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QUESTION:This Sunday morning our natural gas furnace suddenly quit working. No pilot
light. Tried to relight the pilot light and the light would not stay lit
when the pilot button was released.
Removed the pilot light assembly (pilot gas line and thermocouple) from
the furnace, measured the output of the thermocouple and found that
the thermocouple has an output voltage of about 25 millivolts when heated
up by the pilot flame and about zero millivolts at room temperature.
Don't have a good known thermocouple to compare with.
The gas furnace is made by a mfg out in Texas, name not recognized, but
the furnace has a Honeywell gas control valve, and the furnace is about
7-8 years old.
Also, with a fine sandpaper burnished the thermouple contact where it
fits into the gas valve body and that didn't solve the problem.
Any suggestions for the next step to determine what's wrong and needs
fixing or replacing?
ANSWER: Why the speculation on whether the thermocouple is bad?
It probably is and for the $6 or so replacement cost, you'd know
for sure. Many hardware stores, plumbing supply outlets and
the Depot sells them. The thermocouples are pretty standard -
show them the old one and they'll match it.
Those Honeywell gas valves rarely fail.
The 25 mv figure, in any event, may not be relevant - the measurement
will vary with the impedance of your meter. They are nominally rated
at 30 mv - if that means anything. Their current output may also drop
off due to oxidation or contamination. The pros don't test them -
they are simply replaced.
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