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    eagano's Avatar
    eagano Posts: 4, Reputation: 2
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    #1

    Dec 10, 2006, 08:52 PM
    Bryant downflow - constant fan cycle
    Hi -

    I have a Bryant 373LAV/376CAV (not sure exactly which one) mounted horizontally in the attic that is <5 years old and has been functioning perfectly since installed, including for the first few months of this winter. This morning when I woke up, I realized that the fan was in a constant ~1 minute cycle, even with the heat off. This is not the main blower fan, it's the smaller fan (maybe the fan switch?). The fan that is next to the gas manifold, not under the logic board.

    The heat works perfectly when I turn the thermostat on and the temp up high enough for it to turn on:
    - The smaller fan comes on for about 30 seconds
    - The ignitor turns on and eventually glows brightly
    - the gas turns on and all the burners light up
    - the main blower comes on after about 1 minute and blows heat
    - when I turn off the thermostat, the burners go out immediately, the main blower stays on for about 1 minute, then shuts down

    About 3-4 seconds after the main blower fan shuts down, it starts this cycle:
    - The smaller fan comes on for about 30 seconds
    - The ignitor turns on and eventually glows brightly
    - I can hear a relay click and the ignitor goes off
    - the small fan continues to run for about 5 seconds
    - the small fan shuts down for 3-4 seconds (everything is off)
    - the whole process above repeats over and over: the smaller fan comes on for about 30 seconds...

    There are no error codes reported (solid red led). When I power off the unit and power up, I get the normal 90 second main blower operation (and the '12' error code indicating 90 second power-on blower), followed immediately by the fan/ignitor cycle outlined above. If I turn off the gas valve, the switch on the gas valve and then power up, it runs for 90 seconds, tries to startup, and goes to a '33' error code which I assume would be caused by the gas turned off. The logic board seems to be working to me.

    The strange part is that when I turn the temp up high enough, the heat kicks on and works great. But once the heat goes off, the smaller fan cycle just continues over and over.

    I'm pretty handy and have no problem changing any parts if I knew where to start. Does anyone recognize this problem or have a suggestion on how to troubleshoot?

    Thanks!
    Mike
    turnerwest's Avatar
    turnerwest Posts: 8, Reputation: 2
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    #2

    Dec 10, 2006, 09:19 PM
    Quote Originally Posted by eagano
    Hi -

    I have a Bryant 373LAV/376CAV (not sure exactly which one) mounted horizontally in the attic that is <5 years old and has been functioning perfectly since installed, including for the first few months of this winter. This morning when I woke up, I realized that the fan was in a constant ~1 minute cycle, even with the heat off. This is not the main blower fan, it's the smaller fan (maybe the fan switch?). The fan that is next to the gas manifold, not under the logic board.

    The heat works perfectly when I turn the thermostat on and the temp up high enough for it to turn on:
    - The smaller fan comes on for about 30 seconds
    - The ignitor turns on and eventually glows brightly
    - the gas turns on and and all the burners light up
    - the main blower comes on after about 1 minute and blows heat
    - when I turn off the thermostat, the burners go out immediately, the main blower stays on for about 1 minute, then shuts down

    About 3-4 seconds after the main blower fan shuts down, it starts this cycle:
    - The smaller fan comes on for about 30 seconds
    - The ignitor turns on and eventually glows brightly
    - I can hear a relay click and the ignitor goes off
    - the small fan continues to run for about 5 seconds
    - the small fan shuts down for 3-4 seconds (everything is off)
    - the whole process above repeats over and over: the smaller fan comes on for about 30 seconds...

    There are no error codes reported (solid red led). When I power off the unit and power up, I get the normal 90 second main blower operation (and the '12' error code indicating 90 second power-on blower), followed immediately by the fan/ignitor cycle outlined above. If I turn off the gas valve, the switch on the gas valve and then power up, it runs for 90 seconds, tries to startup, and goes to a '33' error code which I assume would be caused by the gas turned off. The logic board seems to be working to me.

