Controlling one fan with two switches

**thebeave** · 03-27-2011, 09:50 AM

I'm considering installing a Panasonic Inline Vent fan (FV-10NLF1) to vent two bathrooms that are currently not vented (much mildew buildup currently). Basically, I'd like the fan to turn on when the main light switch in either (or both) bathroom is turned on. I'd rather not have a switch separate from the light, as there are kids (and adults) with faulty memories who probably won't remember to turn on the fan when taking a shower. These fans advertise themselves as being great for venting two separate rooms, so its not like this idea is out of the ordinary.

What I'm currently thinking of doing is using the first bathroom to provide power (and switching) for the fan, and then replace the switch in the second bathroom with a double pole toggle switch. The first pole would control that bathroom's light, and the second pole of the switch would be wired in parallel with the switch in the first bathroom, to turn on the fan. Does that make sense? Or am I making this unnecessarily complicated? Hopefully, that configuration is NEC compliant, but I've not run across it before. thanks for all your help!

**LLigetfa** · 03-27-2011, 10:35 AM

I'm sure JW will come along and address any code issues. One possible concern I would have is potentially having two different breakers supplying the one box and exceeding the allowed number of conductors in the box. Powering a couple of relays and a timer may be a consideration.

A fan should be left to run for a length of time that may be longer than what the room is occupied. I have a HRV and use low voltage crank timers but of course they require manual activation.

**drick** · 03-27-2011, 10:59 AM

Multiple circuits in one box is not a problem, but your wire count may be. Assuming you can address the box wire count issue and your bathroom outlets are on a separate circuit from the lighting I see no problem with it.

The bigger question I have is why would you want to blow conditioned air in an unused bathroom out of the house. Seems like a waste to me.

-rick

**jwelectric** · 03-27-2011, 11:56 AM

If and only if both bath lights are on the same circuit just run from the light to the fan from both baths. It won’t hurt a thing if both switches are on at the same time but again only if both bath lights are on the same circuit

**thebeave** · 03-27-2011, 01:47 PM

I'm pretty sure the two bathroom lights are on the same circuit, but in case they're not, what are the wiring options available to me?

**jadnashua** · 03-27-2011, 01:49 PM

Originally Posted by jwelectric

If and only if both bath lights are on the same circuit just run from the light to the fan from both baths. It won’t hurt a thing if both switches are on at the same time but again only if both bath lights are on the same circuit

To expound on this, if they aren't on the same circuit, you would be backfeeding the other when a switch was on, potentially exposing the things to both legs (and 240vac), and could really messing things up!

**drick** · 03-27-2011, 01:59 PM

The OP said he was going to use a double pole switch. He would need double pole switches in both bathrooms however - not just one bathroom - to keep the lights and the fan separate. As long as he does this there should be not problem with the lights in the different bathrooms being on different circuits. (Or can you not have a single device connected to two circuits without a handle tie at the breakers?)

-rick

**LLigetfa** · 03-27-2011, 02:11 PM

Originally Posted by jwelectric

If and only if both bath lights are on the same circuit just run from the light to the fan from both baths. It won’t hurt a thing if both switches are on at the same time but again only if both bath lights are on the same circuit

I'd like to see a proposed schematic on that. So, either switch would turn on both lights and the fan? If someone tried to turn off the light in the other bathroom but instead turned it on, when the other bathroom is vacated and the light switched off, confusion may ensue.

At least with relays, the lights could still be independent. They make current sensing relays that remain electrically isolated from the lights. Some even have timers to run the fan for a while after the trigger has been removed.

http://www.google.com/products?q=current+sensor+relay

**jadnashua** · 03-27-2011, 04:16 PM

What he was trying to say was use one pole to turn the local light on, and the other pole to turn the fan on. The light could be on a different leg, but the fan would still need to be on the same leg as from the other on/off location that was controlling the fan. This would not be a typical 3-way installation, but a two location parallel OR'ing of the power. IOW, you couldn't turn the fan off from either location unless it was the only switch that was on.

**jwelectric** · 03-27-2011, 04:26 PM

Originally Posted by LLigetfa

I'd like to see a proposed schematic on that. So, either switch would turn on both lights and the fan? If someone tried to turn off the light in the other bathroom but instead turned it on, when the other bathroom is vacated and the light switched off, confusion may ensue.

didn't say it was a good idea just that it would work

**jch** · 03-31-2011, 08:11 AM

I was going to need a similar setup within *one* bathroom so that either a dehumidistat *or* an electronic timer would force the fan on.

I would run power to both the dehumistat and the timer boxes and they would both be wired in parallel to apply current to the fan's hot wire.

The dehumidistat is mechanical, so should be no problem.

However, the timer is electronic and am not sure whether it'll like having power applied to its output (by the dehumidistat) when the timer is supposedly "off"....

Anyone tried this?