The furnace or air handler (indoor unit of an air conditioner) is required to have a way to disconnect the electricity to it within sight, so that a service technician can safely service the system without someone in another room inadvertently turning the power back on at the electric panel. While the disconnect means can be of several different types, such as a fused or pull disconnect, it can also just be an ordinary switch on the wall next to the unit.

    When the furnace or air handler is in a closet, it’s pretty obvious that the switch is for the furnace—or, at least, something in the closet. But, when the equipment is in the garage, it can be confusing, especially if the switch is right next to the door from the garage into the house. The problem with that location is that occasionally someone in the house mistakenly thinks it is a light switch and shuts it off, after which the furnace or air conditioning system is mysteriously no longer working. It’s not unusual for an a/c tech on a service call to simply throw the switch to “fix” a broken HVAC system.

    This is why we often see a switch in a garage that is taped-over or marked “DO NOT TOUCH” by the homeowner to prevent anyone from unwittingly shutting off the system, like in the photo below. The top arrow points to the problem switch, a shut-off for the furnace/air handler and heavily taped over, while the bottom one is the light switch.

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  To learn more about heating and air conditioning systems, see these other blog posts:

• How can I find out the SEER of my air conditioner? 

• My air conditioner won't turn on. What's wrong? 

• How can I find out the size of my air conditioner? 

• How can I tell whether the condenser (outdoor unit) is an air conditioner or heat pump? 

• Where is the air filter for my central air conditioner and furnace? I can’t find it? 

• Does an old air conditioner use more electricity as it ages? 

• How did homes stay cool in Florida before air conditioning? 

• What is wrong with an air conditioner when the air flow out of the vents is low?

• Why has the thermostat screen gone blank? 

• Why does it take so long to cool a house when an air conditioner has been off for a while? 

• Why is my air conditioner not cooling enough? 

• What are the most common problems with wall/window air conditioners?  

• Will closing doors reduce my heating and cooling costs? 

    Visit our ELECTRICAL and HEATING AND AIR CONDITIONING pages for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.