If your gas furnace doesn't cycle off and on when the home reaches a certain temperature, check the limit switch for problems. The limit switch is a safety mechanism that controls the blower motor by telling it when to come on and when to shut off. But if the limit switch malfunctions, it can't relay messages to the blower motor properly. Your blower motor stays on, even when the home is warm enough. Resetting the limit switch may solve your issues. Here's how you check and reset the limit switch.
Locate the Limit Switch
The limit switch sits just below the draft hood of your furnace. To access the switch, cut power and gas to the furnace, then remove the side paneling from the plenum, which is the large rectangular-shaped container found on the top of the furnace. The limit switch is housed inside a small, metal box. You should see a tiny, white knob or button sticking out of the box. The knob is the piece of the limit switch that controls when the blower motor cycles off and on.
After you locate the limit switch, remove its cover. It's a good idea that you use a screwdriver with an insulated handle during this part of your repairs. The limit switch uses an electronic system and wires to operate. If the switch still has electrical currents flowing through it, it can shock you, even with the power off.
Now, proceed to the steps below.
Clean and Reset the Limit Switch
Before you do anything else, examine the inside of the limit switch for soot, smoke burns, and fire damage. If the internal parts show signs of damage, stop and contact a heating repair technician right away. The gas furnace may have issues with its burners, elements and gas lines. If the internal parts appear in good condition, you can move forward with your repairs.
Use a small duster with soft feathers to clean out the box. If you run into stubborn pieces of lint and dust, gently blow air into the unit with your mouth. Don't apply any type of liquids to the parts of the box, including cleaning solution. You can permanently damage the parts and furnace.
After you complete the steps above, check the position of the button. If the button is pushed into the box, carefully pull it out with the tips of your fingers. In order for the furnace's blower motor to cycle off and on correctly, the button must be set to automatic, or the up position. The blower motor will stay on when the limit switch button is placed in the down position.
Next, put the cover back on the box, then replace the paneling over the plenum. Turn on your furnace's power supply and gas line, then wait about 15 minutes for the appliance to heat up. If the blower motor goes off and on correctly, you solved the issue.
If the blower motor continues to operate, contact a repair technician (such as one from Controlled Comfort) for services. Your furnace may have other problems to repair.