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Old 09-21-2011, 09:13 AM   #1
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Lennox Furnace Dead


Son has a 2 year old Lennox G50UH series furnace that will not fire up.
Thermostat has new batteries.
There is power to the furnace, from the make up box, through to the transformer.
The gas valve is "on"
Mini breaker next to cover switch is not popped
Both LED's are out(red and green).
It worked fine the first year. Last year it wasn't used and the gas to the house was turned off. House was heated with space heaters due to idiocy of occupants.
Any suggestions?
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Old 09-21-2011, 09:55 AM   #2
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Does anything happen when thermostat calls for heat? Does inducer fan start?

If both LEDs on circuit board are out there must not be power at the board. Have you checked the low voltage fuse?
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Old 09-21-2011, 10:06 AM   #3
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Does anything happen when thermostat calls for heat? Does inducer fan start?

If both LEDs on circuit board are out there must not be power at the board. Have you checked the low voltage fuse?
The furnace is silent when the thermostat is raised calling for heat.
The fuse is intact.
I'm going back with a meter to see if there's voltage coming out of the transformer. I'll also check the fuse for continuity in case it's not connected at an end.
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Old 09-21-2011, 11:47 AM   #4
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Lennox Furnace Dead


I would try connecting the thermostat with a short stub of wire, right at the furnace, and see if you can get a call for heat. It is also possible, that the thermostat is fubar.
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Old 09-21-2011, 01:31 PM   #5
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I would try connecting the thermostat with a short stub of wire, right at the furnace, and see if you can get a call for heat. It is also possible, that the thermostat is fubar.
I tried another thermostat an hour ago, no luck. I checked the transformer and had no reading coming out to the mini breaker on the low voltage side. I'll try replacing that. After that, it's a service call.
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Old 09-21-2011, 04:53 PM   #6
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Do you have high voltage at the transformer terminals of the control board?
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Old 09-21-2011, 05:26 PM   #7
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Do you have high voltage at the transformer terminals of the control board?
There is 110 volts on the line side of the transformer.
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Old 09-21-2011, 06:13 PM   #8
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There is 110 volts on the line side of the transformer.

Turn the stat to Auto and Cool and drop the temp. If the cooling works then it's not the transformer.
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Old 09-21-2011, 06:24 PM   #9
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Turn the stat to Auto and Cool and drop the temp. If the cooling works then it's not the transformer.
Will this just operate the fan in the furnace? He has no central A/C.
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Old 09-21-2011, 08:02 PM   #10
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If you have no LED lights that should be flashing slow without a call for heat, then you either have no 24V or you have a bad control board. WIth a call for heat the LED lights should flash fast.
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Old 09-21-2011, 08:03 PM   #11
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Will this just operate the fan in the furnace? He has no central A/C.
Put stat to fan on. If anything works than low voltage is present. Tyranny should be good then.
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Old 09-21-2011, 08:07 PM   #12
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Transformer gives stat control voltage so if any or one call works tyranny is good.
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Old 09-21-2011, 08:08 PM   #13
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Put stat to fan on. If anything works than low voltage is present. Tyranny should be good then.
How do you know that G is wired?
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Old 09-21-2011, 08:12 PM   #14
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How do you know that G is wired?
How much wood could a wood chuck chuck if a wood chuck could chuck wood?
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Old 09-21-2011, 08:16 PM   #15
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Tech, I don't know.
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