I'm planning on replacing two old bathroom fans with a single inline model installed in the attic (Panasonic WhisperLine). It will be connected with the two bathrooms through a Y-duct and exhaust outside through one exhaust vent in the roof.

There is an existing question from many years ago, but I still don't quite understand how to control this from both bathrooms.

If I want a countdown timer, can I install one in each bathroom and connect both sets to the fan in a boolean OR configuration? Do the circuits need to be isolated, like with a diode? Is there a solution that puts a single timer near the fan and is triggered by switches in the bathrooms?

  • I think you'd have better results if each bathroom had its own dedicated fan.
    – JACK
    Commented Aug 17, 2020 at 15:48
  • Can you specify exactly what it is that's confusing you? This sounds like a standard 3-way switch setup, except that your switches are timers instead. I believe you'd need 3-way timers to be able to do this. I don't see anything in the answers to the linked question mentioning anything about diodes, so I can see where you're quite confused...
    – FreeMan
    Commented Aug 17, 2020 at 16:07
  • Will one fan will being able to draw moist air sufficiently enough to evacuate two spaces at the same time? Is it specifically rated for the combined square footage and duct length in question. ?
    – Alaska Man
    Commented Aug 17, 2020 at 19:21
  • The inline fans are made explicitly for this purpose. I'm getting the 240 CFM model. The install kits have a Y-duct for this purpose. The reason I'm doing this instead of replacing the dead fans is that the existing fans are 1950s models that are oddly shaped and we want to keep the retro look of the existing grill. Inline fans allow us to reduce all the cutting and patching of drywall and plaster.
    – guidoism
    Commented Aug 17, 2020 at 20:26

1 Answer 1


This is the classic "DC electronics guy thinking schematic. Here, you need to think wiring diagram due to NEC 300.3(B) and the eddy currents induced by AC EMF. So wiring routes must be a "tree" topology; no loops or bubbles.

All wiring throws EMFs. For steady-state DC power, it's like a refrigerator magnet. Not for AC power; its constant changing causes powerful EMF effects.

As long as all related conductors are tight-packed in the same cable, these EMF effects cancel each other out. Say there's 6A on hot1 and 4A on hot2 both returning 10A on neutral. That's 10A of EMF that are equal and opposite, and cancel each other out, and that is fine.

Anytime AC power goes out one path and back another, that encircled area becomes the core of a transformer. So if hot2 took a different path, now you have 4 ampere-turns being induced into the encirclement, which will cause vibration (leading to wire cracking leading to overheat), eddy current heating in anything ferrous or nonferrous, etc. That's why we don't do that thing.

However, what happens inside a junction box is something we are resigned to. And it helps for EMFs to use a metal box. I don't know why people are so in love with plastic boxes, SMH.

So here's the plan

Wire it with a /2 cable from supply to timer1, then /3 to timer2, then /2 to the fan. This is "tree" topology and complies with 300.3(B).

Supply to time1 carries always-hot (black), neutral (white) and of course safety earthing.

Timer1 to time2 contains always-hot (black), switched-hot (red), neutral (white), and earth.

Timer2 to fan contains switched-hot (black remarked red), neutral (white) and earth.

The remarking of black to red is not a Code requirement but it makes the circuit easier to understand by making colors match wire function.

At timer1, the timer connects to always-hot (black) and switched-hot (red).

At timer2, ditto.

Wow, that was easy.

  • 2
    Fascinating. And it works. I drew it out, because it was a bit confusing. It is functionally: Hot -> T1 & T2 -> Switched Hot. But practically, it is as if that circuit was squished and stretched in order for everything to parallel correctly. Effectively turning the usual 3-way XOR wiring into OR (as requested by OP). Commented Aug 17, 2020 at 19:33
  • This is great. Thank you! I've drawn it out too and trying to understand better.
    – guidoism
    Commented Aug 17, 2020 at 20:28
  • I'm gonna have to draw this out but will give you the benefit of the doubt..lol +1
    – JACK
    Commented Aug 19, 2020 at 19:27
  • Perfect example of why you always turn off the breaker before doing any work. Some hotshot could remove live power to timer 1 and assume the timer was dead. Someone flips timer 2 on and now timer 1 switched hot,red, is hot... and zap!
    – JACK
    Commented Aug 19, 2020 at 22:26

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