Furnace burner remains burning after blower shuts down... intermittent concern

Hello,
My furnace recently started this. It goes thru the normal cycle but intermittently will continue burning after the blower shuts down. It will burn for maybe 30-45 seconds or so after shut down... then burners off. Then after about 30 seconds the blower will come back on for about 30 seconds.
It just had its bi annual checkup with the local HVAC company about 30 days ago and the only notation they made was possible low batteries in the thermostat. I replaced those shortly after the service because I noticed this problem on the first use of the furnace, and unfortunately have procrastinated finding the root cause since.
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I meant to add that this happens sporadically. Sometimes it will go days before I notice it again. GMC furnace. Honeywell digital thermostat.
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This is Turtle.
It would be nice to know a model and serial number of it and about how old is it.
Was it made before 1991 or made after 1991 ?
Does it have a vent fan that blows when the flames comes on. ?
now I will take it that it is a older model made before 1991 by GMC.
You have a fan/limit switch sticking. Change or read ajust the fan/limit switch. if labor cost is high in your area , change the fan/limit out with a new one.
Now if it is a GMC furnace made after 1991 or maybe 1993 that is another horse of a different color. You need it looked at NOW and not later. It could burn your house down doing this.
Reply with answer to the above or i answer e-mails too.
TURTLE
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TURTLE wrote:

Thanks for the reply Turtle!
It is a GMC, Goodman manufacturing, model GU100-4, REV A,S Serial Number 900281904. I could not find a manufactured date. I only know it was here when I purchased this older home. There is an ANS date code it appears of 1988?
Wow, fan limit switch. I was thinking something like gas valve since it was staying burning after fan shut down. Im sure glad we have knowledgable folks to help us from making stupid mistakes! THANK YOU!
Fordtech
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Darn forgot to add that it is a standing pilot light model, with no induction fan. Just the plain old cheapie Ive seen most of my life.
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GRRRR, sorry again, purchased home in 1995!
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And if you listen to what that knuckle head turtle says you just may electrocute yourself or blow yourself up. Your choice.
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This is Turtle.
you say these words about a HVAC business owner and Operator that has been in Business most likely before most of the poster here were born. To be exact 1956 . Whay was you doing in 1956 Elmo ?
TURTLE
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This is Turtle.
You have a Feb. 1990 date of manufactor date code in the serial number.
TUIRTLE
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My furnace did something similar. The burner would shut off. The blower would shut off nearly a minute later (normal). It would then turn on a few seconds later then stop after a couple seconds and may repeat. In my case, I found, on the control board, the power supply filter electrolytic capacitor had lost nearly all of its capacity to hold a charge. I had one in my parts case. I soldered it in and it has been perfect ever since (2 years ago).
John
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Most people would call that HVAC company back and not expect to pay more, since they obviously did not do a thorough job.
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snipped-for-privacy@UNLISTED.com wrote:

Hi, My guess is thermostat is acting up. Tony
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Thanks Turtle. The adjustment of the fan limit switch was all that was necessary to get it working correctly. Been 12 hours since adjusment and it was doing it almost every cycle since the cold wave hit. But since adjustment it hasnt missed a lick.
Much appreciated.
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Thanks for the report. Very helpful.
--

Christopher A. Young
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burners
about
30
in
unfortunately
The first thing I would look at is the fan/limit switch, too...
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but
It
about
batteries
I
Then the blower motor relay. Seen a few with intermittant solder connections where the coil windings attach to the terminals...

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Thanks Rick, that indeed was the problem. Thanks to all those will helpful offers of advice. :)
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I'm wondering about heat anticipator, or a duct blowing directly onto the stat.
--

Christopher A. Young
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