Gas furnaces rely on two simple but critical safety components to keep your home warm and your air moving: the thermocouple and the limit switch. When things go wrong, you usually know it immediately. Either the pilot light won’t stay lit, or the blower runs like a jet engine even when you’re not looking for heat. Both issues are fixable. You just need to know which part is failing and how to adjust it without causing a leak or a short circuit.
The Thermocouple: Why Your Pilot Light Dies
The thermocouple sits right near the pilot light burner. It’s a small safety device made of copper and steel. Its job is simple. It generates a tiny voltage when heated by the pilot flame. This voltage keeps the gas valve open. If the pilot light goes out, the thermocouple cools down. The voltage drops. The gas valve snaps shut. No gas means no explosion. Smart design.
If your pilot light won’t stay lit, the thermocouple is the first suspect. It might be loose. It might be dirty. Or it might be dead. Before you buy a replacement, try an adjustment. Sometimes the connection just needs a nudge.
Grab a wrench. Locate the nut where the thermocouple connects to the gas line. Tighten it slightly. Do not crank it down hard. You’re not building a bridge. Just snug it up. Try lighting the pilot again. If the flame stays lit, you’re done. If it dies again, the thermocouple is faulty. You need a new one.
Replacing a faulty thermocouple is straightforward if you follow the steps.
- Disconnect the power. Turn off the gas supply to the furnace. Safety first.
- Remove the old unit. Unscrew the copper lead and the connection nut inside the threaded connection to the gas line. Under the mounting bracket on the thermocouple tube, unscrew the bracket nut. This holds the tube in place. Pull the old thermocouple out.
- Install the new one. Insert the new thermocouple into the hole in the bracket. Orientation matters. The steel tube must point up. The copper lead must point down. Secure the steel tube by screwing the bracket nut back on under the bracket.
- Connect the gas line. Push the connection nut to the threaded connection where the copper lead meets the gas line. Ensure the connection is clean and dry. Dust or moisture can cause leaks or poor ignition.
- Tighten with care. Screw the nut into place. Tightly, but not so tight you strip the threads. Both the bracket nut and the connection nut should only be a little tighter than hand-tight. Over-tightening cracks the copper. Under-tightening causes a gas leak. Neither is acceptable.
The Limit Switch: Stopping the Noisy Blower
The limit switch is another safety control. It sits on the furnace, just below the plenum. The plenum is where the heated air enters your ductwork. If the plenum gets too hot, the limit switch cuts off the burner. It prevents the furnace from overheating and potentially damaging the heat exchanger.
But the limit switch does more than just shut off the heat. It also controls the blower fan. Once the burner shuts off, the limit switch keeps the blower running until the temperature drops to a safe level. This pushes the remaining heat out of the furnace and into your home.
If your blower runs continuously, you have two potential culprits. The first is your thermostat. Check the blower control setting. If it’s set to “ON,” the fan runs all the time regardless of the temperature. Switch it to “AUTO.” The fan will now only run when the furnace is actively heating or just finished.
If the thermostat is already on “AUTO” and the blower still won’t stop, the limit switch needs adjustment. This is less common but happens over time as components wear.
To adjust the switch, you need to get
