How to Reset a Furnace Limit Switch

Unscrew the panel underneath the plenum, or heating duct, just below the draft hood (the diagonal opening). Take the panel off and set it aside.

Locate the limit switch at the top of the open area where the panel was located. The limit switch should have a silver cover with a white switch sticking out of it.

Remove the cover to the limit switch by lifting it up or pulling it off, depending on the model. You should see a silver dial, with the plastic body of the switch beneath.

Check the dial settings. One metal arm is a number that tells the fan to turn off at a low temperature, while the other arm is a number that tells the burner when to turn off at high temperatures. In between should be a middle setting that marks a temperature at which the fan turns on once the burner warms up.

Hold the dial with one hand while you move the left metal arm to around 90 degrees Fahrenheit. This is the factory setting. At this setting the fan should be off so it doesn't blow cool air through the house.

Hold the dial and set the middle arm to about 30 degrees above the lower arm. In general, you want the fan to come on at around 110 to 130 degrees -- which the middle arm designates -- once the burner warms up.

Set the final arm to mark the burner shut-off temperature. On most furnaces this should be set around 200 degrees or just over. Consult the furnace manual or the side instruction panel for specific information related to the temperature settings of your furnace.

Set the white switch to the proper position for your furnace. Pushing it in will cause the fan to run constantly and thereby disregard the settings of the dial; pulling it out will force the switch into automatic mode to shut on or off based on the dial settings. Leave the switch pulled out to automatic unless the instructions for the furnace or a professional tell you otherwise.

Reattach the limit cover. Reinstall the panel on the furnace, securing is as needed with the screws.