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I just bought a Panasonic FV-20NLF1 inline bath exhaust fan that I planned to used to exhaust 2 different bathrooms. I want each bathroom light switch to control the fan, so when the lighting turns on in either bathroom, the fan will go on. If both light switches are off, the fan will go off. If one light is on the fan will stay on. Could someone share how I could accomplish this, ideally in a diagram?

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  • Do I understand that each bath will have an independent light but a common fan all on the same switch?
    – mikes
    Dec 29, 2012 at 21:16
  • Is the fan large enough to adequately exhaust both rooms at the same time? Do the fan and lights have to be on the same switch, or can there be independent switches for lights and fan?
    – Tester101
    Dec 30, 2012 at 13:22
  • The easiest option is to install two fans, one for each room. If you need a single duct, you may be able to hook that up with dampers to prevent the exhaust from going back into the house.
    – BMitch ♦
    Dec 30, 2012 at 18:04
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    As Tester101 says, if you use a separate switch for the fan (one in each bathroom) this would be quite simple. With this option, you could also use a countdown timer, which is a great way to control bathroom fans. They come with different time ranges: max 15 minutes is great for powder rooms (smells..) while 30 or 60 is good for humidity from showers. This keeps the fan running for a bit after the light shuts off, though you do have to remember to actually turn it on.
    – gregmac
    Dec 31, 2012 at 3:53

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I don't think it's possible to individually control two different lights while controlling a common fan using conventional house wiring devices. Powering the fan will backfeed power to the other light that was supposed to stay off. But I look forward to being proven wrong.

It can be done if both lights and the fan all go on at the same time. This is just a standard 3-way switch circuit. Alternately, wire two standard switches in parallel so one bath user would not accidentally turn off the light for the other. This can be confusing as one switch may not always turn off the lights.

To have only one light go on with a particular switch, you need a pair of single throw-double pole switches where one pole of each switch goes to it's associated light and the other pole of each switch both go to the fan. While such a switch is common in electronics, I don't know if such a device is available for house wiring.

Alternately, a double pole relay could be used to switch the fan. Either pole powers the fan, but the separate coils keep each circuit isolated from the other. Sorry I'm not illustrating this, I need to take some time to learn how to add drawings here, I know it's fairly easy. In the mean time, if anyone gets what I'm saying, feel free to add some illustrations.

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  • +1 for the single throw-double pole switch idea. That's typically used for 220v switches where you have two hots, but electrically it would work for a single hot to two activities. That said, I don't know if it's up to code, and I'm sure the next electrician would be thoroughly confused.
    – BMitch ♦
    Dec 30, 2012 at 18:02
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    I don't see why a DPST switch would be against code. The concern I'd have would be the power source. If the bathrooms are on a different breaker, you cause a potentially dangerous situation: turning off the breaker for bathroom 1 would mean that if someone turned on the switch in bathroom 2, it would energizing one pole of the switch in bathroom 1. If they're on the same breaker, or OP could run a common source for the fan to both switches, this situation wouldn't be a problem. At the VERY least (and this may be a code issue) swap in a double-pole breaker to power both bathrooms.
    – gregmac
    Dec 31, 2012 at 3:44
  • Also, this calls for labels: putting something like "to fan and bathroom 2 switch" on the wire would be immensely helpful to any electrician in the future.
    – gregmac
    Dec 31, 2012 at 3:46
  • You don't want two out of phase circuits supplying each bathroom. If both switches are on, you'll get a 220v short! In phase is better, but you have potentially cross connected circuits, a big non-no. This concept only really works at all if it all involves the same circuit. Only the relay idea would work if multiple circuits are involved.
    – bcworkz
    Dec 31, 2012 at 23:12
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You have one option that you could consider. Leave the existing single pole switches to control the lights in each of the two areas but remove fan wiring from these two switches. Then add a 3-way wiring setup and two switches, one at each location, to control the fan interdependent of the lights. This of course would wire the fans up using conventional 3-way switches and the two wiring travellers running between the switch locations.

This approach as a distinct advantage of allowing the use of low cost readily available components with which to implement the solution. Standard 3-way switches, electrical boxes and multi-conductor wiring is available at most hardware stores and home centers. If installed correctly it will meet your local electrical code and regulations.

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  • This answer was migrated from an identical question.
    – BMitch ♦
    Jan 3, 2013 at 16:31
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A second option that you have is to add a pair of relays driven one each from the two lighting circuits. The relays would have coils sensitive to the AC line voltage applicable to your area. Each coil would be wired in parallel (across) the light fixtures. The output contacts of the two relays would be wired in parallel to the ventilation fan so that either light on would relay over to the fan so it would be on.

One difficulty with this approach is procuring the suitable components and then installing and packaging the solution in a safe and legal manner following all applicable electrical codes and regulations.

If you were to want to attempt to go with this relay approach I would suggest that you deploy some industrial style solid state relays that are designed to accept a wide range AC input and capable of driving the vent fan. One possible component that could be used, as pictured below, can be mounted inside the bottom of a larger metal electrical box, two side by side, and then wired appropriately. This style of component has isolated mounting holes to facilitate mounting to the electrical box and convenient screw terminals for wiring up to the lighting and fan circuits.

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Here is the distributor that could supply the above pictured component.

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  • This answer was migrated from an identical question.
    – BMitch ♦
    Jan 3, 2013 at 16:32
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I had this problem a while back - simple, use a two gang 2-way switch and wire the fan up as if it was a 2-way lighting circuit, one switch controls the fan, the other controls the light - no backfeed, fans operate when needed (client will have no automatic control however, unless you install a flush PIR in between switch and fan) Dont forget your 3-pole isolator switch however!

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There are two approaches to this problem:

  1. use dual pole switches to control the fan and each light simultaneously -- this is the best approach if the lights and fan all are on the same circuit from the panel
  2. use a dual relay to control the fan circuit from the light circuits -- this is the best approach if the fan and lights are on separate circuits, or if the lights in each bathroom are on different circuits from each other.
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