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Thread: Lennox Furnace not kicking on unless thermostat is turned off then on

  1. #1
    DIY Junior Member sharkbait's Avatar
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    Default Lennox Furnace not kicking on unless thermostat is turned off then on

    Issues with furnace not kicking on…. I have a Lennox G12Q4E-137-7 furnace and White Rodgers Digital Comfort II thermostat. All will work fine for most of the day or into the night but then at some point it does not kick on. It usually happens at night. I had the cover off to get the model number off the furnace and it kicked on while I was looking inside. Everything looked and sounded fine to my untrained eyes and ears. Every time we are able to fix it by manually turning the thermostat to OFF, waiting 30-45 seconds and then turning it back on. I replaced the batteries in the thermostat but that did not help. I am tired of waking up cold every morning…any ideas????
    Thanks!
    Dan
    Last edited by sharkbait; 12-27-2010 at 09:04 AM. Reason: O should be Q in model number

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    All a thermostat does is connect the Red (power) to the White (heating, first stage) when it calls for heat. It is effectively a switch that opens and closes (closed should turn on the furnace). But, keep in mind, these colors are what are marked on the thermostat, and they may have used different colored wires in the cable, so be careful. First thing is to make sure the wires are tight in their connections.

    The power on the red is (or should be) around 24vac - enough to spark, but normally not enough to hurt you except under really unusual circumstances. With many of these digital thermostats, removing it leaves the mounting plate and electrical connections on the wall. If you were to take an insulated piece of wire and connect (short) the red to the white, the furnace should come on. If it does, then it is the thermostat. If it does not, then it is the furnace. If you don't feel comfortable doing this, then you need to call in a qualified service man.

    If it ends up being the furnace, it could be numerous things, but that's another whole troubleshooting sequence. Your operator manual for the furnace should help there.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Junior Member sharkbait's Avatar
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    Thanks Jim. That is a good way to see if the thermostat is the problem. (makes me just a little nervous though.....) If I somehow connected the wrong 2 wires.....what is the worst that could happen???? Connecting the red to the white is easy enough, but I have been doing a little research and they have said that the wiring colors on the thermostats are sometimes a little suspect....

    Oh and by the way, I misread the model number. It should be: G12Q4E-137-7

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    Engineer Furd's Avatar
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    Turning the thermostat down below the room temperature and then back up above the room temperature indicates a "soft" lockout of the furnace. Turning the thermostat down (turning off the connection between red and white) resets the soft lockout.

    The most common cause of this soft lockout is failure to sense a flame on start up. Your furnace should have some diagnostic LED lamps that will blink in a code and the key to what code a specific blink pattern means is pasted on the back of the blower compartment door.

    Failure to sense the flame, when one is present, is usually due to oxides on the flame sensing rod that you will find on the burner tube farthest from the ignitor, often the left-most burner. It is normally held in place by one screw and you should remove the rod and clean it by rubbing with a Scotchbrite pad.

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    DIY Junior Member sharkbait's Avatar
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    Furd - that seems to make sense. It is probably a relatively easy fix, but I hate projects that if I make a mistake, then I can potentially burn my family out of the house...... I think that if that is the issue, I really should call in a tech. I do thank you much for your response!

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