Home Repair - Other

How to Replace the Thermocouple in a Furnace



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Here's a quick Q&A about your furnace's thermocouple; what it is, where it is, and how to replace it.


What is a thermocouple?


The thermocouple on your furnace is the safety measure which shuts off the gas supply and the blower if there is no pilot light detected. This keeps gas from leaking into your home protecting you and your family.


How does a thermocouple work?


A thermocouple contains two wires welded together on the ends which are contained inside a protected housing. One end of the wire runs up the center of the pilot light where the flame is the hottest; the other end connects to a solenoid transformer. When the heat is detected it sends an electric signal to the transformer which opens the gas valve.


What happens if it goes bad?


If a thermocouple goes bad the gas valve will not stay open which means neither your pilot light or burner will stay lit and you will be without heat.


How do I replace a thermocouple?

* Before doing any work to your furnace turn off the power at the fuse box and turn the gas line to your furnace to the off position.


Parts and tools: To replace a thermocouple you will need a screwdriver, an adjustable wrench, you furnace's owner manual, and a new thermocouple.


1. Remove the front panel to gain access to the thermocouple.


* Note: Some furnaces require the removal of additional panels. Reference your owner's manual if needed.


2. Locate the thermocouple next to the pilot light and unscrew the gas line connection nut and the bracket nut.


3. Once the thermocouple is disconnected slide it out of place.


4. Slide the new thermocouple onto the mounting bracket and tighten the bracket nut.


* Remember to replace it the same way it came off with the copper tube facing down and the steel tube facing up.


5. Once the thermocouple is in place you can reconnect the gas line and tighten the connection nut.


* Note: Don't over tighten any of the connections.


6. As a final step turn the gas and the power back on and reignite the pilot light.


Remember that there are other problems that can occur resulting in the pilot or burner not staying lit including a faulty limit switch, unadjusted pilot, or an unpaid gas bill. However a thermocouple is simple and easy to replace which makes it good place to start your diagnostics. Whenever you are dealing with a furnace you feel unsure of yourself it is safer and often cheaper to call a professional than to perform a job incorrectly.


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