Pilot will not stay lit after replacing thermocouple


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Old 12-04-09, 12:15 PM
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Pilot will not stay lit after replacing thermocouple

I was reading an older post where a person was having the same issue that I am having. The pilot on my Rheem furnace will not stay lit. I have replaced the thermocouple to no avail. The flame looks perfect, nice and blue, the thermocouple gets glowing hot when attempting to light the pilot. I hold the button down in excess of 3 minutes and as soon as you release, the pilot light goes out immediatly. Reading other information on-line, could this be a faulty 24 volt transfomer not sending voltage to open the gas valve to let the pilot remain lit? I have cleaned everything properly and have tried two different thermocouples (thinking one of the new ones maybe bad) but the pilot will not stay lit. Any assistance with this is greatly appreciated.
 
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Old 12-04-09, 12:21 PM
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Do you turn the knob from pilot to on before you release it?
 
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Old 12-04-09, 12:24 PM
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I am unable to turn the knob until I release the button, it remains locked in place until I release.
 
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Old 12-04-09, 01:29 PM
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The thermocouple generates a small voltage (.015 volt or so) directly from the heat of the pilot flame. This is used to energize an electro magnet in the gas valve.

When you push down on the gas valve control, you are pushing down a stopper which allows gas to pass through the valve. The stopper is held open by the electro magnet.

If the pilot goes out, the voltage goes away, and the stopper snaps closed, preventing gas from flowing to the burners when the pilot light is out.


So this system is i8ndependent of the 24 VAC control system that turns on the gas to the burner.

There can be other control connected into this milivolt circuit, but that would be unusual in a furnace with a 24 VAC control system. I'm presuming that the thermocouple screws into the gas valve with no wires connected to it directly or indirectly.

If that's the case, you no doubt need a new millivolt gas valve. In my view, replacing gas valves is not a suitable task for DIYers, so I'd shop around for a service agency to do that for you.

A service agency should be able to do that task in no more than an hour. A reasonable price for the gas valve might be $150 or so.

Get the BTU input from the rating plate in the burner compartment of the furnace and shop around for a good price for replacing this part. "I'd like to get a price for to have a new millivolt gas valve installed in my gas furnace that has a 100,000 BTU input, parts and labor. No new thermocouple is needed."


I'd be interested to find out how that price shopping experience might go.



Seattle Pioneer
 
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Old 12-04-09, 01:36 PM
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Whoops. You don't have a millivolt gas valve. It sounds like you have a 24 volt gas valve with a standing pilot.

So you need a replacement 24 volt gas valve for a furnace of X thousand BTUs that has a standing pilot.


Another thing to consider would be installing an intermittent pilot ignition control system. This would light the pilot each time the thermostat calls for heat, eliminating gas usage for the pilot light.

This is probably a worthwhile option to consider, since it involves replacing the gas valve when it's done. A service agency should be able to quote you a fixed price for installing that kind of retrofit kit.

I just sold the one I had left over from my furnace repair business on Ebay a week or two ago for $60!
 
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Old 12-04-09, 01:43 PM
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Am I confused or isn't a millivolt system different from a 24v system. I believe a millivolt system is self generating and a 24v needs power to run. Even the t-stats are different, and one cannot be substituted for the other.
 
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Old 12-04-09, 01:46 PM
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I may have jumped the gun. By the time I posted SP had made the correction.
 
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Old 12-09-09, 06:55 AM
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Originally Posted by SeattlePioneer
Whoops. You don't have a millivolt gas valve. It sounds like you have a 24 volt gas valve with a standing pilot.

So you need a replacement 24 volt gas valve for a furnace of X thousand BTUs that has a standing pilot.


Another thing to consider would be installing an intermittent pilot ignition control system. This would light the pilot each time the thermostat calls for heat, eliminating gas usage for the pilot light.

This is probably a worthwhile option to consider, since it involves replacing the gas valve when it's done. A service agency should be able to quote you a fixed price for installing that kind of retrofit kit.

I just sold the one I had left over from my furnace repair business on Ebay a week or two ago for $60!
Seattle Pioneer:

This unit is a standing pilot. Everything looks good and I too think it could be the gas valve that is gone bad. According to the check list posted on the unit, it says that if the thermocouple has been changed and the pilot light will not remian lit, the next item is the valve body. Here is what I have observed and what I have come to understand from other information that I have read so please correct me if I am wrong.

When lighting the pilot the thermocouple generates a small amount of electricity when it reaches the proper temperature and triggers the electomagnic valve to open. This electric signal if I can call it that, remains constant as long as the pilot remains lit and the thermocouple is hot. If the pilot goes out for whatever reason, the signal stops and the valve closes so there is no free gas being emitted.

I have the 24 volt system as you said. There are two lead wires going to the transformer (black and white), 120VAC and two lead wires (yellow) coming from the back side of the transformer which is the DC volts. Those yellow wires go to the Honeywell control board, and from the control board, I have a yellow wire and a light blue wire that goes to the gas valve. I have checked the transformer and there is voltage leaving it and going to the control board, but there is no voltage at the leads on the gas valve. Does the gas valve need this voltage from the transformer to open the valve to keep the pilot lit? Or is it strictly an electromagnet that opens from the electric pulse from the thermocouple? If that is the case, what is the yellow and blue wires that attach to valve body do? Does this open another set of vavles when the furnace ignites?

Please advise.

Thanks
 
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Old 12-09-09, 09:12 AM
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Originally Posted by dmweaver04
I have checked the transformer and there is voltage leaving it and going to the control board, but there is no voltage at the leads on the gas valve. Does the gas valve need this voltage from the transformer to open the valve to keep the pilot lit?

You are correct. The 24 VAC voltages is needed to energize the gas valve solenoid and cause gas to flow to the burners.

You have 24 VAC at the transformer, so that's not the problem.

You need to "walk the cuircit," checking each switch and part to see where the 24 VAC power stops. It may be the thermostat or thermostat wire that's the problem, the limit switch or something else.
 
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Old 12-09-09, 10:40 AM
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Originally Posted by SeattlePioneer
You are correct. The 24 VAC voltages is needed to energize the gas valve solenoid and cause gas to flow to the burners.

You have 24 VAC at the transformer, so that's not the problem.

You need to "walk the cuircit," checking each switch and part to see where the 24 VAC power stops. It may be the thermostat or thermostat wire that's the problem, the limit switch or something else.
SeattlePioneer, thanks for the response, but I have another question. This voltage is needed to engergize the solenoid to cause gas to flow to the burners. But is this same voltage required to keep the pilot lit as well or does the electric generated from the thermocouple do this. I guess my question is, should there always be voltage from the ciruit to the solenoid, or is there voltage only there when the burners are trying to activate as a result of the thermostat triggering the furnace to start?
 
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Old 12-09-09, 01:38 PM
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The thermocouple provides a small amount of voltage to hold the gas safety valve open --- about 15 millivolts (15/1000 of a volt) This is independent of the 24 VAC power supply that energizes the solenoid that opens the gas valve to fire the main burners.

The 15 millivolts is generated directly from the heat of the pilot flame on the thermocouple.
 
 

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