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I have a two-story house with blown-in insulation in the attic. There are at least four wall switches in my house (some on the ground floor, some upstairs) that seem to serve no discernible function. I have asked the previous owners of the house and the owners before that, but none of them know the function of these switches. The first family to occupy the house is no longer alive.

What techniques might I use to figure out what these mystery wall switches are for? I've tried all the outlets, and they don't seem to turn any of them on and off, and I can't find any mounted lights controlled by the switches.

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I had two switches by the front door of my newly purchased house that bedeviled me for weeks. I finally called the previous owner who told me her husband started to run new lights and never got past the switches and unpowered wires dropped to the basement. –  bib Sep 15 '12 at 21:35
If you have any built-ins nearby, they might have covered up a switched outlet without leaving it accessible. –  Jeremy W. Sherman Sep 16 '12 at 19:40
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2 Answers

up vote 7 down vote accepted

First thing I'd check is if the switch is even connected to any wires. You can probably check this just by removing the face plate.

You already checked the receptacles, but did you check both the top and bottom outlet? Sometimes a switch will only control one part of the outlet. In this case, the metal tab connecting both hot terminals is broken so that the top and bottom outlets can be individually powered (and switched).

The next step is to look at the wiring of the switch. First turn off the circuit breaker and test again with a non-contact tester (if you don't know what the switches do, then it could have more than 1 circuit in the box - just to be safe). Remove the switch and take note of the switch type - is it a single pole switch or 3-way switch? (If its a 3-way switch then the color of the traveler might be helpful in finding what it controls.) Is the line (from the circuit breaker) in this box?

If a visual inspection doesn't give you any clues then you need to trace the circuit. This is usually done with the power off using a circuit tracer:

Circuit Tracer

A telecom probe/tracer can also be used in many cases. You hook the tone generator up to the wires at the switch, and then you use the probe to follow the wires through the wall. With any luck you will find your way to a ceiling or wall box. Depending how badly you want to know, you might need to open the wall (or use a scope) at some point to help trace the wire. More expensive versions of this tool exist that might be needed to find really sneaky wires.

It's not unheard of to find out that a previous owner dry walled over an electrical box, so keep this in mind as a possibility.

And if you just can't sleep at night without know where it goes, you could always x-ray your house as a last resort.


(The above image is by photographer Nick Veasey. While this photo was stitched together, there are companies that x-ray buildings )

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Wow, what a thorough answer! –  Daniel Lawson Sep 15 '12 at 3:10
DEFINITELY open the switch plates up and see if there is any wiring at all - I've got spots in my house where instead of getting a blank plate, the previous owner just removed the wiring and closed it back up. Also, look at the wiring on the switch and the placement. It's not impossible that it's part of a 3-way that someone screwed up, and left the unused switch dangling. –  Michael Kohne Sep 15 '12 at 18:19
also remember it could be switching a specific outlet or plug on an outlet. its not unusual to have the top plug to be the only one switched so that it can turn the floor lamp on and off for the room. –  Kendrick Sep 16 '12 at 0:34
It's not uncommon for a half-switched outlet to be replaced without the tab being broken. –  Brad Gilbert Sep 16 '12 at 1:04
Our house has a 4-gang switch in the living room, could not figure out what 2 of them did. Finally pulled the cover and found switches had no wires connected. In the finished basement, there's a blank wallplate on a J-box next to a window, but no wires in the box. We'll probably never know what they were thinking... –  TomG Oct 15 '12 at 0:21
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I had one circuit in my house where the outlets were originally wired as split outlets - the top of all of them was switched, and the bottom was always on. Then, the owner replaced one of the outlets, but they didn't know what they were doing and forgot to break the tab on the outlet connecting the two hot sides together, which made them all on all the time. Breaking the tab on that outlet fixed the issue.

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