A broken pilot light is annoying. A furnace that won’t stop running is worse. Both problems usually point to two specific safety components. Fixing them keeps your home warm and your energy bill from spiraling.
The thermocouple sits near the pilot burner. It is a safety device. If the pilot light goes out or the electric igniter fails, the thermocouple shuts off the gas. No flame means no gas flow. Simple.
If your pilot light won’t stay lit, the thermocouple might be the culprit. It may just need adjustment. Or it might need replacing. Try tightening the nut with a wrench first. Be careful. Do not apply too much pressure. Just tighten it slightly. Then try lighting the pilot again.
If the pilot still dies, you need a new thermocouple. Get one of the same type. Replacing it is not hard.
How to replace a faulty thermocouple
- 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.
- Insert a new thermocouple into the hole in the bracket. The steel tube goes up. The copper lead goes 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.
- Tightly screw the nut into place. Do not over-tighten.
Both the bracket nut and the connection nut should only be a little tighter than if hand-tightened. Over-tightening can crack the fittings.
Why does my blower run continuously?
The limit switch is a safety control. It sits on the furnace just below the plenum. If the plenum gets too hot, the limit switch shuts off the burner. It prevents the house from becoming an oven.
The switch also controls the blower. It turns off the blower when the temperature drops to a certain level after the burner shuts off.
If your blower runs continuously, one of two things is happening. Either the blower control on the thermostat is set to ON. Or the limit control switch needs adjustment.
Check the thermostat first. It is the easiest fix. If the blower control is set to ON, change it to AUTO. If it is already on AUTO, the limit switch is the problem. It needs adjustment.
How to adjust the limit switch
Removing the control’s cover reveals a toothed dial. One side is marked LIMIT. Do not touch this side. The other side is marked FAN.
There are two pointers on the fan side. The blower turns on at the upper pointer setting. It turns off at the lower pointer setting.
The pointers should be set about 25 degrees apart. You want the fan to kick in before the system overheats but shut off before the air gets cold.
Set the upper pointer at about 115 degrees Fahrenheit (46.1 degrees Celsius). Set the lower one at about 90 degrees Fahrenheit (32.2 degrees Celsius).
This setup ensures the blower runs long enough to pull heat from the heat exchanger but stops before the air turns chilly. If you are unsure about your specific furnace model, check the manual. Some units have different tolerances. But for most standard gas furnaces, that 25-degree gap is a good starting point.
Gas work carries risk. Always turn off the gas supply before touching the thermocouple. If you smell gas, leave the house and call the utility company. Do not try to fix a gas leak yourself.
The limit switch adjustment is less dangerous but still involves electrical components. Make sure the power is off before removing the cover. A small shock is not worth the adjustment.
Once the pointers are set, replace the cover. Turn the power back on. Watch the furnace cycle. The blower should start when the heat
