How To Look At A House
McGarry and Madsen's home inspection blog for buyers of
site-built, mobile/manfuactured and modular homes
How can I figure out what a mystery wall switch does?
Tuesday, August 14, 2018
When you first move into a new home, there always seems to be at least one wall switch that’s a puzzle. What is it connected to? Here’s eight pointers to help determine what circuit the switch serves:
1) A switch in the room under an attic hatch opening may turn on a light in the attic. Sometimes the switch is installed in the ceiling next to the hatch opening.
2) Furnaces and air conditioning air handlers are required to have an electrical disconnect device within sight of the unit, so a service technician can shut off the power and be sure that it stays off while working on the system, to avoid an shock hazard. It is usually a circuit breaker or pull-disconnect in a small metal box, but is sometimes a regular wall switch. When it is turned off by someone assuming its a light switch, the furnace or air handler will not run. If your mystery switch is near the indoor unit of your HVAC system, try turning it off while the system is running as a test.
3) Wall switches in some rooms go to a wall or floor receptacle outlet, With the switch in the off position, check for any nearby receptacles that are dead, then turn it on to verify. Still no luck? The switch may be installed upside down, so try putting it in the on position and checking again for a dead receptacle. Also, some electricians indicate that a wall receptacle is switch-controlled by installing it upside down or using a brightly colored screw at the faceplate. And, to make it even more difficult, only the top or bottom outlet of the pair may be switch controlled.
4) The switch may be connected to a ceiling fan/light that requires a hand-held remote to operate after the switch is on. If there are no pull-chains on the side of the fan, that means it uses a remote. Look for a remote and try the switch again.
5) A switch next to an exterior door may serve floodlights under the soffits or post lights in the yard with dead bulbs. Try changing the bulbs in the exterior lights and trying the switch again.
6) The switch may be connected to an attic exhaust fan. Because these fans have a thermostat that acts as a secondary switch to activate the fan only when the attic is hot, the switch may not appear to be functional during cool weather. The fan would be visible in the attic on the underside of the roof sheathing or at a gable end.
7) Kitchen sink disposals and gas fireplaces often have a switch hidden in a nearby cabinet.
8) Also, if the previous owner removed the disposal—or some other long-gone appliance—it may just be an orphan switch.
Also, see our blog post How does a three-way switch work?
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To learn more about electrical wiring, devices, and receptacles, see these other blog posts:
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