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Old 03-15-2010, 08:23 AM   #1
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Wiring inline bath fan for multiple bathrooms


I am planning to install bath fans for my parents' two upstairs bathrooms. Both are below the same unfinished attic, so running ductwork and electrical is no problem. To cut down on noise (and holes in the house), I am thinking of putting one large inline panasonic fan in the attic, and attaching the ducts with a 'Y' to both bathrooms.

Ideally, I would like each bathroom to have a countdown timer so the fan can run for 10-15 minutes after leaving the bathroom. What is the best way to wire it? For my own bathroom, I got one of these Lutron timers:
http://www.amazon.com/Lutron-MA-T51-...8659218&sr=8-1

It only has two wires (+ground), so it is obviously not a 3-way. Would I need to simply find a 3-way countdown timer? Do they exist?

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Old 03-15-2010, 09:01 AM   #2
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Wiring inline bath fan for multiple bathrooms


You can have separate ordinary countdown timers, one in each bathroom. You would need to run 3 wire cable for electronic timers requiring house current power; two wire cable will suffice for spring wound timers and battery operated timers.

At the fan connect the fan's (black; hot) power lead to both blacks going down to the timers. Connect the power feed up at the fan box to both reds (whites if two wire Romex) going down to the timers.

For the 3 wire cable connect neutral to both whites going to the respective timers.

This way, turning either timer will turn on the fan. The fan shuts off when both timers are off. This is not a 3-way and does not need to be.

One disadvantage of a shared fan is that inside heated air or air-conditioned air is being sucked from both bathrooms whenever either bathroom is in use.

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Last edited by AllanJ; 03-15-2010 at 09:10 AM.
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Old 03-15-2010, 09:11 AM   #3
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Wiring inline bath fan for multiple bathrooms


This will be an electronic timer (neutral required) so i'll need 3-wire cable.

So basically, I am running two switches in parallel? What happens when both switches are ON? It gets feeds from two "different" sources? (i know, essentially the same source). This doesn't cause a problem? I assume not or you wouldn't have suggested it.
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Old 03-15-2010, 09:11 AM   #4
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Wiring inline bath fan for multiple bathrooms


and make sure the circuit used at each timer is the same circuit (as in fed from the exact same breaker in the panel)

I know, it seems like a no brainer but trust me, it needs to be said.
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Old 03-15-2010, 09:13 AM   #5
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Wiring inline bath fan for multiple bathrooms


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and make sure the circuit used at each timer is the same circuit (as in fed from the exact same breaker in the panel)

I know, it seems like a no brainer but trust me, it needs to be said.
Of course...i guess then I'd be getting two sources and pushing 240v to the fan?

I never thought of running switch loops in parallel to the same load. That's what I'd be doing, right?
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Old 03-15-2010, 09:16 AM   #6
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Wiring inline bath fan for multiple bathrooms


As long as the power is at the fan as Allan described there is no possibilty of having two sources. With both switches on there will still be only 120 volts.
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Old 03-15-2010, 09:24 AM   #7
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Wiring inline bath fan for multiple bathrooms


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Of course...i guess then I'd be getting two sources and pushing 240v to the fan?

I never thought of running switch loops in parallel to the same load. That's what I'd be doing, right?
2 reasons you run the same circuit.

1. if you pulled circuits from opposing legs, you would trip breakers every time both timers were on

2. if the circuits were on the same leg of the service, you would not trip breakers but you would have to turn off 2 breakers to kill the power to the fan. You also would feed power back to the other timer if both breakers were not turned off. Either situation could cause an unsafe condition if you needed to work on anything involved in the circuitry.
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Old 03-15-2010, 09:25 AM   #8
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Wiring inline bath fan for multiple bathrooms


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As long as the power is at the fan as Allan described there is no possibilty of having two sources. With both switches on there will still be only 120 volts.
that's true but if there are already fans and switches installed, it would be a very easy mistake to make to simply continue the existing wiring to the fan.

we are dealing with DIY. I would much rather take the time to explain what and why than leave it unanswered. I have been around this stuff long enough to know what some folks will do if you do not explain to them what not to do as well as what to do.
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Old 03-15-2010, 09:49 AM   #9
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Wiring inline bath fan for multiple bathrooms


Thanks to everyone for the input.

Now that I think about it, the timer switches do not require a neutral. I may run a neutral to the switches anyway, in case a future timer does...but does this diagram look correct, aside from grounds?
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Old 03-15-2010, 09:49 AM   #10
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Wiring inline bath fan for multiple bathrooms


Wire the switches in parallel. That way if either switch is on the fan will be on. Only if both swtiches are off will the fan go off. You don't want threeways. With threeways if someone goes into the room and the fan is on they could turn it off without knowing.

Diagram will work. However to meet code the power to the switch should be the white and the switched power to the fan should be the black. That way you don't have two whites connecting at the fan.
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Old 03-15-2010, 09:52 AM   #11
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Wiring inline bath fan for multiple bathrooms


I always get that backwards. Thanks Joed.
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Old 03-15-2010, 01:37 PM   #12
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Wiring inline bath fan for multiple bathrooms


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One disadvantage of a shared fan is that inside heated air or air-conditioned air is being sucked from both bathrooms whenever either bathroom is in use.
Do they make a fan grill that you can close? Only one bathroom gets a lot of use, the other one is rarely used, so it'd make sense to just close the unused one.

I can post this question in the HVAC forum, just figured it was worth a quick mention here.
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Old 03-15-2010, 01:54 PM   #13
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Wiring inline bath fan for multiple bathrooms


Now you are getting complicated.


yes, there is such thing as a powered louver or more appropriately, a powered damper. The problem now comes where you can power that damper only when the timer in the involved bathroom is on and not open the damper for the other bathroom at the same time.


now you are going to have to incorporate a couple relays to isolate the two switching systems. I will have to draw something out. The old picture is worth a thousand words and all and hopefully somebody else comes back with a drawing before I do.
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Old 03-15-2010, 01:57 PM   #14
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Wiring inline bath fan for multiple bathrooms


It could also be a manually closed vent grille....but a powered one would be cool

Probably too expensive though.

I'd be interested to see yours (or someone's) drawing.
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Old 03-15-2010, 01:59 PM   #15
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Wiring inline bath fan for multiple bathrooms


Yes, you would get a small louver commonly used for hot air heating registers. With the shared fan and ducts leading to each bathroom, you probably have a custom made box where the duct reaches the wall (or ceiling) and you would choose a shape and size of louver to go on the surface. For a manually operated louver you just have to remember to open and close it at the appropriate times.

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