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

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This is a continuation of the thermocouple instruction page, please click here to go back to the first page.
First remove the cover make sure you have turned the power to the boiler off before you remove the cover
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.
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
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.
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
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.
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.
 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.
 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.
 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.
 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.
 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.
 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 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
 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
 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.
 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.
 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.
 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. 

 

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