

It sound as if the timer has gone on the fan - the fan will need replacing if that is the case. You can get the fan to operate on/off from the light switch (no timer over run) - for it to do that you will need to put a link wire between the switch live and the permanent live at the fan.

... and also (which I assume you intended, but didn't actually say!)disconnect the permanent live feed wire and put it safely into a connector block or something - otherwise the fan (and the light!) would be permanently on, regardless of the switch, even if the timer were OK, wouldn't it?!

Kind Regards, John

 

[bernardgreen]

Have you changed the lamps for energy savers recently ?

The fans rely on a low impedance lamp to bring the potential on the switched live down to neutral to indicate the lamp is off. Energy saving and some other modern lamps do not always bring the switched live potential down to neutral so the fan thinks the lamp is still on.

 

[EFLImpudence]

The fans rely on a low impedance lamp to bring the potential on the switched live down to neutral to indicate the lamp is off. Energy saving and some other modern lamps do not always bring the switched live potential down to neutral so the fan thinks the lamp is still on.

You have said that before.
I don't know what you are thinking of but it isn't extractor fans.

They don't rely on anything to 'indicate the lamp is off', not that they care or know, but for someone to remove the 240V on the switched live.

 

[JohnW2]

The fans rely on a low impedance lamp to bring the potential on the switched live down to neutral to indicate the lamp is off. Energy saving and some other modern lamps do not always bring the switched live potential down to neutral so the fan thinks the lamp is still on.

You have said that before. I don't know what you are thinking of but it isn't extractor fans. They don't rely on anything to 'indicate the lamp is off', not that they care or know, but for someone to remove the 240V on the switched live.

I think he's probably right, at least in theory. Every fan timer module I've seen has a very high impedance 'trigger' input (to which one connects the S/L) which only needs a relatively small voltage on it to tell it that the light is on, and therefore that the fan should be switched on. With an incandescent lamp betwqeen S/L and N, there is no way that there can be a significant pd between S/L and N when the lamp is off (even if there is 'stray pickup' in the cable). However, other types of lamp may appear as very high impedances when 'off', so that stray (e.g. capacitive) pick up by the S/L conductor could probably sometimes be enough to activate the 'trigger'.

Whether that often (or ever) happens in practice, I don't know, but it's certainly a theoretical possibility.

Kind Regards, John

 

[EFLImpudence]

Isn't that a contradiction?

High impedance trigger working when it shouldn't because of high impedance lamps.

Should that still not be the case with no lamp at all?

 

[JohnW2]

Isn't that a contradiction? High impedance trigger working when it shouldn't because of high impedance lamps.

Not at all. It's just like a voltmeter with a high imput impedance giving confusing results due to 'stray pick up' when connected to a long length of ('dead') cable with no (or very high impedance) load connected to it, but with that 'false' reading disappearing if you connect a relatively low impedance something across the meter.

Should that still not be the case with no lamp at all?

Indeed so. I would say that's the worst ('extreme'!) case in terms of the mechanism Bernard was postulating. Has anyone seen that happening, I wonder - fan coming on continuously because there was no lamp (or dead lamp) connected to the S/L?

Kind Regards, John

 

[bernardgreen]

I have seen it so it is both theory and proven by experience from me.

Some fans do now come with a bleed circuit between switched live and neutral to prevent this happening.

 

[JohnW2]

I have seen it so it is both theory and proven by experience from me.

Do you mean with no lamp at all? As I wrote to EFLI, I would expect that to be the 'worst case'.

Some fans do now come with a bleed circuit between switched live and neutral to prevent this happening.

I've never seen one of those - although, admittedly, my experience is limied, since I only usually get an opportunity to dissect them if they die! In the one's I've seen, there has always been a high value resistor in series (IIRC, typically around 270 k&#937 in series with the S/L input and, in most cases, that is then connected to the (extremely high impedance) input of a CMOS gate. One problem with a 'bleed circuit' (presumably just a resistor) between S/L and N would might be heat generation - depending upon how much 'bleeding' was necessary to address the issue we're discussing. I suppose one might contemplate using a capacitor, but physical size might then become an issue.

Kind Regards, John

 

[turip]

Wow, thanks for all the replies.

In answer to the question, no, I have not changed anything, no bulbs or the like.

I am happy for the fan to operate with the light switch, or not at all really. So I probably need to get the control panel off and have a proper look then (obviously with power off). As you can tell, I am no electrician, but a decent DIY'er.