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Old 10-29-2010, 07:21 PM   #1
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lennox mgf furnace


I have a Lennox 80 mgf furnace made in 1995 and it won't stay lit. I cleaned it well and that didn't seem to help, when it turns on the pre-lighting blower comes on to clear out any fumes, the furnace lights and the main blower doesn't come on and the burners go out so the pre-lighting blower will just run.

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Old 10-29-2010, 07:54 PM   #2
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Sounds as though the burners are turning off because the high limit is tripping.

Does the blower come on when you turn the thermostat fan selector to the on position?

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Old 10-29-2010, 08:08 PM   #3
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The blower will run in the "on" position but the burners go out and don't try to relight, usually it will try 3 times total but now the primary blower will just keep running.
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Old 10-29-2010, 08:12 PM   #4
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How long does the burners stay lit?
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Old 10-29-2010, 08:23 PM   #5
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I just turned the blower to the on position and restarted the furnace and it's working like a charm, so the part that I need is a "High Limit Switch' you say.
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Old 10-29-2010, 08:27 PM   #6
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Let's not jump to quick here. I don't think the limit is the problem.

Just to be sure

You make a call for heat, inducing blower comes on, burners lit and burn for some time 5-10 minutes, main blower does not come on and the burners go off.

Do I have this right?
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Old 10-29-2010, 08:40 PM   #7
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The stat calls for heat and the inducing blower comes on for a minute the burners light for about 30 seconds and then go out with the main blower never coming on and the inducing blower continuing to run after the burners go out.
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Old 10-29-2010, 08:51 PM   #8
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Ok

Look at the gas valve and follow where the gas comes in and goes out. Follow the pipe going out to the burners all the way to the last burner. In front of the last or nest to last there should be a rod about 1/8" in diameter with a single wire coming off it. This is the flame sensor. Usually it is a phillips ot a 1/4" nut driver to remove the screw holding it in place.

Remove the flame sensor and clan the rod with steel wool or emory cloth. Clean the rod with a clean cotton cloth. After you clean the rod DO NOT TOUCH THE ROD WITH YOUR HANDS. THe oils of your hands will contaminate the rod. Reinstall the rod and this should cure you problems.
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Old 10-29-2010, 09:15 PM   #9
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Can I use some fine sandpaper?
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Old 10-29-2010, 09:17 PM   #10
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Very fine and make sure you wipe the sensor clean
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Old 10-29-2010, 09:34 PM   #11
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Excellent call, you are the man! It seems to be working fine now but the old flame sensor is discolored even after being cleaned do you think that it could use a new one?

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