How do I Replace a Thermocouple?

Dee S.

Depending on the model, some gas furnaces, water heaters, and broilers ignite by an electrical spark method. In most cases, a gas pilot light is necessary for firing. Occasionally, a unit will not operate properly because the pilot light is out. In that case, it may be necessary to replace a thermocouple. A thermocouple is the copper tube with a bulb-like end that is raises in temperature because of the pilot light.

Thermocouples generate voltage that is proportional to the heat they are measuring or monitoring.
Thermocouples generate voltage that is proportional to the heat they are measuring or monitoring.

It is best to begin by attempting to relight the pilot light following the instructions attached to the furnace, water heater, or broiler unit. Usually, a gas cock with three settings will be evident. Simply turn it to “off,” wait for the gas to clear, and then turn the gas cock to the setting marked “pilot.” Then, hold a match to the pilot while pressing a reset button. If the pilot stays lit, turn the gas cock to “on.” However, if the pilot light will not stay lit, the next step is to replace a thermocouple.

To replace a thermocouple, the gas inlet valve should first be shut off. Then, the access cover should be removed so that the burners are both easy to see and easy to work with. Disconnect the thermocouple from the control unit. This can usually be done by unscrewing a hold-down nut. The other end of the thermocouple may just pull out or there be a second hold-down nut that will need to be unscrewed.

Most home improvement centers will sell thermocouples. A person who works in the store will be able to show customers where to find the materials to replace a thermocouple. As project begins, it is important that the copper tube is not kinked or dented. The other primary concern should be that the bulbous section of the thermocouple should touch the pilot light. If it cannot become heated, the unit will not work.

After the thermocouple is in the correct place, and touching the pilot light, it can be firmly held in place again by re-screwing the hold-down nuts. The gas must then be turned on again and the pilot light should be relit. This time the pilot light should remain lit.

If, by chance, the pilot light does not remain lit, it may be too weak. The flame should be blue with a yellow tip. If it has other colors or too much yellow at the tip, there could be a draft or the pilot tube may need to be cleaned out. This can be done quickly with a needle.

In all, replacing a thermocouple should only take about 15 minutes. However, as with any problem, if additional issues arise, or if other problem solving techniques are needed, it could take much longer. As with any home improvement or repair, always make sure the electricity and gas are off before beginning the project. In addition, call a specialist for help if difficult problems arise.

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