Like the other component parts in your furnace, the thermocouple can wear down over time, producing lower voltage than it should when heated. Thermocouples can last several years, but they don’t typically last as long as the furnace itself.
How Does a Thermocouple Work?
A gas furnace thermocouple is a small electrical device that acts as a safety device on a furnace. It is installed so that its ends are in the pilot light of the furnace, and is considered the right hand man of the pilot light.
The thermocouple responds to changes in temperature by producing an electrical current that causes the gas valve that supplies the pilot light to open when the temperature is high or to shut once there is no direct heat supply.
If the furnace’s pilot goes out, or the gas flow is interrupted then the thermocouple will automatically shut off the gas flow to the pilot, shutting down your furnace. This prevents unburned gas from flowing into the combustion chamber, where a small spark might cause an explosion.
A Dirty Thermocouple
It’s important to know how to keep a thermocouple clean and in working order. If the pilot light is not operating properly, that could mean that soot has built up on the thermocouple. As a result, the thermocouple can’t properly detect heat, or loss thereof. When a malfunctioning thermocouple provides improper readings, it can shut off the gas supply to the furnace even when the pilot light is working.
What does a thermocouple look like and how do you find it?
Before attempting to check or clean the gas furnace thermocouple, you need to shut off the furnace’s power supply. If you can’t find your furnace’s power switch, shut off the circuit breaker that supplies power to the furnace.
You also want to make sure that you have the gas shut off, and the you set the gas valve to the off position. You then need to let the furnace air out and cool down for a few minutes. Once this is done you can remove the pilot cover which will reveal two upright posts, one of which is the gas line for the pilot and the other is your thermocouple.
The thermocouple consists of up of a tube, a bracket, and wires. The tube sits above the bracket, a nut holds the bracket and wires in place, and underneath the bracket, you will see the copper lead wires that connect to the gas valve on the furnace.
Some thermocouples can look differently, so make sure you consult your furnace manual.
Failed Thermocouple Symptoms
Once you have located the thermocouple, do a visual inspection. You are looking for the following:
- Signs of contamination on the tube, e.g. discoloration, cracks, or pinholes.
- Signs of wear or corrosion of the wirings, e.g. missing insulation or bare wire.
- Physical damage to the connectors which can have an effect on the reliability of the test reading.
If you cannot detect any obvious problems then it may be time to call an HVAC contractor to check the functionality of your thermocouple.
How to Clean a Furnace Thermocouple
If there is soot build up on your thermocouple, then the issue might be resolved by cleaning your thermocouple. You will need a screwdriver to loosen the screws and remove the thermocouple from the pilot assembly.
Make sure to handle the thermocouple with care. Use a piece of steel wool to gently remove the soot. Don’t use any cleaning products, especially not those containing flammable chemicals. You can use a pencil eraser to clean between the threads of the screw that attaches the thermocouple.
When the thermocouple is clean, reattach it to the pilot assembly. Turn on the furnace and light the pilot light following the manufacturer’s manual.
If the pilot remains lit, but the main furnace doesn’t stay lit, it may be time to replace the thermocouple. Most hardware and home improvement stores carry replacement thermocouples. However, they come in different sizes, so take your faulty thermocouple to the store with you. Be sure you use the exact furnace details so you get the correct size and type for your thermocouple replacement.
If you are unsure about what’s going on with your furnace or how to inspect or replace your thermocouple, make sure you contact a professional HVAC contractor to do the job safely!
Our HVAC experts at S&C Mechanical Systems provide high quality furnace repairs that come with a 12 month no-hassle guarantee!