    The strange part is that when I turn the temp up high enough, the heat kicks on and works great. But once the heat goes off, the smaller fan cycle just continues over and over.

    I'm pretty handy and have no problem changing any parts if I knew where to start. Does anyone recognize this problem or have a suggestion on how to troubleshoot?

    Thanks!
    Mike
    Mike you either have a loose solder connection on your main control board BUT more likely you have a defective board ASSUMING you have a digital thermostat NOT a analog (Mercury) one. Eric
    eagano's Avatar
    eagano Posts: 4, Reputation: 2
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    #3

    Dec 14, 2006, 02:23 PM
    Thanks for your response, Eric!

    That sounded like the culprit. I have two of these Bryant units in the attic, so I swapped the logic boards between them. After the swap, the units both behave the same: the good unit, with the board from the unit that isn't working, still works great, while the bad unit, with the board from the good unit, still does the 90 second power-on blower and then falls into this strange start/stop igntion cycle. I am using a digital thermostat (Totaline).

    Is there somewhere else I can look? I'm hoping that it is just a sensor that needs to be replaced or something. Are there other troubleshooting steps I can take?

    Thanks,
    Mike
    labman's Avatar
    labman Posts: 10,580, Reputation: 551
    Uber Member
     
    #4

    Dec 14, 2006, 05:33 PM
    To eliminate the thermostat as a cause disconnect either the red and white wires at the furnace. Then check and clean the flame detector or limit switches.
    letmetellu's Avatar
    letmetellu Posts: 3,151, Reputation: 317
    Ultra Member
     
    #5

    Dec 14, 2006, 07:56 PM
    Eagano... the first thing I would do it to disconnect the thermostat, or at least take the wires off the poles that they belong on. Then I would connect the red and white wires, assuming that they were the ones connected to the W pole and the R pole. I would let the furnace go through the operation to the point that the main blower comes on. I would then disconnect the white and red wires from each other and let the main blower do it's thing, Then just wait to see what happens, if the unit does not do this cycling thing you will know that it sounds like something wrong with the thermostat. If it does do it then it sounds like you might have a wire shorting out to the cabinet or something. It this case I would do the same thing I told you above but do it as the circuit board where all of the low voltage wires connect. Let us know what you find out.
    eagano's Avatar
    eagano Posts: 4, Reputation: 2
    New Member
     
    #6

    Dec 14, 2006, 09:35 PM
    Based on all these responses, I am guessing it is probably the thermostat. I realized that when I powered up the 'good unit' it did not do anything (no blower fan, nothing) as I suppose it should.

    When I power up the problem unit, the 'blower on after power up' phase runs for 90 seconds, then it starts into the repeating ignition cycle. The manual says that the 'blower on after power up' only happens when the unit is powered up during a call for heat (R-W closed). This suggests to me that my thermostat is hosed and is therefore always calling for heat?

    I'm going to try to switch the thermostats and see what happens. Thanks everyone for helping me through this! I'll post back what I find.

    Thanks,
    Mike
    labman's Avatar
    labman Posts: 10,580, Reputation: 551
    Uber Member
     
    #7

    Dec 14, 2006, 09:50 PM
    As both letme and I pointed out, pull the red or white wire off the control board . That prevents the thermostat from starting a heat cycle. Much easier than changing thermostats. Even if they unplug from the base, swapping them won't reveal shorted wires.
    eagano's Avatar
    eagano Posts: 4, Reputation: 2
    New Member
     
    #8

    Dec 14, 2006, 10:49 PM
    Labman - Thanks, sometime you have to really spell it out for me. :-)

    Success! I pulled out the red wire, powered up the unit and no blower cycling! At that point it seems to be either wiring or the thermostat based our your input. Since this was our primary heat unit, I swapped the thermostat from the other one, and everything works great! I'll get a new thermostat for the second unit tomorrow.

    Thanks everyone for your help! I really appreciate the guidance (and so does my wife who likes it quite a bit warmer than me!).

    Thanks,
    Mike

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