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Gas Furnace cycles when thermostat is off

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Szolusha
Szolusha Member Posts: 5
in Gas Heating
Hi all hope I posted to right area, I have a 19 yr old Nordyne furnace, it is located in attic and is very well ventilated
if anyone saw it they would say its brand new. Any way I had a problem with it 2 yrs ago and it was the main circuit board had a crack in it, I replaced it and all worked fine. Note its only heating 2 bedrooms and a family rm , with the ac unit outside. Now I am having a problem where the unit cycles by itself meaning thermostat is in off position, we are not running the ac unit or heat , thermo is OFF and about 3 hrs later there's air being blown through the vents? then it shuts off and wont do it again for 2 maybe 3 days and the air will start again then stop? I am confused and wondering what I should replace or ck
any help would be great
Thanks Mike

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  • JUGHNE
    JUGHNE Member Posts: 11,062
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    Would you have a thermostat that has a "recirculate" feature on the fan selection? Or just "fan on" and "auto" selections.
  • Szolusha
    Szolusha Member Posts: 5
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    new thermostat with auto and off and its in the autoposition
  • JUGHNE
    JUGHNE Member Posts: 11,062
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    When this happens is it running the heat or AC with the fan?
  • Szolusha
    Szolusha Member Posts: 5
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    no nothing just air
  • Jamie Hall
    Jamie Hall Member Posts: 23,327
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    Did it start to do this when the new thermostat was installed?
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Szolusha
    Szolusha Member Posts: 5
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    nope replaced it 2 wks ago, it started about april this year , when the ac was on during summer , in the middle of nite when ac would shut off 2 -3 am then I would hear the blower kick on with air, then 10 -30 min later the ac kicked back on so its not something I can pinpoint exactly when it happens. So we would shut off the over ride switch to the unit, then last wk I went up cleaned unit and threw the switch back on and it went 3 days with nothing then at 4 in afternoon it was blowing air all with the thermo off.
  • JUGHNE
    JUGHNE Member Posts: 11,062
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    IIWM, I would disconnect all the tstat wires from the furnace and see if the fan goes off on it's own in the future.
    Possible the R and G are shorted in the tstat cable.
    If good then you could reconnect the wires and disconnect the tstat at the wall.
    This process could be reversed if you wish.
    HVACNUT
  • ratio
    ratio Member Posts: 3,628
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    Check the rollout & limit switches for rusty/corroded connections. Some controls will call for the blower if they think that the HX is hot even if they're not calling for heat.
  • unclejohn
    unclejohn Member Posts: 1,833
    edited October 2019
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    does it have a temp. sensing fan limit switch. may be getting hot enough in attic to cycle fan
  • Szolusha
    Szolusha Member Posts: 5
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    Ratio and uncle john that is exactly what I thought it mite be so I went ahead and ordered one when I replace I will update thanks a lot guys for all help , you too Jughne appreciate all avenues

    Mike
  • HVACNUT
    HVACNUT Member Posts: 5,841
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    You stated you replaced the circuit board, so we're assuming (pic of the wiring diagram would be Gr8) the operating fan controls are relays and not a fan/limit (insertion)control. And the issue started even before you replaced the thermostat.
    Like @JUGHNE wrote, it might be a short. I would just remove the wire from the G terminal on the board, and wait, and wait. Do you have an electrical meter? You also said "main" board. Is there a separate time delay relay or sequencers?

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