Bathroom extractor timing


Postby RoyG » Sun Jan 09, 2011 9:12 am

This is more to satisfy my curiosity than anything else.

The bathroom has the light and a 12v extractor fan both activated by the pullswitch by the door. The fan appears to be wired from the ceiling light with a three core and earth. Permanent live, switched live (with a brown sleeve on the core) and neutral (blue sleeve on the core)

As I understand it, the point of having the live and switched live is to allow the fan to overrun on a timer after the light is switched off. Only it doesn't. It seems to go off with the light.

If I can track down the transformer, which is presumably somewhere in the loft, should there be a control or selector switch for a timer overrun?

Also, while I think of it, the downstairs shower room has an identical fan and light, but wired completely differently. There's a light switch on the outside of the door, but also a fan isolator--presumably a three pole switch--and the ceiling light is fed with one twin and earth cable. That fan does overrun.

Why does the downstairs shower need a separate fan isolator switch when the bathrom upstairs doesn't?
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Postby sparx » Sun Jan 09, 2011 12:56 pm

hi, several issues here.
upstairs and downstairs are both seemingly wired for overrun fan but was overrun unit fitted at each fan?
Possible reason for no 3 pole isolator switch for bathroom but one for shower room may be to do with regs for safe isolation for maintenance.
If natural light available ie a window in the room then you can isolate the lighting circuit at the consumer unit for cleaning fan etc. likely in bathroom.
If no window in shower room then would need light to be on but fan isolated, this is ok for seperate fan & light but not much use with combined unit!
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Postby ericmark » Sun Jan 09, 2011 5:02 pm

Opening window does not require fan so any fan fitted is not covered by Part F.

Non opening window means controlling fan from light switch is no good as can’t turn on fan without lights pneumatic switches are common.

Some fans require 3A fuse so the three pole isolator is not really any good and a FCU is used and a double pole light switch.

In the main fans are fitted to satisfy the building inspector and it is down to what the builder can get away with. And what the supplier had in stock.

Personally I think one needs to be able to switch the fan off as they so often fail and if on same switch as lights then one would have lost lights as well.

314.1 Every installation shall be divided into circuits, as necessary, to: (i) avoid hazards and minimize inconvenience in the event of a fault. (iii) take account of danger that may arise from the failure of a single circuit such as a lighting circuit. (vi) prevent the indirect energizing of a circuit intended to be isolated.

However where the light and fan are combined what ever the regs say it may not be practicable.

I think all electricians have at some time opened the boxes to fit the gear only to find it is not the right gear in the box. They then have to decide if it is worth taking it back or just fit what is supplied. Transport is expensive and so is waiting at a trade counter and so it is always temping to fit what has been sent. If some one complains then return with correct gear what ever it is.

If one can unplug the transformer then that could be considered as an isolator.

I have seen loads of fans fitted into bathrooms with un-opening windows and still wired to light switch. As electricians we read Part P and electrical regulations but Part F is not always on out reading list so many electricians don’t realise the wiring to light switch is not permitted where there is outside light. And really it is up to the LABC to point it out but all they seem to ask is does the fan work and they tick the box.
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Postby RoyG » Mon Jan 10, 2011 9:00 am

Well, I made a bit of an error in my original question. It turns out that the downstairs fan is actually (probably) a mains unit. Although the 2 units have identical fascias (they're from Silavent) the one upstairs is labelled "Low Voltage" (I assume that in context that means 12v) and the one downstairs isn't. I guess that's why it needs a separate isolator. (next time I happen to be up in the loft I'll have a look for the transformer)


For completeness sake, the bathroom has a large window with an opening pane and the small dowstairs shower has a NON opening window.
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Postby ericmark » Tue Jan 11, 2011 10:03 am

[quote="RoyG"]
For completeness sake, the bathroom has a large window with an opening pane and the small dowstairs shower has a NON opening window.[/quote]
This explains why down stairs has over run and up stairs does not. There is no requirement for fan up stairs.

Down stairs should be switched on independent of light switch although it seems rare that this is complied with.
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