How To Repair Gas Furnaces

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How To Replace a Thermocouple

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 thermocouple:

What You'll Need
You'll want to have these tools on hand to replace a thermocouple:
  • Wrench
  • Replacement thermocouple
Step 1: Unscrew copper lead and connection nut inside threaded connection to gas line. Under mounting bracket at thermocouple tube, unscrew bracket nut that holds tube in place.

Step 2: Insert new thermocouple into hole in bracket. Be sure steel tube is up and copper lead is down. Under bracket, screw bracket nut over tube. Push connection nut to threaded connection where copper lead connects to gas line. Make sure connection is clean and dry.

Step 3: Tightly screw nut into place, but do not over-tighten. Both bracket nut and connection nut should be only a little tighter than if hand-tightened.

The thermocouple is installed next to the pilot light on a gas furnace.
© 2006 Publications International, Ltd.
The thermocouple is installed next to the pilot light. A bracket holds
it in place, steel tube up and copper lead down.
Limit Switch

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 adjusting.

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 and the lower one at about 90 degrees Fahrenheit.

If you smell gas while performing these repairs, you'll want to get that leak taken care of right away. Learn how to find gas leaks on the next page.

For more articles on home repair, check out the following links.
  • Furnace Maintenance: Save yourself time and money by learning the steps to keep your furnace in prime condition.
  • Major Appliance Repair: If the furnace isn't the only thing in your house on the fritz, you can learn how to fix other machines in this article.
  • Small Appliance Repair: Once you've tackled the furnace, a toaster or blender seems like child's play. Find out how to fix them here.
  • Thermostat Maintenance: To make sure there's actually a problem with your heating system, you may want to check the thermostat, too. Learn how to calibrate a thermostat.
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