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I have 2 bathrooms, each with both an extractor fan and a ceiling rose mounted light bulb. The fans are Greenwood Airvacs. They are 20 years old. The fans have automatic run ons of about 15 minutes after the bulbs are switched off with their respective cord pulls. It all works fine in both bathrooms.

I have now substituted the bulbs for LED bulbs. The fans now just run on for ever after turning the bulbs off. The only way to switch the fans off is via the isolator switches.

Putting the incandescent bulbs back restores proper fan operation. Swapping the LED bulbs from bathroom to bathroom makes each fan run on for ever. It's definitely something to do with the design of LED light bulbs.

EDIT: Taking the bulbs out, pulling the switch starts the fans. Then they run and stop. They stop with no bulb in the socket. So in summary, the fans operate correctly with incandescent bulbs, and no bulbs. They never stop with LED bulbs(!?)

One LED bulb is from ASDA (own brand) and the other is from Homebase ( a Phillips). They physically look different. There is no connection between them that I can ascertain.

Why do my extractor fans never stop with an LED bulb in the ceiling fitting?

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    What happens if no light bulb is installed? As you probably know, LED (and CFL) bulbs use a lot less power than an incandescent bulb. If no bulb installed also causes the fan to run continuously, then the device somehow depends on the amount of current drawn by the bulb. In that case, you probably have no choice but to either continue using incandescent or replace the fans. – DoxyLover Jan 5 '16 at 23:52
  • I think engineering creativity used to be more visible than it is now. The modern way to do that would probably have them entirely independent from each other except for a common go/stop control, but I'm guessing that this one uses the incandescent light bulb as part of a timing circuit. You can test this to see if a different-size incandescent bulb also produces a different time. – AaronD Jan 5 '16 at 23:52
  • If they can use more than one bulb, probably the easiest way to "solve" this would be to use a low-wattage (25W?) incandescent bulb for one of the bulbs, and an LED for the other. It's not ideal and may look strange, but it beats having to swap out the whole unit. – Johnny Jan 6 '16 at 1:09
  • @DoxyLover Will try your no bulb experiment in the daytime and report... – Paul Uszak Jan 6 '16 at 1:13
  • @DoxyLover Tried it. They stop with no bulb. The explanations here seems plausible till this test. – Paul Uszak Jan 6 '16 at 22:04
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schematic

The fan controller is monitoring the lamp circuit to know when the lamp turns off. It seems as though it relies on a low-resistance path to neutral through the lamp filament to run the timer.

The LED lamps have fancier electronics in them so there may not be a direct path to neutral if the electronics is not running.

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