Dear Mr. Electrician: How do I go about adding a bathroom fan switch to my existing bathroom light switch to control my bathroom fan separately?
Answer: Depending on how the home was originally wired, adding a bathroom fan switch could just be a matter of changing some connections at the existing light switch. Then you could install a combination device with two single pole switches on one strap.
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ADDING BATHROOM FAN SWITCH
I have had this request a few times, and in some cases, all I needed to do was open up the existing wall switch and make some changes. I would remove the wall plate, unscrew the screws holding the switch in the switch box, and pull the switch away from the wall to access the wires. Sometimes, the most difficult part is getting the paint off the wall plate screws.
Suppose there is a cable inside the electrical switch box that goes to the bathroom light fixture and another cable that goes to the bathroom fan. In that case, it is quite possible that a second switch could be added without installing additional wiring. See the bathroom fan wiring diagram below.
The original wiring would consist of a cable that brings the power to the switch box and the two cables that go to the fan and the light separately. Other cables could be present that supply power to other parts of the home. They would just be connected to the hot and neutral.
All of the white neutral wires would be spliced together using a wire nut or another type of wire connector. The black wires that go to the fan and to the light would be spliced together with a black pigtail which would be connected to the single wall switch.
The black wire from the power cable is connected to the other terminal on the switch. When the switch is turned on, power flows to both the fan and light at the same time.
Inside the wall switch electrical box, I would separate the black wire that goes to the bathroom fan from the black wire that goes to the light fixture. Then a combination switch device such as the one below is installed to replace the old single pole switch.
In the replacement scenario above, the existing hot wire in the switch box would connect to one of the black screw terminals. Then the black wire for the fan goes under one of the brass screws. The other black wire for the lighting goes under the other brass screw. The grounding conductor only goes under the green screw.
In the photo below, you can see the wiring terminals. Both screws on the left side are the hot connection which is obvious because of the break-off tab between them. It is possible to have two separate circuits feeding this device just by breaking off that one little tab.
ADDING WIRING FOR A NEW SWITCH
Because of the original installer’s wiring method, a quick wiring connection change is not always possible. In that case, the next choice would be to install a new separate cable up to the fan or to the light fixture to make it possible to have two separate switches.
Depicted below is a job where I replaced the entire bath fan unit and also installed a separate wall switch for the fan. The old existing bathroom exhaust fan was wired from the ceiling light fixture next to it. Therefore a new cable was needed from the switch location to the new bathroom fan.
The replacement of the fan unit was considered a repair and was not required to be inspected by the city electrical inspector. The new wiring and new switch box did have to be inspected, but because it was a small job, no rough-in inspection was needed. I applied for the electrical permit a few weeks earlier before starting the work.
The existing switch electrical box was full to capacity with wires, so it was necessary to install a two-gang old work switch box to be code compliant. Article 314.16 in the National Electrical Code tells how to calculate the cubic inch capacity required for each wire size.
I developed my own method of cutting access holes in walls where I needed to install wiring. I cut the hole holding my compass saw at an approximately 45-degree angle with the blade tilted inward. This makes it much easier to patch. See the photos at the end for the patch jobs.
There was a total of six wires plus grounds in the existing switch box. One black and white wire pair was the electrical feed supplying power to the box. Another black and white pair was supplying power from this switch box to the switch box for the hall light.
The last pair of wires went to the existing bathroom ceiling light. I labeled the black wire that is the LOAD that went to the light fixture with red electrical tape.
I needed to cut access holes in the ceiling and the wall to get the new cable down to the existing switch location. The ceiling had trusses for joists which made it easy to fish the new cable across.
The wall was more difficult because of the medicine cabinet, the closet behind this wall, and the close proximity to the door. The holes above were cut with the saw angled at 45 degrees.
I used to use my keyhole or my compass saw to cut these holes. However, since I purchased my oscillating multi-tool, I have been using it for cutting access holes in drywall.
After I cut the access holes in the drywall, I had to drill some holes using my half-inch Milwaukee Angle Drill. I had to drill up through the wall’s top plate to get into the ceiling. I also had to drill through some wall studs to get the new cable over to the existing switch location.
Above is the new Romex cable that I installed next to the existing wiring, which will be used again when the new switches are installed.
Before I removed the old switch box, I held the new old work plastic switch box on the wall and traced a line for cutting out the opening. It is easier to remove the old box with the bigger hole.
I start to pull the wires through into the new two-gang old-work switch box a little at a time. As I push the box closer to the wall, I continue to pull each wire until the box sits inside the wall.
Notice the sheet metal screw driven through the old-work box into the wood stud on the right side. This gives the box more stability.
PATCHING THE HOLES
The almost finished switch installation. When I added the bathroom fan switch, the homeowner wanted her existing locator light switch installed.
The holes above were patched by using the old pieces of drywall that were cut out. The drywall edges on the wall and the cut pieces were each buttered with a thick coat of joint compound. After the piece is pushed into place, some of the compound will ooze out. Use a wide-blade putty knife and smooth it down.
The joints do not need to be taped due to the angle cut of the drywall. Let the joint compound dry overnight. Next day smooth it a little with a damp sponge or fine sandpaper and then apply a second coat. A third coat may be needed to finish.
For large ceiling pieces, I will install a block of wood or two to screw the piece of drywall into to keep it in place until the compound dries.
The adding bathroom fan switch finished product above.
The old ceiling fan had a plastic housing which made removal easier. I just kept breaking pieces off with my pliers.
The new Panasonic bathroom fan housing with a dusty interior from my work. I usually wipe them down before installing the motor and grill. Panasonic fans are made for retrofitting into existing fan locations.
I particularly like the WhisperFit Panasonic fan models as they have a low profile and are rated for use with either 3-inch or 4-inch air ducts.
Above is the finished Panasonic bathroom fan grill, which is controlled by its own separate wall switch.
Below is a photo from a different bathroom remodeling job where the drywall was removed from the walls. You can see the wiring route the original electrician had to take to avoid the medicine cabinet.
The first thing that I did when I started this job was move that outlet up to the same height as the switches.
Below is my short video from YouTube about the same bath fan switch job.
My post with light switch wiring diagrams may be useful to you when adding a bathroom fan switch.
Click to see my blog post about grounding outlets and switches properly.
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