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#687876 - 10/10/11 05:30 PM Pilot only goes out at end of cycle
Louis Qatty Offline
newbie

Registered: 10/10/11
Posts: 3
I have a Lennox G12 furnace. I keep the pilot clean with annual maintenance. The filter is new. The thermocouple appears to be working as it kept the pilot lit all summer. Now, when the thermostat is turned on, everything does as it should, all thru the blower cycle. However, when it's time for the burners to turn off, the doggone pilot light goes out with it. The pilot re-lights easily, and stays on until the next cycle when the control valve turns off the gas to the manifold and burners, and then the pilot goes out as well. That's the only time the pilot goes out, and I'm pretty sure the control valve and pilot are separate circuits. Any ideas on this.?

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#687885 - 10/10/11 06:54 PM Re: Pilot only goes out at end of cycle [Re: Louis Qatty]
Punky Offline
Carpal Tunnel

Registered: 06/01/05
Posts: 3859
Loc: Springville, NY
Hi Louis,

The most common problem when this happens is a failed gas valve or thermocouple/Baso controller. You should first clean the pilot and orifice and install a new thermocouple. Make sure the flame of the pilot just barely wraps the thermocouple. This will ensure proper power generation. If the pilot goes out again after all this checking and cleaning you most probably need a new gas valve.

Your old G12 may have the separate Baso and gas valves. A new standing pilot gas valve would replace all of these older style controls.

You should have a tech inspect this furnace. The older G1* series furnaces were known to have cracked heat exchangers near the rear turns after just a few years of use.
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#687902 - 10/10/11 07:54 PM Re: Pilot only goes out at end of cycle [Re: Punky]
Louis Qatty Offline
newbie

Registered: 10/10/11
Posts: 3
Hi Punky: Thanks for the input. The flames are all pretty blue, the thermocouple sits nicely within the flame, and my 3 CO detectors have never indicated a problem around the house.
I can see how a bad valve or controller could cause problems, but why would it effect the pilot only at the end of the burner cycle.?. Why wouldn't it shut off at arbitrary times.?

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#687904 - 10/10/11 08:06 PM Re: Pilot only goes out at end of cycle [Re: Louis Qatty]
Punky Offline
Carpal Tunnel

Registered: 06/01/05
Posts: 3859
Loc: Springville, NY
I don't know why it does it that way. Hot water tank controls do it too. Sometimes they DO go out at arbitrary times but it's usually just after the main burners go out and there is a a hard draft on the pilot assembly. The pilot controller is weak and the wavering flame causes it to quit.

Just because you have CO detectors does not mean that your furnace is safe. When the main blower activates the pressure outside the exchanger is higher than inside it. CO would get pushed out the flue pipe. I have seen hundreds, literally, of these furnaces that are bad yet have NOT set off any detectors. Just a thought...
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#687907 - 10/10/11 08:21 PM Re: Pilot only goes out at end of cycle [Re: Punky]
Louis Qatty Offline
newbie

Registered: 10/10/11
Posts: 3
That's interesting info. A neighbor tossed out a thought, that it may be a problem with a 'high limit safety switch'. I thought I knew my furnace pretty well, but I don't know what that is, or even if my G12 furnace has one.. Is there any merit to that thought.?

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#687911 - 10/10/11 08:51 PM Re: Pilot only goes out at end of cycle [Re: Louis Qatty]
Punky Offline
Carpal Tunnel

Registered: 06/01/05
Posts: 3859
Loc: Springville, NY
no, the high limit only causes the 24 volt circuit - the one that makes the main gas solenoid fire - to open, thus cancelling the main burner.

the pilot is controlled solely by the thermocouple and pilot controller. its responsibility is to "tell" the gas valve that there is an ignition source for main burner operation.

a high limit problem would most likely be caused by a plugged filter or heat exchanger failure. the g12 has 1 high limit
_________________________
Opinions should be verified with your local town Board/Inspector/Review Panels

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#688104 - 10/12/11 09:15 AM Re: Pilot only goes out at end of cycle [Re: Louis Qatty]
dora Offline
Search and Rescue
Carpal Tunnel

Registered: 02/16/02
Posts: 21264
Loc: Somewhere under the sun
Quote:
when it's time for the burners to turn off, the doggone pilot light goes out with it.


Is the blower still on when the pilot goes out, it's supposed to be to cool the heat exchanger? Wondering if you may have a backdraft problem or possibly a cracked heat exchanger. Try starting the blower without the burners on and see if the pilot maintains a steady state or does it move outward.
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