Confusion re old gas furnace thermocouples


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Old 05-13-12, 04:54 PM
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Question Confusion re old gas furnace thermocouples

My house has a Lennox GH5 furnace thatI suspect is as old as I am (and I'm not young). Cleaning out the pilot orifice solved a recent problem with the pilot thermocouple relay resetting. But while removing the pilot light assembly I noticed a couple of things that have me confused, and I'm hoping someone can explain them to me.

First, there are actually 2 orifices, one in line with the pilot thermocouple to its left and another about 90 degrees behind it. Two, there's a 2nd thermocouple to the right and a couple of inches in front connected to the main gas valve, nowhere near any flame that would be generated by the pilot.

So - is this 2nd thermocouple a safety sensor designed to close the main valve when things get too hot? Or has the pilot been mistakenly rotated and both orifices meant to be in line with both thermocouples? If not, why the dual orifices?

Thanks for any info.
 
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Old 05-13-12, 07:57 PM
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I would probably suggest (without pictures) that the one out of the flame is actually a vent tube from the gas valve or regulator. If you look closely you will notice it has an open end. Now I'm confused with the double orifices, it may be a restricter rather then an orifice. Are you able to post some pics?
 
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Old 05-13-12, 11:55 PM
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Sorry for the crappy pic - I'll post a better one tomorrow if needed after I take the assembly apart again.





The flame in the foreground is enveloping the hard-to-see thermocouple. Note the flame behind it. In the right foreground is what I've assumed is a second thermocouple.



Thermocouple connection to the relay.



Second thermocouple (?) connection to top of main valve.

Thanks for taking the time to look at this all!
 
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Old 05-14-12, 09:28 AM
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The main valve (control valve) is on the right and on the left is the pressure regulator. If you are referring to the copper tube coming out of the regulator that is a vent to allow the diaphragm to move in response to differing inlet gas pressure AND to vent any gas if the diaphragm ruptures. Venting to the vicinity of the pilot ensures that the gas will be burnt rather than accumulate and possibly explode.
 
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Old 05-14-12, 04:41 PM
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Thanks - vent tube it is.

Any ideas about the dual orifice? Everything works ok, I'm just trying to understand why I'd want a 2nd flame pointed at nothing.

Taken from about 10 o'clock position (with autofocus camera, sorry); so the right orifice is the one aligned with the thermocouple while the left one points more or less to the back.


 
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Old 05-14-12, 06:14 PM
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It is not a dual orifice but a "wing" pilot burner. One side should be towards the thermocouple and the other towards the main burner. This keeps the thermocouple somewhat away from the main flame while still monitoring the pilot and ensuring reliable ignition of the main burner.
 
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Old 05-14-12, 06:22 PM
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I think I understand your "two" orifices now. The pilot burner provides two flames, one to heat the thermocouple and the other for burner ignition. Are you calling those two flame openings orifices?

Looks like Furd beat me to it, shouldn't have stopped to move the hose.
 
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Old 05-14-12, 06:39 PM
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Furd, mbk3: Many thanks for your explanations - it all makes sense now. I'll watch my assumptions re terminology in the future!

Richard
 
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Old 05-14-12, 09:26 PM
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That furnace is probably over 50 years old.

Based on where the pilot tube is, the gas to the pilot won't shut off if the flame goes out. The thermocouple most likely just prevents the main valve from opening when the pilot is off.

By today's standards, your furnace isn't safe even if the heat exchanger is still okay - it should have been replaced a long time ago.
 

Last edited by user 10; 05-14-12 at 09:45 PM.
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Old 05-15-12, 03:26 PM
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Thanks - yes, I expect it is that old. And you're right, the thermocouple relay only controls the main valve. It's been hard to justify replacing a furnace that worked so well without problems - especially as we know the hot-damn new one will be lucky to last half as long - but I'll probably do so this summer. I was mostly just curious as to what was going on once I took a close look at the pilot assembly.
 
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Old 05-15-12, 09:15 PM
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A new high efficiency furnace can cut your gas bills by at least 1/3. (even if it's 70-80% efficient when running, the average efficiency over the heating season is most likely 55-65% due to pilot and cycling losses)

I can appreciate wanting to keep a safer 1980s or early 90s furnace going, but not something which is over 40 years old.

You're probably right about lifespan though; the metal on new ones is thin and they have more electrical components to go bad.
 
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Old 05-18-12, 01:03 AM
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Yeah, I'm onside. Time to get some estimates.
Thanks.
 
 

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