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There is one switch that controls the lights and fan in the bathroom. Users turn the light/fan on when they enter, and off when they leave.

I'd like for the bathroom fan to run for a time after they leave, after turning off the lights.

Is there a device, some sort of delay relay perhaps, that would help me do this without altering user behavior?

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What ever device you find, make sure it's rated for motor loads. – Tester101 Jun 6 '12 at 15:58
Yes, there're consumer devices exactly for this application. No idea how they are properly named in English. – sharptooth Jun 6 '12 at 16:23
What about using an occupancy sensor like this – Tester101 Jun 6 '12 at 16:30
A recent comparison of two timer switches: marco.org/2012/05/06/bathroom-fan-timer-switches – William Jackson Jun 6 '12 at 20:18
I was thinking about a vacancy sensor (manual on - sensor off) that is rated for a motor load, but I can't find any. – lqlarry Jun 7 '12 at 0:31
up vote 11 down vote accepted

You want a timer switch. They are quite common. As Tester101 states, you have to make sure the switch is either:

a. rated for the load of the motor (most should be for a standard fan) or, if not...

b. you need to have the switch control a relay, which in turn will turn the fan off/on. I did this to install the lutron timers that I like on a ceiling heater unit (which was more of a load than the fan):

enter image description here

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You are looking for a "delayed off switch"

This is the first one that I found on a Google search

Fan Delay Timer Switch

I have not used this product before.

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I find is strange it is not already fitted with one. Fan timers that run of the lighting circuits have been around for a long time and they are very standard in all homes and commercial property.

If your fan does not have one it is normally just a case of replacing the fan itself as the timer is normally built into it. You do get standalone timers that you can add to the fan but they are a lot less common. It might be more cost effective to just replace the fan unit as they are fairly cheap. I would hope also you have a triple pole isolator as well but since it does not have the timer control it makes me dubious that you would. They are meant to be fitted within a close distance of the fan itself.

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Here in the USA I have never seen a bathroom fan timer in a residential bathroom. I see them in hotels and commercial settings all the time though. How would just replacing the fan work? If the wall switch removes power completely from the fan/light unit, then the timer does no good. – Dave Nay Jun 6 '12 at 16:30
Odd I see them in all domestics. Guess thats a regional thing. As for the power issue, Fans have a L1, L2, N connections. They take a neutral and a live from the lighting circuit and the light switch switches the L2 connection therefore the fan has power after the switch is turned off but it starts the timer. that is also why you need a triple pole isolator to make it safe if working on. – Ryan Walkowski Jun 6 '12 at 16:44
A timer switch costs pennies more. At least in America, being the land of cheaply built houses, those pennies mean many do not come built with those standard. – DA01 Jun 6 '12 at 18:09
Well while I was originally from the U.K I have lived in Massachusetts for 7 years and the above is still common here. It was the standard back home also. As for been cost effective the above method does not require a separate or additional switch/controller it works of a standard light fitting thus your only purchase is the fan which has the electronic timer built in. But all in all it's just semantics. They do the same job – Ryan Walkowski Jun 6 '12 at 19:28
Broan makes a vent fan that has a motion switch built into the fan. Also moisture sensing available. – lqlarry Jun 7 '12 at 0:33

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