Wiring multifunction fixture to 2 switches, 2-wire cables

Question

I'm replacing a fan in a bathroom. The setup I have currently cannot be changed with new wiring. I can add pigtails but I can't run a new 3-wire cable through the chase. Power supply runs to the fan box first. The old combo fan and light were independently operated on a double switch. 2 separate 2-wire (plus ground) cables are running from the fan box to each side of the double switch. I don't know how the old fan-light combo was wired but the double switch had the grounds tied together in the switch box (but not grounded to anything); 2 white wires hooked up to two black screws on the right side of the double switch; and 2 black wires hooked up to the brass-colored screws on the left side. There were no other wires in the switch box and no jumpers or pigtails there.

The fan that I purchased is the Delta Breeze Green Builder with combination LED light, fan, and automatic humidity sensor GBR80HLED. The fan comes with three pairs of wire and a ground. One pair is white and black labeled power. One pair is white and blue labeled mode. One pair is white and red and not labeled. Instructions do not show how to wire with power to fixture first. They show using either three or two switches connecting blue to blue and red to red, neither of which I have in my cabling.

I would like to use existing wires and set it up so that mode (humidity sensor) is always on, one switch controls light, and one switch controls manual on fan for odors. Help with a diagram?

Answer 1

Color-coding. It is your friend. You'll need some black, red and blue electrical tape.

1. Go into the switch box. Confirm that, in fact, the white and black from cable 1 are across switch 1... And the white and black from cable 2 are across switch 2... (that is to say, they are not criss-crossed). If they are criss-crossed, that is bad, make them square.
2. Also in the switch box, tape both of the white wires black -- by which I mean wrap several wraps of tape around the wire an inch or two back from the ends.
3. Up in the fan box: Note the two cables coming up from the switch. Tape both of the white wires black. We are re-designating these white wires to be "always-hot". **
4. Note the two black wires coming from the switch (partners to the whites you just taped). Tape one of those black wires red.
5. Tape the other black wire blue. This is also a switched-hot.
6. Note the third cable coming from supply. It has a black wire. Nut it together with the two whites now taped black.
7. Now look at the wires which remain loose. You see a white wire (neutral, from supply), a "red" wire (switched hot from switch), and a "blue" wire (switched hot from other switch). Also a black bundle pushed back into the box. These are exactly the same wires you'd have on a /4 cable.
8. Wire it per factory instructions.
9. If the fan needs "always-hot" for some reason, tap it off the black/"black"/"black" bundle you pushed into the back of the box in step 3. It looks like for the humidity sensor to work, the black wire must be connected to power. The blue wire is an override telling it to run full speed regardless of humidity.

If you are not happy with which switch is the fan and which the light, switch them up top. Do not switch hots at the switch, that would create the crisscross situation we labored to avoid in step 1.

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** When you are forced to use a white wire as "hot", like in a switch loop, there's an advantage to using the white as the always-hot wire. It's far easier to detect with a voltage tester since it's always-hot. If it was used for switched-hot, a person might mistake it for a neutral since it is not energized with the switch off. White wires reused as hots MUST be taped.