Things You'll Need
Furnace instruction manual
Consult with an electrician before adjusting your furnace if it's more than 30 years old.
The function of your furnace limit switch is to tell the fan when to turn off and on depending on the temperature in the home. It's possible to reset the switch at home when necessary by following a few simple steps. However, for your own safety, don't attempt to reset the switch without first consulting your furnace's instruction manual.
Locate your furnace's heating duct. Heating ducts are typically located in a home's basement or utility closet. The panel will be located underneath the duct. Open the panel with a screwdriver.
Locate the white limit switch. It will be located at the top of the panel. Depending on your furnace model, the switch will be surrounded by a silver or white cover. Remove the cover by lifting it upwards or pulling it off to reveal the body of the switch and two metal arms.
Look at each arm to identify the settings. One arm controls the furnace's low temperatures and the other arm controls the high temperatures.
Identify the component (setting) located in between the two arms. This setting controls the furnace's fan.
Set the temperature on the middle component to approximately 105 to 115 degrees Fahrenheit. Setting the middle arm at this temperature ensures that the fan will switch on at the correct time (when the furnace warms ups).
Set the arm to left of the middle component to approximately 90 degrees Fahrenheit. Once your furnace reaches 90 degrees, the left arm signals the furnace to stop blowing cold air through house.
Set the arm to the right of the middle component to approximately 200 degrees Fahrenheit. When the burner reaches this temperature, this arm signals the furnace to shut off completely. The shut-off temperature will vary depending on the type of furnace you have, so consult your instruction manual before proceeding.
Pull out the white switch to set the furnace to the automatic mode. Automatic mode means the furnace will adhere to the settings you just implemented. If you push the white switch in, your settings will be cleared.
Close the panel with the screwdriver.