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:

 • What is the difference between what trips a GFCI (ground fault) receptacle and a circuit breaker?

What is the code requirement for GFCI protection for receptacles near a wet bar sink? 

What is the requirement for a service receptacle outlet for heating, air conditioning, and refrigeration (HACR) equipment? 

Why is an opening in the wall around the side of an electrical receptacle outlet a safety defect?  

When was GFCI-protection for kitchen dishwasher receptacle outlet first required? 

What is allowable voltage range at a wall receptacle outlet in a house?

When should I replace electric receptacle outlets? 

Does an electric receptacle outlet in a storage shed require GFCI protection? 

What are "self-contained" electrical receptacle outlets and switches?

What is the difference between an electrical receptacle, an outlet, and a plug?

Does a washing machine receptacle outlet require GFCI protection?

What is the building code requirement for receptacle outlets at stairs and stair landings?

Can I remove a 240-volt range receptacle and hard-wire the range? 

What is a "backstab" receptacle outlet? 

Why are some electric receptacle outlets upside down (ground slot up) in a house?   

What is the height requirement for an electric receptacle outlet? 

Where are GFCI receptacle outlets required?

When were GFCI receptacle outlets first required?

 Does a home inspector remove receptacle outlet cover plates?

What is the minimum height for an exterior receptacle outlet? 

When was the current receptacle/outlet spacing of 12-feet first required? 

When was the three-slot (grounding) outlet/receptacle first required? 

Why does painting an electric receptacle (outlet) make it unsafe?

Why are electrical outlets and plugs polarized? 

How many electrical receptacles (outlets) are required in a hallway?  

What problems does having too many electric receptacle outlets on a single circuit cause? 

Is a house required to have outdoor electric receptacle outlets?

How I can tell if a receptacle outlet is tamper resistant?

Why is there a GFCI breaker in the electric panel for the bathroom shower light and exhaust fan?

What is a false ground, bootleg ground, or cheated ground receptacle? 

How can adding wood paneling or a wainscot create an electrical safety hazard?

How far apart should kitchen counter receptacles be spaced?  

How far above a kitchen countertop do electrical outlets have to be?

What is reversed polarity at an outlet/receptacle? Why is it dangerous? 

How high above the floor do electric outlets/receptacles in a garage have to be?

How far apart should electric receptacles be spaced in a bathroom? 

Is an ungrounded electric receptacle outlet dangerous?

My bathroom electric receptacle/outlet is dead and there are no tripped breakers in the electric panel. What's wrong?  

Is there an adapter that can be placed on a two-slot receptacle to make it safe? 

How do the new tamper-resistant electric outlets work?

Why is there no bathroom electric receptacle in this old house?

How can I tell if the electric receptacle outlets are grounded? 

How far apart should the electrical receptacles be placed?

What are the most common problems/defects found with electric receptacle outlets during a home inspection?

   Visit our ELECTRICAL page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles. 

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