Extractor fan and light

[myhrlfc]

I am converting our utility room into a cloakroom, and plan to put a timed fan in there. I have read the instructions for wiring in this via the light, which is okay, but we also want to be able to turn the fan on while the light is off. What is the best way to achieve this?
Thanks.

 

[securespark]

Not without a gr8 deal of thunking and maybe some speshul equipment!

 

[ban-all-sheds]

You could use the lighting switched live to control a relay. This would give you the isolation that you need to be able to have a second way of providing switched live to the fan without back-feeding to the light.

But I'd also be interested in the functionality you wish to achieve, not just how you could implement a solution that you think would provide it.

I don't know why you want do do what you describe, but...

1) If it's so that the fan comes on when people are in there but don't need the light on, have you considered a PIR switch?

2) If it's so that the fan comes on if you're running appliances in there but don't need the light on, have you considered a fan with a humidistat?

3) If it's neither of those, have you considered just having a separate switch for the fan, and a length of 4x2 which you apply to the heads of people until they remember to turn the fan on and off as required?

 

[pipme]

Hi,
If using a dedicated fan pull cord sw. with a timed over run, You would want one with an 'indicator' (on / off). Why ? Because if the fan is running ... you'll maybe not know if it has been switched off and is on time out, or has been left 'on'. Generally happens just as you are about to leave the house in a hurry ... isolator handy in this instance.

The question is, can you buy an indicator type pull switch other than the heavy looking high amperage types generally available ?

P

 

[Big_Spark]

There is a very simple way to achieve this, no complicated relays, contactors or fancy wiring at all.

When you connect the fan up, you will be taking the following cores to it.

L1 = Permanent Live Feed (to allow timer over-run)
L2 = Switched Live Feed
N = Neutral
E = Earth or CPC

Now the simple way to achieve what you want it to wire a push button switch (Push to Make) in parallel with the normal switched feed.

When you push the switch the fan gets a momentary feed on the trigger or switched line, this will fool the fan timer into thinking the light has been turned on when it has not, the fan will then run for the preset over-run period.

 

[myhrlfc]

Thanks for replies. There will be a washer/dryer in there, so hence the reason for wanting to have the fan on without the light on. Already have a fan, so don't really want to go buy another, but the humidistat idea sounds good, is there an adapter you can get?

 

[Big_Spark]

You can buy seperate humidistats. You wire the supply through the stat so it acts as an automatic switch.

TLC have this http://www.tlc-direct.co.uk/Products/BGREMHM.html

and then this http://www.minispecialists.co.uk/cat/Humidistats/standard_robust.htm

 

[securespark]

FWL

Won't that turn the light on, too??

 

[ban-all-sheds]

A brief flash is all.

A bigger problem is where do you find a suitable momentary action push-button switch.

 

[breezer]

here

 

[Big_Spark]

Why will it turn the light on?

If wired correctly it will not effect the light as no circuit will be completed.

 

[ban-all-sheds]

So, referring to your diagram, when the momentary switch is closed, thus making the cable from the bottom terminal live and supplying a feed to the fan, why does that live feed not also make its way along the connection between the fan and the output of the pull-switch, which is connected to the light?

[code:1] /
_________/ _________
| (momentary) |----- TO FAN
| |
LIVE FEED | |
--------------------| |--------------- TO LIGHT
| |
| / |
|_________/ _________|
(pullcord)[/code:1]

If the pull cord is going to work the light and the fan, why isn't the momentary switch going to work the fan and the light (albeit only momentarily).

You could use a DP pullcord so that you could also switch the neutral for the light - that would get round the problem.

 

[breezer]

So, referring to FWL_Engineer's diagram, when the momentary switch is closed, thus making the cable from the bottom terminal live and supplying a feed to the fan, why does that live feed not also make its way along the connection between the fan and the output of the pull-switch, which is connected to the light?

As I see it, it will, but a the switch is off it wont do anything

also 2nd answer is it does (you said why isn't the momentary switch going to work the fan and the light (albeit only momentarily).

 

[ban-all-sheds]

As I see it, it will, but a the switch is off it wont do anything

Yes - the pullswitch is off, so there is no connection made between COM and L1, so there is no live feed to the light. Until you press the momentary switch which provides another route for electricity to reach the L1 terminal of the pullswitch. So the fact that the pullswitch is off is irrelevant - the light will be energised via the momentary switch.

 

[Big_Spark]

There may be a flow of electricity to the light, but it should not be enough to cause it to operate.

However if it did become an issue, some fans need the supply on for about 2 seconds in order to operate, then the simple expediance of placing an electrolytic capacitor on the feed from the pullcord leading to the fan would do the trick as it would block the voltage running from the fan to the pullcord, the wrong way.

Please ignore..this is not a serious suggestion, I'm just having a silly five minutes and being daft