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

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.

Burnham Gas Valve

First thing that I need to make clear is the difference between a as 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. This is a gas control Valve.

Gas Cock

This is a gas cock, or turn off valve. 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.

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.

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.

How to replace a thermocouple on a Weil McLain gas boiler.

Weil McLain boiler cover

First remove the cover make sure you have turned the power to the boiler off before you remove the cover

Weil McLain Thermocouple

This is what it looks like with the cover removed note the gas valve has a copper colored wire and a silver colored wire looking tube going to the burners. This is a standing pilot gas valve.

Weil McLain Pilot tubing

On the gas valve there is a brown knob turn this knob so that the arrow is pointing to pilot. Now you need to remove the burner cover, warning it may be very hot loosen the screws on both sides of cover.

Weil McLain burner tube

The newer versions of this type of boiler has a roll out switch be careful not to damage this switch as you remove the cover. Older boilers do not have this safety device.

Weil McLain roll out switch

If you did not turn the power off and you touch this roll out switch to any metal part of the boiler it will spark and can cause the transformer to but out. So make sure you turn the power off.

24 volt gas valve

If you are replacing the thermocouple before it has failed you will notice the pilot is still burning. This is a great opportunity to test if the gas valve needs to be replaced. With the knob turned to the pilot setting mentioned before.

Test the thermocouple

Take your 3/8" wrench and turn the thermocouple nut (the copper one) to the left loosening the nut, this should cause the pilot to go out. If you have removed the thermocouple nut from the gas valve and the pilot stays on, The gas valve is defective and must be replaced. Stop Turn the GAS COCK off and Call a professional.

Remove the Pilot tube

Most of the time the pilot goes out and that is what it is designed to do. now that the pilot is out and there is no more flame it is safe to remove the pilot tubing from the gas valve. in most cases you will need to remove the burner to replace the thermocouple and so you need to remove the pilot tubing from the gas valve as well. use your 7/16" wrench to do this.

Remove burner tube

Now to remove the burner, This type Weil McLain boiler has simple aluminum burners that come out easy just grab the burner lift so that the rear of the burner comes up then pull it forward to clear the brass fitting it is resting on.

Pilot tube from burner tube

Be careful not to damage the pilot tubing when taking this burner out sometimes it runs under the burner manifold and other times it goes over. now you have the burner with the pilot assembly in your hand.

Thermocouple from burner tube

Removing the old thermocouple is easy it either will unscrew from the bottom or just pull out, Just throw it away once you have it out. because you have the pilot assembly in your hand it is a good time to clean out the pilot orifice.

Pilot tube from assembly

Remove the 7/16" nut from the pilot assembly, being careful not to bend it away from the burner, and also be careful not to drop and lose the orifice it will be full of a white powdery substance clean all that up and make sure the hole in the orifice is clear. be sure to put it back and make the tubing connection tight when you are done.

Pilot assembly orifice.

This is the orifice for this type of pilot assembly. they are not all like this some need to be remover with a wrench this type will come out just by removing the pilot tubing from under it.

Your new Thermocouple

Your New thermocouple comes with a little small piece of metal and a threaded adaptor. some applications require one or the other. This type uses the threaded adaptor. If the hole in the pilot assembly is to small for the threaded adaptor, the thermocouple just slides up into the assembly.

Install new Thermocouple

Be sure the thermocouple is pushed as shown in the picture above so that it will send a good amount of voltage to the gas valve.

Weil McLain burner tubes.

This is the back of the boiler with all the burner tubes removed. notice the slits in the metal, The end of the burner tube goes into this slit. the hole in other side of the burner tube goes over the brass orifice on the burner manifold first then the other end drops into the slit.

Burner tube up side down

With this series of this type of Weil McLain Boiler it is not possible to put the burners in upside down. but older versions of this series boiler you can put the burners in upside down and this is very dangerous.

Don't forget the orifice

Make sure the burner is properly over the orifice and all the way down into the slit. and all the burners are face up as in the above image. If you have removed all the burners to clean them, make sure the pilot burner goes back into the same place on the manifold that you removed it from.

Test gas tube with soap

Now reconnect the pilot tubing to the gas valve and make sure it is tight. then connect the new thermocouple nut to the gas valve. Make it snug do not over tighten. Mix a little dishwashing liquid with a little water with a sponge or small brush put this soapy water around the nut that ties the pilot tubing to the gas valve.

Push and hold the red button and look to see if the soap starts to make bubbles. If no bubbles are present. release the button and WAIT 5 Minutes Then using a match or other open flame. push and hold the red button again  and light the pilot once the pilot is lit hold the button for one minute. After you have held the button for one minute relieve the button The pilot should stay on. If not try again if it continues to fail call me  once the pilot stays on DO NOT TURN THE BROWN KNOB BACK TO THE ON POSITION. First replace the burner cover making sure it is in it's proper place with the screws in the slots. then standing up with your face away from the front of the boiler turn the brown knob to the on position. then turn the power back on. you are done put the boiler cover back on. The gas valve in this example has a brown knob with a separate red button to push for the pilot. But some only have one knob with out the red button, with this type you push the know when it is in the pilot position Not all the knobs are brown. Again If you have any doubt call a Professional. 

If you need help with this, please click on the Answers Now link at the top of the page.