Dear Mr. Electrician: How do I add a switch to my existing bathroom light switch to control my bathroom fan separately?
Answer: Depending on how the home was originally wired, it could just be a matter of changing some connections at the existing light switch. Then you could install a device with two single pole switches on one strap. NOTE: Some text links below go to applicable products on Amazon or EBay.
METHODS OF ADDING A SEPARATE BATH FAN SWITCH
I have had this request a few times and in some cases all I needed to do was open the existing switch up. I removed the wall plate, unscrewed the screws holding the switch in the switch box, and pulled the switch out away from the wall so I could access the wires. Sometimes getting the paint off the wall plate screws is the most difficult part.
Inside of the wall switch electrical box I separated 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 light 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 BATH FAN SWITCH
Because of the original installer’s method of wiring, 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 existing bathroom exhaust fan was wired from the ceiling light fixture next to it.
The replacement of the fan unit was considered a repair and not required to be inspected by a 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 saw at an approximate 45 degree angle with the blade tilted inward. This makes it much easier for patching. See the photos at the end for the patch jobs.
I labeled the black wire that is the LOAD that goes to the existing light fixture with red electrical tape.
I needed to cut access holes in the ceiling and the wall in order 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. However the wall was more difficult because of the medicine cabinet, 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 for cutting 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 Milwaukee Angle Drill. I had to drill up through the top plate of the wall to get into the ceiling. I also had to drill through some of the wall studs in order 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.
I start to pull the wires through into the new 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.
PATCHING THE HOLES
The almost finished switch installation. The homeowner wanted her old locater light switch installed with the new bath fan switch.
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 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 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 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. I usually wipe them down before installing the motor and grill. Panasonic fans are made for retro fitting into existing fan openings.
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 job.
My post with light switch wiring diagrams may be useful to you.
I am currently writing a book about replacing bathroom fans. I have no idea when it will be completed. If you want to repair an existing bathroom fan, visit my post and you can download my FREE bath fan repair book.