Gas Furnaces: Thermocouple and Limit Switch
The thermocouple is a gas furnace component located near the pilot light burner. It is a safety device that shuts off the gas if the pilot light goes out or the electric igniter fails.
If the pilot light won't stay lit, the thermocouple may be faulty and should be adjusted or replaced. To adjust the thermocouple, you must tighten the thermocouple nut with a wrench. Take care not to apply too much pressure to the nut -- just tighten it slightly. Then try lighting the pilot. If the pilot won't stay lit, replace the thermocouple with a new one of the same type. Here's how to replace a faulty thermocouple:
Step 1: Unscrew the copper lead and connection nut inside the threaded connection to the gas line. Under the mounting bracket at the thermocouple tube, unscrew the bracket nut that holds the tube in place.
Step 2: Insert a new thermocouple into the hole in the bracket. Be sure the steel tube is up and the copper lead is down. Under the bracket, screw the bracket nut over the tube. Push the connection nut to the threaded connection where the copper lead connects to the gas line. Make sure the connection is clean and dry.
Step 3: Tightly screw the nut into place, but do not over-tighten. Both the bracket nut and connection nut should be only a little tighter than if hand-tightened.
The limit switch is a safety control switch located on the furnace just below the plenum. If the plenum gets too hot, the limit switch shuts off the burner. It also shuts off the blower when the temperature drops to a certain level after the burner has shut off. If the blower runs continuously, either the blower control on the thermostat has been set to the ON position or the limit control switch needs adjustment. Check the thermostat first. If the blower control has been set to ON, change it to AUTO; if the blower control is already on AUTO, the limit switch needs to be adjusted.
To adjust the switch, remove the control's cover. Under it is a toothed dial with one side marked LIMIT; don't touch this side. The other side of the control is marked FAN. There are two pointers on the fan side; the blower goes on at the upper pointer setting and turns off at the lower pointer setting. The pointers should be set about 25 degrees apart. Set the upper pointer at about 115 degrees Fahrenheit (46.1 degrees Celsius) and the lower one at about 90 degrees Fahrenheit (32.2 degrees Celsius).