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.

I meant to add that this happens sporadically. Sometimes it will go
days before I notice it again. GMC furnace. Honeywell digital
thermostat.

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

TURTLE wrote:
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

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

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.

GRRRR, sorry again, purchased home in 1995!

"FordTech" wrote in message
oups.com...
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.

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

On 6 Dec 2005 16:38:54 -0800, "FordTech" wrote:

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.

Most people would call that HVAC company back and not expect to pay
more, since they obviously did not do a thorough job.

wrote:
On 6 Dec 2005 16:38:54 -0800, "FordTech" wrote:


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.


Most people would call that HVAC company back and not expect to pay
more, since they obviously did not do a thorough job.
Hi,
My guess is thermostat is acting up.
Tony

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.

Thanks for the report. Very helpful.

--

Christopher A. Young
Do good work.
It's longer in the short run
but shorter in the long run.
..
..


"FordTech" wrote in message
oups.com...
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.

"FordTech" wrote in message
oups.com...
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.


The first thing I would look at is the fan/limit switch, too...

"Rick" wrote in message
.net...

"FordTech" wrote in message
oups.com...
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.


The first thing I would look at is the fan/limit switch, too...

Then the blower motor relay. Seen a few with intermittant solder
connections where the coil windings attach to the terminals...

Thanks Rick, that indeed was the problem.
Thanks to all those will helpful offers of advice.

I'm wondering about heat anticipator, or a duct blowing directly onto the
stat.

--

Christopher A. Young
Do good work.
It's longer in the short run
but shorter in the long run.
..
..


"Rick" wrote in message
.net...

"FordTech" wrote in message
oups.com...
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.


The first thing I would look at is the fan/limit switch, too...