How can I wire my bath fan to have automatic humidity control and manual switching?

Question

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)?

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.