question about gas fireplace pilot light

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RustyShackleford

Minister of Fire
Jan 6, 2009
1,331
NC
I'm troubleshooting a gas (propane) fireplace. The pilot won't stay lit (after I ignite it with piezo start and let it burn 30-60 secs. I pulled the unit out to have a better look, and then decided to try again to light the pilot - this time it worked.

Attached is a picture of the pilot assembly, from the manual, and a photo of a replacement unit for sale (the exact spec'd part#).

A few questions:

1. Why does this thing have a thermopile AND a thermocouple ? I thought they did the same thing: detect a lit pilot flame and allow a valve to continue providing gas to the pilot. The "thermopile" seems to connect with two wires, as I expect, as it provides a signal of some millivolts to enable gas flow. The "thermocouple" seems to connect with a small gas line of some kind. What's the deal here ?

2. Is intermittent failure common in a thermocouple/thermopile, or should I search for alternative causes of the intermittent failure ?

3. You can't see it too well in the photo, but the thermopile is semi-permanently connected to the bracket which also holds the igniter etc; rather than with a locknut securing it into a small hole (as seen in photos of typical units). Is it a mistake to try to just replace the thermopile ? (I'm sure I can do it without damaging things).

gas control 2.png PXL_20221114_024340497.jpg
 
I'm troubleshooting a gas (propane) fireplace. The pilot won't stay lit (after I ignite it with piezo start and let it burn 30-60 secs. I pulled the unit out to have a better look, and then decided to try again to light the pilot - this time it worked.

Attached is a picture of the pilot assembly, from the manual, and a photo of a replacement unit for sale (the exact spec'd part#).

A few questions:

1. Why does this thing have a thermopile AND a thermocouple ? I thought they did the same thing: detect a lit pilot flame and allow a valve to continue providing gas to the pilot. The "thermopile" seems to connect with two wires, as I expect, as it provides a signal of some millivolts to enable gas flow. The "thermocouple" seems to connect with a small gas line of some kind. What's the deal here ?

2. Is intermittent failure common in a thermocouple/thermopile, or should I search for alternative causes of the intermittent failure ?

3. You can't see it too well in the photo, but the thermopile is semi-permanently connected to the bracket which also holds the igniter etc; rather than with a locknut securing it into a small hole (as seen in photos of typical units). Is it a mistake to try to just replace the thermopile ? (I'm sure I can do it without damaging things).

View attachment 302780 View attachment 302781
1. Why does this thing have a thermopile AND a thermocouple ?

Thermocouple generates 28-31mV to hold the electromagnets open for the pilot.
Thermocouple generates 550+-/ mV to hold the electromagnets for the burner open.

2. Is intermittent failure common in a thermocouple/thermopile…

Yeah, sometimes the TP or TC will start to fail slowly, maybe even intermittently. Sometimes they just fail completely.
The TP is wired to the valve terminal block. The TC screws into the valve. The copper cladding looks like gas line, but it’s not. It’s also an electrical connection.

You CAN replace either mV generator separately.
 
Awesome, thanks for the comprehensive answer (all the people at stackexchange could do was whine about the fact I'd asked 3 questions in one).

I decided to go ahead and spring for $50 for the entire assembly. They don't mean for you to replace them separately, although I suspect the reasons are mostly economic. But I figure why do anything even slightly half-assed with propane.
 
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So just double-redundancy, I guess. Seems like thermocouple and thermopile are both activated when there's a good pilot flame. Since they are both right there at the pilot, it's not like one of them would be able to detect the fault condition that the pilot is lit but somehow it fails to ignite the main burner.