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The Boiler or furnace Thermocouple, what it does why it fails and how to replace it.

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The thermocouple is a probe that sits directly in the flame of the pilot light of your gas fired boiler or furnace or water heater or unit heater. That has a copper wire that goes from the probe to your gas valve. It is important to note: not all gas boilers, furnaces and so on, have a thermocouple or a light able pilot. Some have electronic ignitions and some have spark ignitions. trying to light this type of system can de dangerous and a total waste of time. Lighting a pilot light and or replacing a thermocouple is potentially dangerous. Fire, Explosion, death and destruction of property can result. If you have any doubt in your ability to do this safely stop and call a professional to replace thermocouple. Master Plumber and I are not responsible for any problems you may have as a result of reading this information and using it.Working on any gas appliance can be dangerous. The few things that I am warning you about may not cover all the possible things that can go wrong. again if you are not 100% sure of your ability CALL A PROFESSIONAL!!! To replace thermocouple Click On Any Image To See A Larger Image.

Hydrotherm boiler thermocouple

A warning about Propane gas: Propane is heavier then air and can sit on your floor all around your boiler and or furnace or water heater, and can explode when you go to light the pilot light, and if that is not bad enough it can, and does, loose it's odor, making it impossible to detect and an extremely dangerous situation. Natural gas can build up in a room and can explode. it has lifted houses off of there foundations. I'm not trying to scare you, Gas appliances are perfectly safe under normal operating procedures. The danger comes in when someone who does not know what they are doing messes around with them, changes settings or removes safety items that are required.

Gas Valve
Gas Valve

What does it do?

A thermocouple send a low voltage signal to the gas valve, letting the gas valve know there is a flame and it can stay on.
Why do they go out? Nothing lasts forever. after a while a thermocouple is no longer able to send enough voltage to the gas valve to keep the pilot button set. and it needs to be replaced. Some times you can just relight the pilot and it will hold for a few days then go out again. Most of the time this is a week thermocouple.
How to replace thermocouple scroll down for instructions 

Gas Cock
Gas Cock

First thing that I need to make clear is the difference between a "Gas Valve" and a "Gas Cock" Many people call the gas cock a gas valve, but for the purposes of this page I will refer to it as the gas cock to avoid confusion. all images on this page are thumb nails click on the image to see a larger version of it.


The Gas valve is the main control of the gas going to your appliance. There are many different kinds of gas valves. The one in this picture is from a Weil McLain boiler, it is a 24 volt standing pilot gas valve. Not all gas valves are 24 volt standing pilot. and not all Weil McLain boilers have this type of gas valve. Some are mili volt and use a thermopile instead of a thermocouple and are not interchangeable. Others are 120 volts. and are also not interchangeable.

The gas cock turns the gas to the appliance off. This gas cock should be located out side the jacket of the appliance it is serving.
There are many different types of gas cocks, but most of them turn off with a 1/4 turn the gas cock in the image above is in the off position. parallel with the pipe is open across the pipe is off.

Ok lets get started If you are unsure if you have a thermocouple or other type of gas control you can send me a picture of the gas valve and lower front of your appliance and I can tell you what type of system it is. Send pictures to

For most thermocouple replacements You will need a 3/8" open end wrench and a 7/16" open end wrench. And for water heaters you will need a 3/4" open end wrench as well. Turn the power to the appliance off if applicable. from this point we will have to go to 1 specific boiler. The boiler I have chosen to use as the example for this is a Weil McLain cg series boiler. once again I need to mention that not all Weil McLain cg boilers have the same gas valves they are not all standing pilot and they are not all 24 volts If you are not sure send me the pictures. and I will help you. The following image is from the Weil McLain boiler. Click here to continue.

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