I just purchased a new bathroom fan/light with a 110 CFM fan, a Broan AER110SLW. I had a 50 CFM fan/light but it wasn’t powerful enough and was having mold issues on the ceiling. The problem is the new comes with a humidity sensor.

I have 2 three way switches. The power comes up on a 2 wire (black/white/ground) to one switch box (box A, bathroom). There’s a 3 wire (black/white/red/ground) going to the other switch box(box B, hallway). There’s an existing 2 wire going up from box A to fan/light and I just ran a new 3 wire from the power feed box A to the fan/light. How can I put constant power on the humidity sensor and have the fan/light come on together from the 2 three way switch locations? The new unit has a black, white, brown, gray and a ground.

  • 4
    Why are you mentioning "outlets"? Are you referring to the 3-way switch boxes?
    – nobody
    Apr 28, 2023 at 0:13
  • Yes I meant switch boxes. I just corrected it. Thank you!
    – Eric B
    Apr 28, 2023 at 3:32

1 Answer 1


Caveat: According to 2020 NEC, switch boxes in many living areas must have a neutral wire available in the box. Your box B does not (it only has hot and the traveler wires), but may be grandfathered in.

The online manual for this unit shows two sample wiring options:

From this, we can determine that the fan unit wires are as follows:

White - neutral (unswitched)

Ground - ground (unswitched)

Brown - auto humidity control (connect to hot to activate)

Grey - fan override (connect to hot to force fan on)

Black - LED light (connect to hot to turn on light)

Seems like you would connect your switched hot (output from the 2 3-way switches) to black and to grey (all 3 together) and an unswitched hot to the brown.

So, you need to run to the fan a white unswitched neutral, a ground, a black unswitched hot, and a wire that is the switched hot from your downstream 3 way switch.

It would be nice, though, to have some way to depower the hot to the humidity sensor without having to climb up and access the fan unit - perhaps add a separate humidity sensor single pole shutoff switch at switch boxe A?

Sample Diagrams From Manual: enter image description here

Edit: Proposed solution

There are two parts: switching the LED light/fan and supplying power to the humidity sensor.

Part 1: LED light/fan switching:

I found this diagram (annotation added by me) at MikeHolt.com showing how your light/fan switching should work. As noted above, your black switched hot should be connected to both black and gray on your light/fan.

Part 2: unswitched hot for humidity sensor:

Brown on light/fan should be connected to unswitched hot from box A

Pulling it all together for your wiring:

(Note: some connections in box A and light/fan are 3-wire connections)

  1. Ignore/disconnect/cap off your current 2-conductor black/white/ground wires running from box A to light/fan
  2. Make sure existing 3-conductor black/red/white/ground running from box A to box B is connected as shown in Mikeholt.com diagram to switch hot (black) with red and red-taped (important!) wires as the 2 travelers: Its black at box A is connected to incoming black hot; its black at box B is connected to 3rd terminal on 3-way switch; Its red and red-taped white at boxes A and B are connected to terminals 1 and 2 on both switches (travelers).
  3. Connect new 3-conductor black/red/white/ground running from box A to light/fan as follows: its black wire is connected to 3rd terminal on box A 3-way switch (switched hot); its black wire at light/fan is connected to both black and gray of light/fan; its white wire at box A is connected to incoming white neutral; its white wire at light/fan is connected to white wire there; its red wire at box A is connected to incoming black hot; its red wire at light/fan (unswitched hot) is connected to brown wire there to power humidity function.
  4. Grounds are connected together AND to metal boxes at all locations as standard wiring practice.

enter image description here

  • Ok I see that but do I need the 2 wire going up to the fan/light. Or can I use just the 2 3 wires?
    – Eric B
    Apr 28, 2023 at 4:07
  • @EricB Please post a diagram of the various wiring and boxes - I'm a bit confused on what is where.
    – Armand
    Apr 28, 2023 at 4:08
  • I posted the power box which is in the bathroom. There’s a 2 wire bringing power up. A 3 wire going to the switch box in the hallway. A 2 wire that was going up to the old fan and the blue 3 wire I ran up to the fan/light.
    – Eric B
    Apr 28, 2023 at 4:14
  • @I don't see any diagram or photo yet. If you could just name the boxes and fan unit (eg A B C etc) and indicate which wires go into each box and out of each box.
    – Armand
    Apr 28, 2023 at 4:27
  • Box A(Bathroom)- 2 wire feed, 3 way switch, 3 wire going to box B, 2 wire going up to fan/light, 3 wire I just ran to fan/light. Box B(Hallway)- 3 wire coming from box A directly hooked up to a 3 way switch. Box C(Fan/Light)- 3 wire coming from box A and a 2 wire coming from box A. I just need to know how to hook the wires up. Thank you for trying to help.
    – Eric B
    Apr 28, 2023 at 6:44

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