I bought a bathroom vent fan with humidity sensor but no light. It unexpectedly arrived with 4 wires: ground, black, white, and blue. I only have 12-2 cable in the bathroom and no extra switch. This is a bugger of a space to run another wire and I am not inclined to do so for this.

I believe the blue wire is to run on automatic mode to sense humidity while the others are for manual operation. How do I wire it to existing 12-2 cable so that the humidity sensor is always on and the switch turns the fan on manually when I want to run it for other purposes (odors)?

  • 6
    Did you read the instructions? Aug 3, 2017 at 17:32
  • 3
    What is the make and model of the fan? So that we can read the instructions.
    – Tester101
    Aug 3, 2017 at 17:47
  • Can you post photos of the insides of the boxes? Aug 12, 2017 at 4:00

1 Answer 1


Option 1: smart switches

In this case you locate and obtain a "smart switch" transmitter/receiver combo, where the transmitter sits in a wired switch location and the receiver sits up in the fan housing. The switch and receiver must communicate wirelessly.

Then you re-task the black wire in your switch run to be always hot for all of these loads: the smart switch, the reciever, and the automatic humdity sensor.

Option 2: you are lucky and the power goes to the fan first

It sounds like you don't... But if the power comes initially to the fan housing (2 Romex's there and only one in the switch box) then the fan housing already contains everything you need. The supply gives ground, neutral and always-hot. The switch loop provides your switched-hot. You are all set.

(Though I don't think you'd be here if that was the case).

Option 3: You don't.

Aside from the smart-switch method I described, you are out of luck. You cannot bring always-hot and also switched-hot using a single black wire. White must be neutral and bare must be ground. Cheating ground in some way is a recipe for disaster in a bathroom. Even a GFCI does not make this safe.

  • If his switch is on a loop, then he can get this done just fine without a smart switch... Aug 4, 2017 at 0:56
  • @ThreePhaseEel good point. But I doubt he'd be here if that was the case. Aug 4, 2017 at 1:11
  • Delta BreezGreenBuilder GBR80H 80 CFM Exhaust Bath Fan with Adjustable Humidity Sensor and Speed Control Manual:deltabreez.com/Upload/SalesBinder/5_IM/3_GreenBuilder/…
    – H. F.
    Aug 4, 2017 at 18:06
  • And look what I found for a single switch (still requiring 12/3): deltabreez.com/Upload/SalesBinder/5_IM/…
    – H. F.
    Aug 4, 2017 at 18:06
  • I am not sure how to know if it is on a switch loop. The wiring in this house is wonky and last time I converted a single pole switch to a timer I had a heckuva time figuring out what they had done. I ran a new line then and it was crazy difficult. Took months to figure out and I was very frustrated. They had some cheesy jumper in the switch box that everyone said was WRONG. I got rid of that. I would have to check if the power goes to the fan or the switch first. It is easy to change either way since I put that new line in and the junction box for power is right by the fan itself.
    – H. F.
    Aug 4, 2017 at 18:10

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