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Hi, thank you for contacting justanswer.
Seems you may be having the troubles caused by a thermocouple starting to go bad/needs cleaning or has moved out of position.
1-Check position of thermocouple to be sure it is positioned in flame at all times
2- Take some 00 Steel wool and clean thermocouple to see if build up has decreased sensitivity.
3- If you have a strong pilot once the fireplace lites and are still experiencing problems you probably have a thermocouple that is going bad.
Clean and reposition thermocouple, if problem persist, replace thermocouple.
If you do not have a strong pilot, clean pilot orifice before you replace thermocouple.
I don't think there is a problem with the thermocouple because the pilot flame burns steady. The thermocouple controls the pilot flame. I think there is a problem with the thermopile which controls the main burner. Most likely I fixed the problem myself. The wires connecting the main switch on the fireplace weren't quite connected. After I wriggled the wires on the main switch and turned on the main switch the burner went on, and I was also able to turn off the fireplace using the main switch as I supposed to do. I believe I may need to replace the main switch and the wires controling the gas valve on the main burner. This explain the freaky operation using the wall thermostat. Once there was a connection the fireplace worked, but once the connection wasn't quite right it didn't work.
I am not quite satisfied with the answer you provided because it didn't provide the solution.
A thermopile is a collection of thermocouples junction pairs connected electrically in series forming a thermopile, (pile of thermocouples).
Your thermocouple senses your pilot flame and allows your main gas valve to open. Your main gas valve will not open if the thermocouple does not sense the pilot.You can have a bad thermocouple and still have a strong pilot. But a bad thermocouple will not allow your main burner to fire. As I am aware of your system you have only 1 thermocouple.
As with all gas/electrical problems you have to start with the more probable cause and work your way to the trouble from there to find the solution (source of the problem). Since you believe you have found your problem I am responding with a need info tab, (not an accept or answer tab). You do not have to do anything, just let the question time out.
thanks for following up on my question regarding the thermocouple/thermopile problem with my Napoleon GDS60 gas fireplace (2 yrs old). Currently, the fireplace works okay. The burner lights when the thermostat is 3 degrees below RT. However, it still doesn't work as reliable as it should be. At times, the main burner doesn't light, but I can fix it by turning on the main switch to fire it up; turning off the main switch the burner stay lite until the set temp is reached by the thermostat. I checked the instruction manual: it shows a thermocouple (2 pieces) AND a separate thermopile (has separate part no). I am still wondering about the thermopile. According to you there's no thermopile. Could it be that the pilot flame needs to be adjusted to generate enough mV to start the main burner more reliably?
You are exactly right, a weak pilot means weak voltage from the thermocouple. The stronger the pilot the more output (voltage) you get from your thermocouple (flame sensor) to the gas valve. If your voltage is fluxuating from the thermocouple your gas valve will drop out. The flame sensor can be dirty and need cleaning that can cause this also. It can be out of position, loose connections, or starting to go bad.
The pilots thermocouple would be the place to start. Aways make sure the thermocouple is working correctly and you move on from there.
Thanks for your further explanantions which was helpful. In order to clean the thermocouple, can I access it from underneath my fireplace? (unscrew the thermocouple and remove it - correct?) In other words, I don't have to open up the fireplace, remove the glass door and fire logs and clean it from the inside, correct?
Correct, you should be able to remove the thermocouple without having to take the logs out. Normally there is just a clamp holder with a screw in it. Loose the screw, slide the thermocouple out. I normally just use some 00 steel wool to clean without even taking the leads loose. When you reposition be sure to get the thermocouple positioned so that the flame burns on the thermocouple and none of the flame misses the thermocouple.
A thermopile is just another thermocouple. You have the thermostat control wall switch. This is just 2 thermocouples used to generate enough voltage for the fas valve and thermostat, Etc. Clean and position both of them If you had a multi-meter you could read the output of the two. You should be getting somewhere between 300-600 millivolts from the two.
When I first looked up the schematic on your system I pulled up the unit without the thermostat control. This model I pulled up was the same as you have but didn't have the thermostat and it had only 1 thermocouple. So, I was wrong by telling you that your unit had only one thermocouple previously.
But yes, clean both and reposition both. If you still have a problem I would read the output voltage from the 2.