[                 ]
             ========[ Inline Vent Fan ]========> 
             ||      [_________________]
             ||                     ||
             ||                     ||
             ||                     ||
      ----------------- -------------------------
      |    Room A     |         |     Room B    |
      |               |         |               |
      |               |         |               |
      |               |         |               |
      |    SWITCH     |         |     SWITCH    |
      |               |         |               |
      -----------------         -----------------

It is possible to wire the diagram above, without electronics, just regular 120v Romex cables, such that when:

Room A (OFF) & Room B (OFF) = Fan (OFF)

Room A (ON) & Room B (OFF) = Fan (ON)

Room A (OFF) & Room B (ON) = Fan (ON)

Room A (ON) & Room B (ON) = Fan (ON) <<<< IMPORTANT

EDIT: Most importantly, I do not want one room to be able to turn OFF the fan, if the other room needs it on.

EDIT #2: Would this wiring diagram work? Should I use a double pole switch instead? Does it violate any electric codes?

enter image description here

EDIT #3: Foud a clear answer here: https://www.hvacquick.com/howtos/howto_wiring.php

  • This is basically a 3-way setup. Except for one slight complication. With 3-way switches, each switch flip toggles the fan on or off. Which means each switch itself isn't like a single switch ON/OFF but rather dependent on the others. So you don't get that last ON + ON = ON. Which is good if you want to be able to turn on/off from either location. But bad if you want either location to be able to force the fan on. Mar 3 at 5:47
  • 1
    I'm definitely looking for the latter case. I don't want one room to be able to switch off the fan, if the other room needs it on.
    – YarGnawh
    Mar 3 at 5:51

1 Answer 1


Easiest way is to run power to the fan and two ordinary switch loops from there, one to each switch.

Don't forget to run neutrals to the switches even if you don't need them. But a good idea for bathrooms would be to use timed switches rather than simple on/off ones. A person in either room can run the fan for 5, 10, 30 minutes or permanently. You can cancel the timer in a room but in this configuration it won't stop the fan if the other room still calls for it.

Note, this solution brings 3 cables with 11 wires into the fan in addition to its own internal wires. The fan's built-in junction box may be too small for that. If so, place a larger junction box nearby attached to a joist. I'm assuming it's an unfinished attic with available access. If that assumption is wrong, there are other ways.

  • I edited the question a bit, is there a way to accomplish using 3 way switches?
    – YarGnawh
    Mar 3 at 5:54
  • No. I missed an important point in your question and I have now changed the answer accordingly.
    – jay613
    Mar 3 at 6:09
  • In my defense, when I missed the important point, it wasn't literally labeled "important". That came later. There is hope for me. :)
    – jay613
    Mar 3 at 6:13
  • Haha, no worries. I added a wiring diagram. Is this what you meant by "two switch loops"?
    – YarGnawh
    Mar 3 at 13:23
  • 2
    The diagram added to the question is not how to wire switch loops and in fact is dangerous. The attic junction box should be the hub for everything. Nothing from the switches to the fan. The cables from the box to the panel and fan stay. Add a cable from the junction box to each of the switches. Those two cables should be /3, including a red wire. In the box connect the panel black to the two switch blacks, the fan black to the reds, and all the whites together. Look for a tutorial on switch loops. This q&a forum isn't great for tutorials.
    – jay613
    Mar 3 at 13:49

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