Extractor fan strangeness

[steenjo]

Hi. I know there have been posts on this topic here before, but I can't see my specific issue addressed. I'm replacing a bathroom extractor fan. The old one [EnviroVent silent 100] worked fine including the timer overrun, but I'm redecorating so I thought I'd replace with a better looking and apparently quieter product i.e Expelair C4HTSR. I installed the new fan but the overrun didn't work; the light and fan were both on when they were on but turning the light off also turned the fan off; no overrun. So I sent the fan back and got a new one from somewhere else. But I have exactly the same problem.
The wiring to the unit has a brown wire, a grey wire and a black wire. I am wiring the new unit the same as the old, that is brown to live, grey to neutral and black to switched live. This wiring worked fine with the old unit. When I test with a meter I can see there is current between brown and grey when the light and fan are on, and current to brown and black (switched live) when I turn the light off. So there is current but no overrun. Given this has happened with two different units now it seems unlikely the unit is at fault. The real strangeness is that I tried putting the old unit back and this is now doing exactly the same thing even though I've wired it back just as it was before. I am now doubting my sanity. Has anyone got any ideas?

 

[EFLImpudence]

The wiring to the unit has a brown wire, a grey wire and a black wire. I am wiring the new unit the same as the old, that is brown to live, grey to neutral and black to switched live.

That seems correct.

This wiring worked fine with the old unit. When I test with a meter I can see there is current between brown and grey when the light and fan are on,

Do you mean Voltage?

and current to brown and black (switched live) when I turn the light off.

Voltage??? What do you mean "TO brown and black"? Where from?

So there is current but no overrun. Given this has happened with two different units now it seems unlikely the unit is at fault. The real strangeness is that I tried putting the old unit back and this is now doing exactly the same thing even though I've wired it back just as it was before. I am now doubting my sanity. Has anyone got any ideas?

I would think you have the Brown and Black the wrong way round at the fan.

 

[steenjo]

Hi thanks for your reply. Ok sounds like I am saying current when I mean Voltage. My voltmeter shows 240V between brown and grey when the light is switched on. At this point the fan is also on. If I switch the light off I then see voltage between brown and black (wired to the switched live terminal) but the fan does not operate. I tried wiring the brown and black the other way around. When I do this the fan turns on when the light is switched off; if I then turn the light on the fan switches off.

 

[EFLImpudence]

My voltmeter shows 240V between brown and grey when the light is switched on. At this point the fan is also on.

Ok, that is as it should be.
There should be 240V Brown to Grey ALL the time.

Is there also 240V between Black and Grey with light on but not when off?'

If I switch the light off I then see voltage between brown and black (wired to the switched live terminal) but the fan does not operate.

I suppose that could be because of a path through the fan or light to Neutral.

I tried wiring the brown and black the other way around. When I do this the fan turns on when the light is switched off; if I then turn the light on the fan switches off.

That's confusing - will have a think.

 

[steenjo]

Hi - thanks for sticking with this

"There should be 240V Brown to Grey ALL the time." - no. there is 240V when light is on but zero when light is off.

"Is there also 240V between Black and Grey with light on but not when off?'" - there is 240V between Black and Grey at all times

 

[EFLImpudence]

"There should be 240V Brown to Grey ALL the time." - no. there is 240V when light is on but zero when light is off.

"Is there also 240V between Black and Grey with light on but not when off?'" - there is 240V between Black and Grey at all times

That means that the Black is Permanent Line and the Brown is Switched Line.


This is not the normal way of wiring it but would make no difference once connected as such.

So - Black to L and Brown to timer.



However, that doesn't fully explain the way the fan is working.

 

[RandomGrinch]

Instead of measuring the voltage between the brown, grey and black wires - can you measure the voltages of these wires, with one probe connected to earth (with the switch both in the on and off position)?

 

[EFLImpudence]

Unless, of course the Black is Neutral but wouldn't that have damaged the fan?

 

[RandomGrinch]

but wouldn't that have damaged the fan?

There's usually something like a 270K resistor on the switched live input.
If neutral was connected to the S/L terminal, there may possibly be some damage, but no dead short.

 

[EFLImpudence]

I am puzzled why, if that is the case, the fan ran at all.

They won't usually turn on unless the Line(live) is connected to both PL and Timer terminals.

 

[steenjo]

Update - following the logic of your response @EFLImpudence: I tried reversing brown and black again. This time the fan remained on for a few seconds when the light was turned off. I then stripped all the wiring back a couple of inches and wired it up again. Bingo! all (hopefully - seemingly) working as it should. I just need to check the duration of the timer overrun (and the humidistat is working) and we should be all good.

many thanks @EFLImpudence and

 

[EFLImpudence]