Furnace has issues (pilot light and more)

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  #1  
Old 11-05-04, 09:18 PM
rabbit16v
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Furnace has issues (pilot light and more)

Ok first off I would like to thank all of you who that work on these things for a living and take time out of your day to help those of use who are just trying to get on appliance working. It is very much appreciated!

Ok here is my story. I have a SEARS model 867.761111. I am pretty certain that the first three digits indicate the manufacturer. I could not find a production date on the unit. It is a gas powered furnace. I know its pretty old because it does not have electronic ignition and the last (oldest) service on the sticker on the cover is dated 1992. It states they replaced a fuse. All other entries on the service sticker are illegible.

Here is how it all goes down. I had the gas turned off about a year ago (they were charging me monthly even though I didnt use the gas). I had them come out about 4 days ago to turn it back on. The tech turned on the main valve and proceeded to light the pilot. I came home later that day and turned on the thermostat. Everything worked fine with this unit before, so I didnt suspect that there might be a problem. We can see where this is going After turning the thermostat up, I heard a click through the ductwork. No fire though. I went under the house to see what was up. Pilot is out. Ok no prob, I relight the pilot. Go back upstairs and turn thermostat up. Everything is good. The refreshing smell of old metal heating up is good. The furnace fired and after a minute or two the blower came on and we were happy. Bedtime comes around and I switch the thermostat off. Get home the next day and turn the thermostat up. Nothing. Ok well, maybe its just cause it hasnt run in a while. I go downstairs and relight the pilot which is out. Go back upstairs and turn the thermostat up again. Nothing. Go back downstairs and see that the pilot isnt lit. 4 unsuccessful tries later I deem it necessary for a smoke. Go back under the house and the pilot lights this time. I bang on the ductwork for my gf to turn the thermostat up so I can watch the burners light. Sure enough, they light this time. Sweet! I go to leave and just when I get near the door, I hear it turn off. GRRRRR!!!! I turn around and attempt to light it again (3 times). No love. Have another smoke. Go back and the pilot lights. Bang on duct work. This time I decided I was getting way too much excercise running up and down stairs, so I waited for the blower. While I waited for the blower (mind you the burners are running) I noticed a thermostatic device bolted to the burner chamber. This device I later find out is the one that turns on the blower. I watch it until the little peg setting gets to a certain point and the blower comes on. Immediately after the blower came on, the temp on the thermostatic device drops. I figure this is normal due to the air being vented from the chamber. It stabilizes around 130 degrees. Ok cool. Now I can sit down and watch TV. Works great for about 45 minutes. Then the whole thing shut off. I had the thermostat set to 90 degrees. It didnt even come close to 90 degrees. I was too tired of dealing with it last night so I went to bed (that and the downstairs is scary, nasty spiders and webs everywhere). Today I come home and try 3 times to light the pilot. No love again. Have another smoke. Try again and the pilot lights. Bang on the duct work. Burners fire. For about 15 seconds. Thats where I am at right now.

Any help or input would be greatly appreciated. The pilot knob is the kind that you have to push in and hold. The thermocouple (I think thats what it is) has a copper tube that follows the pilot feed tube and terminates right near the tip of the pilot feed tube. The thermocouple end does not glow red hot (I saw a post that said it was supposed to). I suspect the thermocouple is the problem. Is there a way to test it? I have a Fluke 88 DMM and am pretty familiar with how it to use it. It also has a temp monitoring kit that I bought for it. Can I use that to check the thermocouple. I hate to just buy a part for an ancient furnace and not know if it is going to fix it. I would prefer to properly diagnose the problem. Am I looking in the wrong direction? Do thermocouples just fail? Why is the sky blue?

Thanks again. Props to those of you that actually took the time to read all of that. I know it was rather long. I tried to keep it somewhat entertaining.

regards,

Chris
 
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  #2  
Old 11-06-04, 07:34 AM
Ed Imeduc's Avatar
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Wink

Do thermocouples just fail? Why is the sky blue?
How high is up and Ill tell you.

Yes all the time. Get a new one to start here ,what 4 or 5 bucks????? Make sure the pilot light is clean and the T/C gets red.Is the air filter clean and new?. When the blower comes on and change in the flame on the burners?? You can have a lot of Ifs here.

ED
 
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Old 11-06-04, 05:28 PM
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replace the tcouple, as ed said, only a few bucks,

watch what happens when someone else turns the tstat on, let us know
 
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Old 11-06-04, 07:03 PM
rabbit16v
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Ok, I put in a new thermocouple. No love again. Although, it did keep the pilot lit. That could be because its warmer out today and also it hasnt been tried since last night.

Anyway, lit the pilot after replacing the thermocouple. Then watched as the GF turned the thermostat up. Burners lit. Cool. Wait for the fan thermostat to turn the blower on. Blower comes on as qued. Cool. Ok lets see what happens when the thermostat is turned off. Burners shut off. Blower continues to run. Pilot is still lit. 15 seconds later the pilot goes out. Then the blower thermostat shuts the blower down. Tried to relight the pilot but my lighter is out of fluid. Thats where I am at right now.

Any other ideas??

regards,

chris
 
  #5  
Old 11-06-04, 07:28 PM
Gas-repairs4U
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Dear Rabbit,

I get this all the time up here in Maine. Here is the scoop. If you have already replaced the thermocouple, you have made it 50% of the way. Whenever I replace a thermocouple, I clean the pilot orifice. Especially in your case, where the furnace sat for a year with no gas through the orifices. Here's what to look for...
1. The flame in the pilot MUST be in full contact with the thermocouple.
2. If #1 is true, then watch the pilot flame as the blower comes on. Does it pull away from the thermocouple? If so, you have a "Cracked" Heat Exchanger.
3. If #1 & 2 are not a problem, look at the flame itself, is it blue all the way, or does it turn yellow at the end?
4. If #3 is true, you can use compressed air to blow out the dust or debris in the pilot assembly by forcing the air directly into the pilot assy' where the flame comes out.

Either way, enjoy a smoke again, and good luck to you.
 
  #6  
Old 11-09-04, 05:41 PM
rabbit16v
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Well here is an update. I dont know how this happened but it did and I am happy.

I started to take the burner assy out so I could take it to my shop and blow it out. I then decided that was more work than I felt like doing at the time. So I put it all back together and turned it on. Amazingly, the pilot stayed lit and the furnace worked. I figured it would be good to have it run full blast for a while. So I let it run with the thermostat cranked all the way up for about 5 hours. Man it was hot! Anyway, I turned it off before I went to bed. Came home today and turned it on. IT WORKS!!

Thanks again for all the help and ideas.
 
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