Pilot way too big

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Old 09-29-18, 09:04 AM
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Pilot way too big

I have a gas fireplace insert that looks to be about 20-30 years old. I think it is a Mendota. I replaced the thermopile because I couldn't get more than 220 mV from it. I bench-tested it and got over 500 mV. The burner will fire off at about 280 mV, but in order to get that I need about a 4-inch yellow pilot flame. Approx 3/8" flame contact with the thermopile. I gave it a boost with a lighter and the mV rapidly increased well over 400. I turned down the pilot adjustment until the flame was blue, but then I get less than 175 mV. Not sure what to do next as that about exhausts my fireplace knowledge.
 
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Old 09-29-18, 11:11 AM
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Update:

I detached the gas tube from the gas valve to the pilot and blew air through it. A small particle blew out near the pilot. I now get a high enough mV generated (280 mV) to get the burner going with half as much pilot. Still more pilot flame than I would like though.
 
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Old 09-30-18, 07:48 PM
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The pilot orifice is very tiny and easily plugged or blocked and especially more so with propane.
Just had this discussion in another thread.... those pilot lights are operated larger than it appears they should be. That is normal. My home unit is the same way. I recommend keeping a spare thermocouple around as they do run hot.
 
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Old 10-01-18, 06:49 AM
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Q: Isn't the temperature at the center of a blue gas flame the same whether it's a 1" or a 4" flame?

I was thinking about this in regards to my Too-Big Pilot issue in the other thread and came to the conclusion you can't "wear out" or "burn out" a thermopile as long as the temperature is below the point the metal actually melts--which isn't possible with NG or LPG.
 
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Old 10-01-18, 08:46 AM
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You don't physically melt the outside case although it can rust thru.
The heat kills the dielectric wires inside.
 
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Old 10-06-18, 03:50 AM
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If the pilot flame is yellow , the pilot office is dirty or partially plugged and needs cleaned. You should remove the pilot tubing where it connects to the pilot , then remove the orfice fitting. From there you can blow out the orfice backwards from it's discharge to the tubing connection with either air, water, or both. Myself, I like to use water first, followed by compressed air. also, clean the whole pilot assembly with the air/water combo and reinstall. This should then allow the pilot to produce a hotter mostly blue flame and a higher MV output. If the MV output is still to low replacing the thermopile is needed.
 
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